//===-- lib/Evaluate/check-expression.cpp ---------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "flang/Evaluate/check-expression.h"
#include "flang/Evaluate/characteristics.h"
#include "flang/Evaluate/intrinsics.h"
#include "flang/Evaluate/tools.h"
#include "flang/Evaluate/traverse.h"
#include "flang/Evaluate/type.h"
#include "flang/Semantics/semantics.h"
#include "flang/Semantics/symbol.h"
#include "flang/Semantics/tools.h"
#include <set>
#include <string>

namespace Fortran::evaluate {

// Constant expression predicates IsConstantExpr() & IsScopeInvariantExpr().
// This code determines whether an expression is a "constant expression"
// in the sense of section 10.1.12.  This is not the same thing as being
// able to fold it (yet) into a known constant value; specifically,
// the expression may reference derived type kind parameters whose values
// are not yet known.
//
// The variant form (IsScopeInvariantExpr()) also accepts symbols that are
// INTENT(IN) dummy arguments without the VALUE attribute.
template <bool INVARIANT>
class IsConstantExprHelper
    : public AllTraverse<IsConstantExprHelper<INVARIANT>, true> {
public:
  using Base = AllTraverse<IsConstantExprHelper, true>;
  IsConstantExprHelper() : Base{*this} {}
  using Base::operator();

  // A missing expression is not considered to be constant.
  template <typename A> bool operator()(const std::optional<A> &x) const {
    return x && (*this)(*x);
  }

  bool operator()(const TypeParamInquiry &inq) const {
    return INVARIANT || semantics::IsKindTypeParameter(inq.parameter());
  }
  bool operator()(const semantics::Symbol &symbol) const {
    const auto &ultimate{GetAssociationRoot(symbol)};
    return IsNamedConstant(ultimate) || IsImpliedDoIndex(ultimate) ||
        IsInitialProcedureTarget(ultimate) ||
        ultimate.has<semantics::TypeParamDetails>() ||
        (INVARIANT && IsIntentIn(symbol) && !IsOptional(symbol) &&
            !symbol.attrs().test(semantics::Attr::VALUE));
  }
  bool operator()(const CoarrayRef &) const { return false; }
  bool operator()(const semantics::ParamValue &param) const {
    return param.isExplicit() && (*this)(param.GetExplicit());
  }
  bool operator()(const ProcedureRef &) const;
  bool operator()(const StructureConstructor &constructor) const {
    for (const auto &[symRef, expr] : constructor) {
      if (!IsConstantStructureConstructorComponent(*symRef, expr.value())) {
        return false;
      }
    }
    return true;
  }
  bool operator()(const Component &component) const {
    return (*this)(component.base());
  }
  // Forbid integer division by zero in constants.
  template <int KIND>
  bool operator()(
      const Divide<Type<TypeCategory::Integer, KIND>> &division) const {
    using T = Type<TypeCategory::Integer, KIND>;
    if (const auto divisor{GetScalarConstantValue<T>(division.right())}) {
      return !divisor->IsZero() && (*this)(division.left());
    } else {
      return false;
    }
  }

  bool operator()(const Constant<SomeDerived> &) const { return true; }
  bool operator()(const DescriptorInquiry &x) const {
    const Symbol &sym{x.base().GetLastSymbol()};
    return INVARIANT && !IsAllocatable(sym) &&
        (!IsDummy(sym) ||
            (IsIntentIn(sym) && !IsOptional(sym) &&
                !sym.attrs().test(semantics::Attr::VALUE)));
  }

private:
  bool IsConstantStructureConstructorComponent(
      const Symbol &, const Expr<SomeType> &) const;
  bool IsConstantExprShape(const Shape &) const;
};

template <bool INVARIANT>
bool IsConstantExprHelper<INVARIANT>::IsConstantStructureConstructorComponent(
    const Symbol &component, const Expr<SomeType> &expr) const {
  if (IsAllocatable(component)) {
    return IsNullObjectPointer(expr);
  } else if (IsPointer(component)) {
    return IsNullPointer(expr) || IsInitialDataTarget(expr) ||
        IsInitialProcedureTarget(expr);
  } else {
    return (*this)(expr);
  }
}

template <bool INVARIANT>
bool IsConstantExprHelper<INVARIANT>::operator()(
    const ProcedureRef &call) const {
  // LBOUND, UBOUND, and SIZE with truly constant DIM= arguments will have
  // been rewritten into DescriptorInquiry operations.
  if (const auto *intrinsic{std::get_if<SpecificIntrinsic>(&call.proc().u)}) {
    const characteristics::Procedure &proc{intrinsic->characteristics.value()};
    if (intrinsic->name == "kind" ||
        intrinsic->name == IntrinsicProcTable::InvalidName ||
        call.arguments().empty() || !call.arguments()[0]) {
      // kind is always a constant, and we avoid cascading errors by considering
      // invalid calls to intrinsics to be constant
      return true;
    } else if (intrinsic->name == "lbound") {
      auto base{ExtractNamedEntity(call.arguments()[0]->UnwrapExpr())};
      return base && IsConstantExprShape(GetLBOUNDs(*base));
    } else if (intrinsic->name == "ubound") {
      auto base{ExtractNamedEntity(call.arguments()[0]->UnwrapExpr())};
      return base && IsConstantExprShape(GetUBOUNDs(*base));
    } else if (intrinsic->name == "shape" || intrinsic->name == "size") {
      auto shape{GetShape(call.arguments()[0]->UnwrapExpr())};
      return shape && IsConstantExprShape(*shape);
    } else if (proc.IsPure()) {
      for (const auto &arg : call.arguments()) {
        if (!arg) {
          return false;
        } else if (const auto *expr{arg->UnwrapExpr()};
                   !expr || !(*this)(*expr)) {
          return false;
        }
      }
      return true;
    }
    // TODO: STORAGE_SIZE
  }
  return false;
}

template <bool INVARIANT>
bool IsConstantExprHelper<INVARIANT>::IsConstantExprShape(
    const Shape &shape) const {
  for (const auto &extent : shape) {
    if (!(*this)(extent)) {
      return false;
    }
  }
  return true;
}

template <typename A> bool IsConstantExpr(const A &x) {
  return IsConstantExprHelper<false>{}(x);
}
template bool IsConstantExpr(const Expr<SomeType> &);
template bool IsConstantExpr(const Expr<SomeInteger> &);
template bool IsConstantExpr(const Expr<SubscriptInteger> &);
template bool IsConstantExpr(const StructureConstructor &);

// IsScopeInvariantExpr()
template <typename A> bool IsScopeInvariantExpr(const A &x) {
  return IsConstantExprHelper<true>{}(x);
}
template bool IsScopeInvariantExpr(const Expr<SomeType> &);
template bool IsScopeInvariantExpr(const Expr<SomeInteger> &);
template bool IsScopeInvariantExpr(const Expr<SubscriptInteger> &);

// IsActuallyConstant()
struct IsActuallyConstantHelper {
  template <typename A> bool operator()(const A &) { return false; }
  template <typename T> bool operator()(const Constant<T> &) { return true; }
  template <typename T> bool operator()(const Parentheses<T> &x) {
    return (*this)(x.left());
  }
  template <typename T> bool operator()(const Expr<T> &x) {
    return common::visit([=](const auto &y) { return (*this)(y); }, x.u);
  }
  bool operator()(const Expr<SomeType> &x) {
    return common::visit([this](const auto &y) { return (*this)(y); }, x.u);
  }
  bool operator()(const StructureConstructor &x) {
    for (const auto &pair : x) {
      const Expr<SomeType> &y{pair.second.value()};
      const auto sym{pair.first};
      const bool compIsConstant{(*this)(y)};
      // If an allocatable component is initialized by a constant,
      // the structure constructor is not a constant.
      if ((!compIsConstant && !IsNullPointer(y)) ||
          (compIsConstant && IsAllocatable(sym))) {
        return false;
      }
    }
    return true;
  }
  template <typename A> bool operator()(const A *x) { return x && (*this)(*x); }
  template <typename A> bool operator()(const std::optional<A> &x) {
    return x && (*this)(*x);
  }
};

template <typename A> bool IsActuallyConstant(const A &x) {
  return IsActuallyConstantHelper{}(x);
}

template bool IsActuallyConstant(const Expr<SomeType> &);
template bool IsActuallyConstant(const Expr<SomeInteger> &);
template bool IsActuallyConstant(const Expr<SubscriptInteger> &);
template bool IsActuallyConstant(const std::optional<Expr<SubscriptInteger>> &);

// Object pointer initialization checking predicate IsInitialDataTarget().
// This code determines whether an expression is allowable as the static
// data address used to initialize a pointer with "=> x".  See C765.
class IsInitialDataTargetHelper
    : public AllTraverse<IsInitialDataTargetHelper, true> {
public:
  using Base = AllTraverse<IsInitialDataTargetHelper, true>;
  using Base::operator();
  explicit IsInitialDataTargetHelper(parser::ContextualMessages *m)
      : Base{*this}, messages_{m} {}

  bool emittedMessage() const { return emittedMessage_; }

  bool operator()(const BOZLiteralConstant &) const { return false; }
  bool operator()(const NullPointer &) const { return true; }
  template <typename T> bool operator()(const Constant<T> &) const {
    return false;
  }
  bool operator()(const semantics::Symbol &symbol) {
    // This function checks only base symbols, not components.
    const Symbol &ultimate{symbol.GetUltimate()};
    if (const auto *assoc{
            ultimate.detailsIf<semantics::AssocEntityDetails>()}) {
      if (const auto &expr{assoc->expr()}) {
        if (IsVariable(*expr)) {
          return (*this)(*expr);
        } else if (messages_) {
          messages_->Say(
              "An initial data target may not be an associated expression ('%s')"_err_en_US,
              ultimate.name());
          emittedMessage_ = true;
        }
      }
      return false;
    } else if (!CheckVarOrComponent(ultimate)) {
      return false;
    } else if (!ultimate.attrs().test(semantics::Attr::TARGET)) {
      if (messages_) {
        messages_->Say(
            "An initial data target may not be a reference to an object '%s' that lacks the TARGET attribute"_err_en_US,
            ultimate.name());
        emittedMessage_ = true;
      }
      return false;
    } else if (!IsSaved(ultimate)) {
      if (messages_) {
        messages_->Say(
            "An initial data target may not be a reference to an object '%s' that lacks the SAVE attribute"_err_en_US,
            ultimate.name());
        emittedMessage_ = true;
      }
      return false;
    } else {
      return true;
    }
  }
  bool operator()(const StaticDataObject &) const { return false; }
  bool operator()(const TypeParamInquiry &) const { return false; }
  bool operator()(const Triplet &x) const {
    return IsConstantExpr(x.lower()) && IsConstantExpr(x.upper()) &&
        IsConstantExpr(x.stride());
  }
  bool operator()(const Subscript &x) const {
    return common::visit(common::visitors{
                             [&](const Triplet &t) { return (*this)(t); },
                             [&](const auto &y) {
                               return y.value().Rank() == 0 &&
                                   IsConstantExpr(y.value());
                             },
                         },
        x.u);
  }
  bool operator()(const CoarrayRef &) const { return false; }
  bool operator()(const Component &x) {
    return CheckVarOrComponent(x.GetLastSymbol()) && (*this)(x.base());
  }
  bool operator()(const Substring &x) const {
    return IsConstantExpr(x.lower()) && IsConstantExpr(x.upper()) &&
        (*this)(x.parent());
  }
  bool operator()(const DescriptorInquiry &) const { return false; }
  template <typename T> bool operator()(const ArrayConstructor<T> &) const {
    return false;
  }
  bool operator()(const StructureConstructor &) const { return false; }
  template <typename D, typename R, typename... O>
  bool operator()(const Operation<D, R, O...> &) const {
    return false;
  }
  template <typename T> bool operator()(const Parentheses<T> &x) const {
    return (*this)(x.left());
  }
  bool operator()(const ProcedureRef &x) const {
    if (const SpecificIntrinsic * intrinsic{x.proc().GetSpecificIntrinsic()}) {
      return intrinsic->characteristics.value().attrs.test(
          characteristics::Procedure::Attr::NullPointer);
    }
    return false;
  }
  bool operator()(const Relational<SomeType> &) const { return false; }

private:
  bool CheckVarOrComponent(const semantics::Symbol &symbol) {
    const Symbol &ultimate{symbol.GetUltimate()};
    const char *unacceptable{nullptr};
    if (ultimate.Corank() > 0) {
      unacceptable = "a coarray";
    } else if (IsAllocatable(ultimate)) {
      unacceptable = "an ALLOCATABLE";
    } else if (IsPointer(ultimate)) {
      unacceptable = "a POINTER";
    } else {
      return true;
    }
    if (messages_) {
      messages_->Say(
          "An initial data target may not be a reference to %s '%s'"_err_en_US,
          unacceptable, ultimate.name());
      emittedMessage_ = true;
    }
    return false;
  }

  parser::ContextualMessages *messages_;
  bool emittedMessage_{false};
};

bool IsInitialDataTarget(
    const Expr<SomeType> &x, parser::ContextualMessages *messages) {
  IsInitialDataTargetHelper helper{messages};
  bool result{helper(x)};
  if (!result && messages && !helper.emittedMessage()) {
    messages->Say(
        "An initial data target must be a designator with constant subscripts"_err_en_US);
  }
  return result;
}

bool IsInitialProcedureTarget(const semantics::Symbol &symbol) {
  const auto &ultimate{symbol.GetUltimate()};
  return common::visit(
      common::visitors{
          [&](const semantics::SubprogramDetails &subp) {
            return !subp.isDummy() && !subp.stmtFunction() &&
                symbol.owner().kind() != semantics::Scope::Kind::MainProgram &&
                symbol.owner().kind() != semantics::Scope::Kind::Subprogram;
          },
          [](const semantics::SubprogramNameDetails &x) {
            return x.kind() != semantics::SubprogramKind::Internal;
          },
          [&](const semantics::ProcEntityDetails &proc) {
            return !semantics::IsPointer(ultimate) && !proc.isDummy();
          },
          [](const auto &) { return false; },
      },
      ultimate.details());
}

bool IsInitialProcedureTarget(const ProcedureDesignator &proc) {
  if (const auto *intrin{proc.GetSpecificIntrinsic()}) {
    return !intrin->isRestrictedSpecific;
  } else if (proc.GetComponent()) {
    return false;
  } else {
    return IsInitialProcedureTarget(DEREF(proc.GetSymbol()));
  }
}

bool IsInitialProcedureTarget(const Expr<SomeType> &expr) {
  if (const auto *proc{std::get_if<ProcedureDesignator>(&expr.u)}) {
    return IsInitialProcedureTarget(*proc);
  } else {
    return IsNullProcedurePointer(expr);
  }
}

// Converts, folds, and then checks type, rank, and shape of an
// initialization expression for a named constant, a non-pointer
// variable static initialization, a component default initializer,
// a type parameter default value, or instantiated type parameter value.
std::optional<Expr<SomeType>> NonPointerInitializationExpr(const Symbol &symbol,
    Expr<SomeType> &&x, FoldingContext &context,
    const semantics::Scope *instantiation) {
  CHECK(!IsPointer(symbol));
  if (auto symTS{
          characteristics::TypeAndShape::Characterize(symbol, context)}) {
    auto xType{x.GetType()};
    auto converted{ConvertToType(symTS->type(), Expr<SomeType>{x})};
    if (!converted &&
        symbol.owner().context().IsEnabled(
            common::LanguageFeature::LogicalIntegerAssignment)) {
      converted = DataConstantConversionExtension(context, symTS->type(), x);
      if (converted &&
          symbol.owner().context().ShouldWarn(
              common::LanguageFeature::LogicalIntegerAssignment)) {
        context.messages().Say(
            "nonstandard usage: initialization of %s with %s"_port_en_US,
            symTS->type().AsFortran(), x.GetType().value().AsFortran());
      }
    }
    if (converted) {
      auto folded{Fold(context, std::move(*converted))};
      if (IsActuallyConstant(folded)) {
        int symRank{GetRank(symTS->shape())};
        if (IsImpliedShape(symbol)) {
          if (folded.Rank() == symRank) {
            return ArrayConstantBoundChanger{
                std::move(*AsConstantExtents(
                    context, GetRawLowerBounds(context, NamedEntity{symbol})))}
                .ChangeLbounds(std::move(folded));
          } else {
            context.messages().Say(
                "Implied-shape parameter '%s' has rank %d but its initializer has rank %d"_err_en_US,
                symbol.name(), symRank, folded.Rank());
          }
        } else if (auto extents{AsConstantExtents(context, symTS->shape())}) {
          if (folded.Rank() == 0 && symRank == 0) {
            // symbol and constant are both scalars
            return {std::move(folded)};
          } else if (folded.Rank() == 0 && symRank > 0) {
            // expand the scalar constant to an array
            return ScalarConstantExpander{std::move(*extents),
                AsConstantExtents(
                    context, GetRawLowerBounds(context, NamedEntity{symbol}))}
                .Expand(std::move(folded));
          } else if (auto resultShape{GetShape(context, folded)}) {
            if (CheckConformance(context.messages(), symTS->shape(),
                    *resultShape, CheckConformanceFlags::None,
                    "initialized object", "initialization expression")
                    .value_or(false /*fail if not known now to conform*/)) {
              // make a constant array with adjusted lower bounds
              return ArrayConstantBoundChanger{
                  std::move(*AsConstantExtents(context,
                      GetRawLowerBounds(context, NamedEntity{symbol})))}
                  .ChangeLbounds(std::move(folded));
            }
          }
        } else if (IsNamedConstant(symbol)) {
          if (IsExplicitShape(symbol)) {
            context.messages().Say(
                "Named constant '%s' array must have constant shape"_err_en_US,
                symbol.name());
          } else {
            // Declaration checking handles other cases
          }
        } else {
          context.messages().Say(
              "Shape of initialized object '%s' must be constant"_err_en_US,
              symbol.name());
        }
      } else if (IsErrorExpr(folded)) {
      } else if (IsLenTypeParameter(symbol)) {
        return {std::move(folded)};
      } else if (IsKindTypeParameter(symbol)) {
        if (instantiation) {
          context.messages().Say(
              "Value of kind type parameter '%s' (%s) must be a scalar INTEGER constant"_err_en_US,
              symbol.name(), folded.AsFortran());
        } else {
          return {std::move(folded)};
        }
      } else if (IsNamedConstant(symbol)) {
        if (symbol.name() == "numeric_storage_size" &&
            symbol.owner().IsModule() &&
            DEREF(symbol.owner().symbol()).name() == "iso_fortran_env") {
          // Very special case: numeric_storage_size is not folded until
          // it read from the iso_fortran_env module file, as its value
          // depends on compilation options.
          return {std::move(folded)};
        }
        context.messages().Say(
            "Value of named constant '%s' (%s) cannot be computed as a constant value"_err_en_US,
            symbol.name(), folded.AsFortran());
      } else {
        context.messages().Say(
            "Initialization expression for '%s' (%s) cannot be computed as a constant value"_err_en_US,
            symbol.name(), folded.AsFortran());
      }
    } else if (xType) {
      context.messages().Say(
          "Initialization expression cannot be converted to declared type of '%s' from %s"_err_en_US,
          symbol.name(), xType->AsFortran());
    } else {
      context.messages().Say(
          "Initialization expression cannot be converted to declared type of '%s'"_err_en_US,
          symbol.name());
    }
  }
  return std::nullopt;
}

static bool IsNonLocal(const semantics::Symbol &symbol) {
  return semantics::IsDummy(symbol) || symbol.has<semantics::UseDetails>() ||
      symbol.owner().kind() == semantics::Scope::Kind::Module ||
      semantics::FindCommonBlockContaining(symbol) ||
      symbol.has<semantics::HostAssocDetails>();
}

static bool IsPermissibleInquiry(const semantics::Symbol &firstSymbol,
    const semantics::Symbol &lastSymbol, DescriptorInquiry::Field field,
    const semantics::Scope &localScope) {
  if (IsNonLocal(firstSymbol)) {
    return true;
  }
  if (&localScope != &firstSymbol.owner()) {
    return true;
  }
  // Inquiries on local objects may not access a deferred bound or length.
  // (This code used to be a switch, but it proved impossible to write it
  // thus without running afoul of bogus warnings from different C++
  // compilers.)
  if (field == DescriptorInquiry::Field::Rank) {
    return true; // always known
  }
  const auto *object{lastSymbol.detailsIf<semantics::ObjectEntityDetails>()};
  if (field == DescriptorInquiry::Field::LowerBound ||
      field == DescriptorInquiry::Field::Extent ||
      field == DescriptorInquiry::Field::Stride) {
    return object && !object->shape().CanBeDeferredShape();
  }
  if (field == DescriptorInquiry::Field::Len) {
    return object && object->type() &&
        object->type()->category() == semantics::DeclTypeSpec::Character &&
        !object->type()->characterTypeSpec().length().isDeferred();
  }
  return false;
}

// Specification expression validation (10.1.11(2), C1010)
class CheckSpecificationExprHelper
    : public AnyTraverse<CheckSpecificationExprHelper,
          std::optional<std::string>> {
public:
  using Result = std::optional<std::string>;
  using Base = AnyTraverse<CheckSpecificationExprHelper, Result>;
  explicit CheckSpecificationExprHelper(
      const semantics::Scope &s, FoldingContext &context)
      : Base{*this}, scope_{s}, context_{context} {}
  using Base::operator();

  Result operator()(const CoarrayRef &) const { return "coindexed reference"; }

  Result operator()(const semantics::Symbol &symbol) const {
    const auto &ultimate{symbol.GetUltimate()};
    if (const auto *assoc{
            ultimate.detailsIf<semantics::AssocEntityDetails>()}) {
      return (*this)(assoc->expr());
    } else if (semantics::IsNamedConstant(ultimate) ||
        ultimate.owner().IsModule() || ultimate.owner().IsSubmodule()) {
      return std::nullopt;
    } else if (scope_.IsDerivedType() &&
        IsVariableName(ultimate)) { // C750, C754
      return "derived type component or type parameter value not allowed to "
             "reference variable '"s +
          ultimate.name().ToString() + "'";
    } else if (IsDummy(ultimate)) {
      if (ultimate.attrs().test(semantics::Attr::OPTIONAL)) {
        return "reference to OPTIONAL dummy argument '"s +
            ultimate.name().ToString() + "'";
      } else if (!inInquiry_ &&
          ultimate.attrs().test(semantics::Attr::INTENT_OUT)) {
        return "reference to INTENT(OUT) dummy argument '"s +
            ultimate.name().ToString() + "'";
      } else if (ultimate.has<semantics::ObjectEntityDetails>()) {
        return std::nullopt;
      } else {
        return "dummy procedure argument";
      }
    } else if (&symbol.owner() != &scope_ || &ultimate.owner() != &scope_) {
      return std::nullopt; // host association is in play
    } else if (const auto *object{
                   ultimate.detailsIf<semantics::ObjectEntityDetails>()}) {
      if (object->commonBlock()) {
        return std::nullopt;
      }
    }
    if (inInquiry_) {
      return std::nullopt;
    } else {
      return "reference to local entity '"s + ultimate.name().ToString() + "'";
    }
  }

  Result operator()(const Component &x) const {
    // Don't look at the component symbol.
    return (*this)(x.base());
  }
  Result operator()(const ArrayRef &x) const {
    if (auto result{(*this)(x.base())}) {
      return result;
    }
    // The subscripts don't get special protection for being in a
    // specification inquiry context;
    auto restorer{common::ScopedSet(inInquiry_, false)};
    return (*this)(x.subscript());
  }
  Result operator()(const Substring &x) const {
    if (auto result{(*this)(x.parent())}) {
      return result;
    }
    // The bounds don't get special protection for being in a
    // specification inquiry context;
    auto restorer{common::ScopedSet(inInquiry_, false)};
    if (auto result{(*this)(x.lower())}) {
      return result;
    }
    return (*this)(x.upper());
  }
  Result operator()(const DescriptorInquiry &x) const {
    // Many uses of SIZE(), LBOUND(), &c. that are valid in specification
    // expressions will have been converted to expressions over descriptor
    // inquiries by Fold().
    // Catch REAL, ALLOCATABLE :: X(:); REAL :: Y(SIZE(X))
    if (IsPermissibleInquiry(x.base().GetFirstSymbol(),
            x.base().GetLastSymbol(), x.field(), scope_)) {
      auto restorer{common::ScopedSet(inInquiry_, true)};
      return (*this)(x.base());
    } else if (IsConstantExpr(x)) {
      return std::nullopt;
    } else {
      return "non-constant descriptor inquiry not allowed for local object";
    }
  }

  Result operator()(const TypeParamInquiry &inq) const {
    if (scope_.IsDerivedType() && !IsConstantExpr(inq) &&
        inq.base() /* X%T, not local T */) { // C750, C754
      return "non-constant reference to a type parameter inquiry not "
             "allowed for derived type components or type parameter values";
    }
    return std::nullopt;
  }

  Result operator()(const ProcedureRef &x) const {
    bool inInquiry{false};
    if (const auto *symbol{x.proc().GetSymbol()}) {
      const Symbol &ultimate{symbol->GetUltimate()};
      if (!semantics::IsPureProcedure(ultimate)) {
        return "reference to impure function '"s + ultimate.name().ToString() +
            "'";
      }
      if (semantics::IsStmtFunction(ultimate)) {
        return "reference to statement function '"s +
            ultimate.name().ToString() + "'";
      }
      if (scope_.IsDerivedType()) { // C750, C754
        return "reference to function '"s + ultimate.name().ToString() +
            "' not allowed for derived type components or type parameter"
            " values";
      }
      if (auto procChars{characteristics::Procedure::Characterize(
              x.proc(), context_, /*emitError=*/true)}) {
        const auto iter{std::find_if(procChars->dummyArguments.begin(),
            procChars->dummyArguments.end(),
            [](const characteristics::DummyArgument &dummy) {
              return std::holds_alternative<characteristics::DummyProcedure>(
                  dummy.u);
            })};
        if (iter != procChars->dummyArguments.end()) {
          return "reference to function '"s + ultimate.name().ToString() +
              "' with dummy procedure argument '" + iter->name + '\'';
        }
      }
      // References to internal functions are caught in expression semantics.
      // TODO: other checks for standard module procedures
    } else { // intrinsic
      const SpecificIntrinsic &intrin{DEREF(x.proc().GetSpecificIntrinsic())};
      inInquiry = context_.intrinsics().GetIntrinsicClass(intrin.name) ==
          IntrinsicClass::inquiryFunction;
      if (scope_.IsDerivedType()) { // C750, C754
        if ((context_.intrinsics().IsIntrinsic(intrin.name) &&
                badIntrinsicsForComponents_.find(intrin.name) !=
                    badIntrinsicsForComponents_.end())) {
          return "reference to intrinsic '"s + intrin.name +
              "' not allowed for derived type components or type parameter"
              " values";
        }
        if (inInquiry && !IsConstantExpr(x)) {
          return "non-constant reference to inquiry intrinsic '"s +
              intrin.name +
              "' not allowed for derived type components or type"
              " parameter values";
        }
      }
      // Type-determined inquiries (DIGITS, HUGE, &c.) will have already been
      // folded and won't arrive here.  Inquiries that are represented with
      // DescriptorInquiry operations (LBOUND) are checked elsewhere.  If a
      // call that makes it to here satisfies the requirements of a constant
      // expression (as Fortran defines it), it's fine.
      if (IsConstantExpr(x)) {
        return std::nullopt;
      }
      if (intrin.name == "present") {
        return std::nullopt; // always ok
      }
      // Catch CHARACTER(:), ALLOCATABLE :: X; CHARACTER(LEN(X)) :: Y
      if (inInquiry && x.arguments().size() >= 1) {
        if (const auto &arg{x.arguments().at(0)}) {
          if (auto dataRef{ExtractDataRef(*arg, true, true)}) {
            if (intrin.name == "allocated" || intrin.name == "associated" ||
                intrin.name == "is_contiguous") { // ok
            } else if (intrin.name == "len" &&
                IsPermissibleInquiry(dataRef->GetFirstSymbol(),
                    dataRef->GetLastSymbol(), DescriptorInquiry::Field::Len,
                    scope_)) { // ok
            } else if (intrin.name == "lbound" &&
                IsPermissibleInquiry(dataRef->GetFirstSymbol(),
                    dataRef->GetLastSymbol(),
                    DescriptorInquiry::Field::LowerBound, scope_)) { // ok
            } else if ((intrin.name == "shape" || intrin.name == "size" ||
                           intrin.name == "sizeof" ||
                           intrin.name == "storage_size" ||
                           intrin.name == "ubound") &&
                IsPermissibleInquiry(dataRef->GetFirstSymbol(),
                    dataRef->GetLastSymbol(), DescriptorInquiry::Field::Extent,
                    scope_)) { // ok
            } else {
              return "non-constant inquiry function '"s + intrin.name +
                  "' not allowed for local object";
            }
          }
        }
      }
    }
    auto restorer{common::ScopedSet(inInquiry_, inInquiry)};
    return (*this)(x.arguments());
  }

private:
  const semantics::Scope &scope_;
  FoldingContext &context_;
  // Contextual information: this flag is true when in an argument to
  // an inquiry intrinsic like SIZE().
  mutable bool inInquiry_{false};
  const std::set<std::string> badIntrinsicsForComponents_{
      "allocated", "associated", "extends_type_of", "present", "same_type_as"};
};

template <typename A>
void CheckSpecificationExpr(
    const A &x, const semantics::Scope &scope, FoldingContext &context) {
  if (auto why{CheckSpecificationExprHelper{scope, context}(x)}) {
    context.messages().Say(
        "Invalid specification expression: %s"_err_en_US, *why);
  }
}

template void CheckSpecificationExpr(
    const Expr<SomeType> &, const semantics::Scope &, FoldingContext &);
template void CheckSpecificationExpr(
    const Expr<SomeInteger> &, const semantics::Scope &, FoldingContext &);
template void CheckSpecificationExpr(
    const Expr<SubscriptInteger> &, const semantics::Scope &, FoldingContext &);
template void CheckSpecificationExpr(const std::optional<Expr<SomeType>> &,
    const semantics::Scope &, FoldingContext &);
template void CheckSpecificationExpr(const std::optional<Expr<SomeInteger>> &,
    const semantics::Scope &, FoldingContext &);
template void CheckSpecificationExpr(
    const std::optional<Expr<SubscriptInteger>> &, const semantics::Scope &,
    FoldingContext &);

// IsContiguous() -- 9.5.4
class IsContiguousHelper
    : public AnyTraverse<IsContiguousHelper, std::optional<bool>> {
public:
  using Result = std::optional<bool>; // tri-state
  using Base = AnyTraverse<IsContiguousHelper, Result>;
  explicit IsContiguousHelper(FoldingContext &c) : Base{*this}, context_{c} {}
  using Base::operator();

  template <typename T> Result operator()(const Constant<T> &) const {
    return true;
  }
  Result operator()(const StaticDataObject &) const { return true; }
  Result operator()(const semantics::Symbol &symbol) const {
    const auto &ultimate{symbol.GetUltimate()};
    if (ultimate.attrs().test(semantics::Attr::CONTIGUOUS)) {
      return true;
    } else if (!IsVariable(symbol)) {
      return true;
    } else if (ultimate.Rank() == 0) {
      // Extension: accept scalars as a degenerate case of
      // simple contiguity to allow their use in contexts like
      // data targets in pointer assignments with remapping.
      return true;
    } else if (ultimate.has<semantics::AssocEntityDetails>()) {
      return Base::operator()(ultimate); // use expr
    } else if (semantics::IsPointer(ultimate) ||
        semantics::IsAssumedShape(ultimate) || IsAssumedRank(ultimate)) {
      return std::nullopt;
    } else if (ultimate.has<semantics::ObjectEntityDetails>()) {
      return true;
    } else {
      return Base::operator()(ultimate);
    }
  }

  Result operator()(const ArrayRef &x) const {
    if (x.Rank() == 0) {
      return true; // scalars considered contiguous
    }
    int subscriptRank{0};
    auto baseLbounds{GetLBOUNDs(context_, x.base())};
    auto baseUbounds{GetUBOUNDs(context_, x.base())};
    auto subscripts{CheckSubscripts(
        x.subscript(), subscriptRank, &baseLbounds, &baseUbounds)};
    if (!subscripts.value_or(false)) {
      return subscripts; // subscripts not known to be contiguous
    } else if (subscriptRank > 0) {
      // a(1)%b(:,:) is contiguous if and only if a(1)%b is contiguous.
      return (*this)(x.base());
    } else {
      // a(:)%b(1,1) is (probably) not contiguous.
      return std::nullopt;
    }
  }
  Result operator()(const CoarrayRef &x) const {
    int rank{0};
    return CheckSubscripts(x.subscript(), rank).has_value();
  }
  Result operator()(const Component &x) const {
    if (x.base().Rank() == 0) {
      return (*this)(x.GetLastSymbol());
    } else {
      if (Result baseIsContiguous{(*this)(x.base())}) {
        if (!*baseIsContiguous) {
          return false;
        }
        // TODO could be true if base contiguous and this is only component, or
        // if base has only one element?
      }
      return std::nullopt;
    }
  }
  Result operator()(const ComplexPart &x) const {
    return x.complex().Rank() == 0;
  }
  Result operator()(const Substring &) const { return std::nullopt; }

  Result operator()(const ProcedureRef &x) const {
    if (auto chars{characteristics::Procedure::Characterize(
            x.proc(), context_, /*emitError=*/true)}) {
      if (chars->functionResult) {
        const auto &result{*chars->functionResult};
        if (!result.IsProcedurePointer()) {
          if (result.attrs.test(
                  characteristics::FunctionResult::Attr::Contiguous)) {
            return true;
          }
          if (!result.attrs.test(
                  characteristics::FunctionResult::Attr::Pointer)) {
            return true;
          }
          if (const auto *type{result.GetTypeAndShape()};
              type && type->Rank() == 0) {
            return true; // pointer to scalar
          }
          // Must be non-CONTIGUOUS pointer to array
        }
      }
    }
    return std::nullopt;
  }

  Result operator()(const NullPointer &) const { return true; }

private:
  // Returns "true" for a provably empty or simply contiguous array section;
  // return "false" for a provably nonempty discontiguous section or for use
  // of a vector subscript.
  std::optional<bool> CheckSubscripts(const std::vector<Subscript> &subscript,
      int &rank, const Shape *baseLbounds = nullptr,
      const Shape *baseUbounds = nullptr) const {
    bool anyTriplet{false};
    rank = 0;
    // Detect any provably empty dimension in this array section, which would
    // render the whole section empty and therefore vacuously contiguous.
    std::optional<bool> result;
    bool mayBeEmpty{false};
    auto dims{subscript.size()};
    std::vector<bool> knownPartialSlice(dims, false);
    for (auto j{dims}; j-- > 0;) {
      std::optional<ConstantSubscript> dimLbound;
      std::optional<ConstantSubscript> dimUbound;
      std::optional<ConstantSubscript> dimExtent;
      if (baseLbounds && j < baseLbounds->size()) {
        if (const auto &lb{baseLbounds->at(j)}) {
          dimLbound = ToInt64(Fold(context_, Expr<SubscriptInteger>{*lb}));
        }
      }
      if (baseUbounds && j < baseUbounds->size()) {
        if (const auto &ub{baseUbounds->at(j)}) {
          dimUbound = ToInt64(Fold(context_, Expr<SubscriptInteger>{*ub}));
        }
      }
      if (dimLbound && dimUbound) {
        if (*dimLbound <= *dimUbound) {
          dimExtent = *dimUbound - *dimLbound + 1;
        } else {
          // This is an empty dimension.
          result = true;
          dimExtent = 0;
        }
      }

      if (const auto *triplet{std::get_if<Triplet>(&subscript[j].u)}) {
        ++rank;
        if (auto stride{ToInt64(triplet->stride())}) {
          const Expr<SubscriptInteger> *lowerBound{triplet->GetLower()};
          const Expr<SubscriptInteger> *upperBound{triplet->GetUpper()};
          std::optional<ConstantSubscript> lowerVal{lowerBound
                  ? ToInt64(Fold(context_, Expr<SubscriptInteger>{*lowerBound}))
                  : dimLbound};
          std::optional<ConstantSubscript> upperVal{upperBound
                  ? ToInt64(Fold(context_, Expr<SubscriptInteger>{*upperBound}))
                  : dimUbound};
          if (lowerVal && upperVal) {
            if (*lowerVal < *upperVal) {
              if (*stride < 0) {
                result = true; // empty dimension
              } else if (!result && *stride > 1 &&
                  *lowerVal + *stride <= *upperVal) {
                result = false; // discontiguous if not empty
              }
            } else if (*lowerVal > *upperVal) {
              if (*stride > 0) {
                result = true; // empty dimension
              } else if (!result && *stride < 0 &&
                  *lowerVal + *stride >= *upperVal) {
                result = false; // discontiguous if not empty
              }
            } else {
              mayBeEmpty = true;
            }
          } else {
            mayBeEmpty = true;
          }
        } else {
          mayBeEmpty = true;
        }
      } else if (subscript[j].Rank() > 0) {
        ++rank;
        if (!result) {
          result = false; // vector subscript
        }
        mayBeEmpty = true;
      } else {
        // Scalar subscript.
        if (dimExtent && *dimExtent > 1) {
          knownPartialSlice[j] = true;
        }
      }
    }
    if (rank == 0) {
      result = true; // scalar
    }
    if (result) {
      return result;
    }
    // Not provably discontiguous at this point.
    // Return "true" if simply contiguous, otherwise nullopt.
    for (auto j{subscript.size()}; j-- > 0;) {
      if (const auto *triplet{std::get_if<Triplet>(&subscript[j].u)}) {
        auto stride{ToInt64(triplet->stride())};
        if (!stride || stride != 1) {
          return std::nullopt;
        } else if (anyTriplet) {
          if (triplet->GetLower() || triplet->GetUpper()) {
            // all triplets before the last one must be just ":" for
            // simple contiguity
            return std::nullopt;
          }
        } else {
          anyTriplet = true;
        }
        ++rank;
      } else if (anyTriplet) {
        // If the section cannot be empty, and this dimension's
        // scalar subscript is known not to cover the whole
        // dimension, then the array section is provably
        // discontiguous.
        return (mayBeEmpty || !knownPartialSlice[j])
            ? std::nullopt
            : std::make_optional(false);
      }
    }
    return true; // simply contiguous
  }

  FoldingContext &context_;
};

template <typename A>
std::optional<bool> IsContiguous(const A &x, FoldingContext &context) {
  return IsContiguousHelper{context}(x);
}

template std::optional<bool> IsContiguous(
    const Expr<SomeType> &, FoldingContext &);
template std::optional<bool> IsContiguous(const ArrayRef &, FoldingContext &);
template std::optional<bool> IsContiguous(const Substring &, FoldingContext &);
template std::optional<bool> IsContiguous(const Component &, FoldingContext &);
template std::optional<bool> IsContiguous(
    const ComplexPart &, FoldingContext &);
template std::optional<bool> IsContiguous(const CoarrayRef &, FoldingContext &);
template std::optional<bool> IsContiguous(const Symbol &, FoldingContext &);

// IsErrorExpr()
struct IsErrorExprHelper : public AnyTraverse<IsErrorExprHelper, bool> {
  using Result = bool;
  using Base = AnyTraverse<IsErrorExprHelper, Result>;
  IsErrorExprHelper() : Base{*this} {}
  using Base::operator();

  bool operator()(const SpecificIntrinsic &x) {
    return x.name == IntrinsicProcTable::InvalidName;
  }
};

template <typename A> bool IsErrorExpr(const A &x) {
  return IsErrorExprHelper{}(x);
}

template bool IsErrorExpr(const Expr<SomeType> &);

// C1577
// TODO: Also check C1579 & C1582 here
class StmtFunctionChecker
    : public AnyTraverse<StmtFunctionChecker, std::optional<parser::Message>> {
public:
  using Result = std::optional<parser::Message>;
  using Base = AnyTraverse<StmtFunctionChecker, Result>;
  StmtFunctionChecker(const Symbol &sf, FoldingContext &context)
      : Base{*this}, sf_{sf}, context_{context} {
    if (!context_.languageFeatures().IsEnabled(
            common::LanguageFeature::StatementFunctionExtensions)) {
      severity_ = parser::Severity::Error;
    } else if (context_.languageFeatures().ShouldWarn(
                   common::LanguageFeature::StatementFunctionExtensions)) {
      severity_ = parser::Severity::Portability;
    }
  }
  using Base::operator();

  template <typename T> Result operator()(const ArrayConstructor<T> &) const {
    if (severity_) {
      auto msg{
          "Statement function '%s' should not contain an array constructor"_port_en_US};
      msg.set_severity(*severity_);
      return parser::Message{sf_.name(), std::move(msg), sf_.name()};
    } else {
      return std::nullopt;
    }
  }
  Result operator()(const StructureConstructor &) const {
    if (severity_) {
      auto msg{
          "Statement function '%s' should not contain a structure constructor"_port_en_US};
      msg.set_severity(*severity_);
      return parser::Message{sf_.name(), std::move(msg), sf_.name()};
    } else {
      return std::nullopt;
    }
  }
  Result operator()(const TypeParamInquiry &) const {
    if (severity_) {
      auto msg{
          "Statement function '%s' should not contain a type parameter inquiry"_port_en_US};
      msg.set_severity(*severity_);
      return parser::Message{sf_.name(), std::move(msg), sf_.name()};
    } else {
      return std::nullopt;
    }
  }
  Result operator()(const ProcedureDesignator &proc) const {
    if (const Symbol * symbol{proc.GetSymbol()}) {
      const Symbol &ultimate{symbol->GetUltimate()};
      if (const auto *subp{
              ultimate.detailsIf<semantics::SubprogramDetails>()}) {
        if (subp->stmtFunction() && &ultimate.owner() == &sf_.owner()) {
          if (ultimate.name().begin() > sf_.name().begin()) {
            return parser::Message{sf_.name(),
                "Statement function '%s' may not reference another statement function '%s' that is defined later"_err_en_US,
                sf_.name(), ultimate.name()};
          }
        }
      }
      if (auto chars{characteristics::Procedure::Characterize(
              proc, context_, /*emitError=*/true)}) {
        if (!chars->CanBeCalledViaImplicitInterface()) {
          if (severity_) {
            auto msg{
                "Statement function '%s' should not reference function '%s' that requires an explicit interface"_port_en_US};
            msg.set_severity(*severity_);
            return parser::Message{
                sf_.name(), std::move(msg), sf_.name(), symbol->name()};
          }
        }
      }
    }
    if (proc.Rank() > 0) {
      if (severity_) {
        auto msg{
            "Statement function '%s' should not reference a function that returns an array"_port_en_US};
        msg.set_severity(*severity_);
        return parser::Message{sf_.name(), std::move(msg), sf_.name()};
      }
    }
    return std::nullopt;
  }
  Result operator()(const ActualArgument &arg) const {
    if (const auto *expr{arg.UnwrapExpr()}) {
      if (auto result{(*this)(*expr)}) {
        return result;
      }
      if (expr->Rank() > 0 && !UnwrapWholeSymbolOrComponentDataRef(*expr)) {
        if (severity_) {
          auto msg{
              "Statement function '%s' should not pass an array argument that is not a whole array"_port_en_US};
          msg.set_severity(*severity_);
          return parser::Message{sf_.name(), std::move(msg), sf_.name()};
        }
      }
    }
    return std::nullopt;
  }

private:
  const Symbol &sf_;
  FoldingContext &context_;
  std::optional<parser::Severity> severity_;
};

std::optional<parser::Message> CheckStatementFunction(
    const Symbol &sf, const Expr<SomeType> &expr, FoldingContext &context) {
  return StmtFunctionChecker{sf, context}(expr);
}

} // namespace Fortran::evaluate