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//===--- ImplicitConversionInLoopCheck.cpp - clang-tidy--------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "ImplicitConversionInLoopCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Decl.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/ASTMatchers/ASTMatchers.h"
#include "clang/Lex/Lexer.h"
using namespace clang::ast_matchers;
namespace clang {
namespace tidy {
namespace performance {
// Checks if the stmt is a ImplicitCastExpr with a CastKind that is not a NoOp.
// The subtlety is that in some cases (user defined conversions), we can
// get to ImplicitCastExpr inside each other, with the outer one a NoOp. In this
// case we skip the first cast expr.
static bool isNonTrivialImplicitCast(const Stmt *ST) {
if (const auto *ICE = dyn_cast<ImplicitCastExpr>(ST)) {
return (ICE->getCastKind() != CK_NoOp) ||
return false;
void ImplicitConversionInLoopCheck::registerMatchers(MatchFinder *Finder) {
// We look for const ref loop variables that (optionally inside an
// ExprWithCleanup) materialize a temporary, and contain a implicit
// conversion. The check on the implicit conversion is done in check() because
// we can't access implicit conversion subnode via matchers: has() skips casts
// and materialize! We also bind on the call to operator* to get the proper
// type in the diagnostic message. We use both cxxOperatorCallExpr for user
// defined operator and unaryOperator when the iterator is a pointer, like
// for arrays or std::array.
// Note that when the implicit conversion is done through a user defined
// conversion operator, the node is a CXXMemberCallExpr, not a
// CXXOperatorCallExpr, so it should not get caught by the
// cxxOperatorCallExpr() matcher.
void ImplicitConversionInLoopCheck::check(
const MatchFinder::MatchResult &Result) {
const auto *VD = Result.Nodes.getNodeAs<VarDecl>("faulty-var");
const auto *Init = Result.Nodes.getNodeAs<Expr>("init");
const auto *OperatorCall =
if (const auto *Cleanup = dyn_cast<ExprWithCleanups>(Init))
Init = Cleanup->getSubExpr();
const auto *Materialized = dyn_cast<MaterializeTemporaryExpr>(Init);
if (!Materialized)
// We ignore NoOp casts. Those are generated if the * operator on the
// iterator returns a value instead of a reference, and the loop variable
// is a reference. This situation is fine (it probably produces the same
// code at the end).
if (isNonTrivialImplicitCast(Materialized->getSubExpr()))
reportAndFix(Result.Context, VD, OperatorCall);
void ImplicitConversionInLoopCheck::reportAndFix(const ASTContext *Context,
const VarDecl *VD,
const Expr *OperatorCall) {
// We only match on const ref, so we should print a const ref version of the
// type.
QualType ConstType = OperatorCall->getType().withConst();
QualType ConstRefType = Context->getLValueReferenceType(ConstType);
const char Message[] =
"the type of the loop variable %0 is different from the one returned "
"by the iterator and generates an implicit conversion; you can either "
"change the type to the matching one (%1 but 'const auto&' is always a "
"valid option) or remove the reference to make it explicit that you are "
"creating a new value";
diag(VD->getBeginLoc(), Message) << VD << ConstRefType;
} // namespace performance
} // namespace tidy
} // namespace clang