[clang][bytecode] Fix CXXConstructExpr for multidim arrays (#164760)

This is a thing apparently.

Fixes https://github.com/llvm/llvm-project/issues/153803

clang/lib/AST/ByteCode/Compiler.cpp

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 836d22f..fc5c782 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -3273,34 +3273,43 @@
   }
 
   if (T->isArrayType()) {
-    const ConstantArrayType *CAT =
-        Ctx.getASTContext().getAsConstantArrayType(E->getType());
-    if (!CAT)
-      return false;
-
-    size_t NumElems = CAT->getZExtSize();
     const Function *Func = getFunction(E->getConstructor());
     if (!Func)
       return false;
 
-    // FIXME(perf): We're calling the constructor once per array element here,
-    //   in the old intepreter we had a special-case for trivial constructors.
-    for (size_t I = 0; I != NumElems; ++I) {
-      if (!this->emitConstUint64(I, E))
-        return false;
-      if (!this->emitArrayElemPtrUint64(E))
-        return false;
+    if (!this->emitDupPtr(E))
+      return false;
 
-      // Constructor arguments.
-      for (const auto *Arg : E->arguments()) {
-        if (!this->visit(Arg))
-          return false;
+    std::function<bool(QualType)> initArrayDimension;
+    initArrayDimension = [&](QualType T) -> bool {
+      if (!T->isArrayType()) {
+        // Constructor arguments.
+        for (const auto *Arg : E->arguments()) {
+          if (!this->visit(Arg))
+            return false;
+        }
+
+        return this->emitCall(Func, 0, E);
       }
 
-      if (!this->emitCall(Func, 0, E))
+      const ConstantArrayType *CAT =
+          Ctx.getASTContext().getAsConstantArrayType(T);
+      if (!CAT)
         return false;
-    }
-    return true;
+      QualType ElemTy = CAT->getElementType();
+      unsigned NumElems = CAT->getZExtSize();
+      for (size_t I = 0; I != NumElems; ++I) {
+        if (!this->emitConstUint64(I, E))
+          return false;
+        if (!this->emitArrayElemPtrUint64(E))
+          return false;
+        if (!initArrayDimension(ElemTy))
+          return false;
+      }
+      return this->emitPopPtr(E);
+    };
+
+    return initArrayDimension(E->getType());
   }
 
   return false;

clang/test/AST/ByteCode/arrays.cpp

diff --git a/clang/test/AST/ByteCode/arrays.cpp b/clang/test/AST/ByteCode/arrays.cpp
index 22a4b41..eaf9559 100644
--- a/clang/test/AST/ByteCode/arrays.cpp
+++ b/clang/test/AST/ByteCode/arrays.cpp
@@ -820,3 +820,14 @@
     return 1;
   }
 }
+
+namespace MultiDimConstructExpr {
+  struct a {
+    a *p = this;
+  };
+  struct b {
+    a m[3][3];
+  };
+  constexpr b d;
+  static_assert(d.m[2][1].p == &d.m[2][1]);
+}