[Clang][Sema] placement new initializes typedef array with correct size (#83124)

When in-place new-ing a local variable of an array of trivial type, the
generated code calls 'memset' with the correct size of the array,
earlier it was generating size (squared of the typedef array + size).

The cause: `typedef TYPE TArray[8]; TArray x;` The type of declarator is
Tarray[8] and in `SemaExprCXX.cpp::BuildCXXNew` we check if it's of
typedef and of constant size then we get the original type and it works
fine for non-dependent cases.
But in case of template we do `TreeTransform.h:TransformCXXNEWExpr` and
there we again check the allocated type which is TArray[8] and it stays
that way, so ArraySize=(Tarray[8] type, alloc Tarray[8*type]) so the
squared size allocation.

ArraySize gets calculated earlier in `TreeTransform.h` so that
`if(!ArraySize)` condition was failing.
fix: I changed that condition to `if(ArraySize)`. 


Fixes #41441

Author: mahtohappy

clang/docs/ReleaseNotes.rst

@@ -539,6 +539,8 @@ Bug Fixes to C++ Support
   Fixes (#GH70604), (#GH79754), (#GH84163), (#GH84425), (#GH86054), (#GH86398), and (#GH86399).
 - Fix a crash when deducing ``auto`` from an invalid dereference (#GH88329).
 - Fix a crash in requires expression with templated base class member function. Fixes (#GH84020).
+- placement new initializes typedef array with correct size
+  (`#GH41441 <https://github.com/llvm/llvm-project/issues/41441>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/TreeTransform.h

@@ -12802,6 +12802,19 @@ TreeTransform<Derived>::TransformCXXNewExpr(CXXNewExpr *E) {
     ArraySize = NewArraySize.get();
   }
 
+  // Per C++0x [expr.new]p5, the type being constructed may be a
+  // typedef of an array type.
+  QualType AllocType = AllocTypeInfo->getType();
+  if (ArraySize) {
+    if (const ConstantArrayType *Array =
+            SemaRef.Context.getAsConstantArrayType(AllocType)) {
+      ArraySize = IntegerLiteral::Create(SemaRef.Context, Array->getSize(),
+                                         SemaRef.Context.getSizeType(),
+                                         E->getBeginLoc());
+      AllocType = Array->getElementType();
+    }
+  }
+
   // Transform the placement arguments (if any).
   bool ArgumentChanged = false;
   SmallVector<Expr*, 8> PlacementArgs;
@@ -12863,7 +12876,6 @@ TreeTransform<Derived>::TransformCXXNewExpr(CXXNewExpr *E) {
     return E;
   }
 
-  QualType AllocType = AllocTypeInfo->getType();
   if (!ArraySize) {
     // If no array size was specified, but the new expression was
     // instantiated with an array type (e.g., "new T" where T is

clang/test/SemaCXX/instantiate-new-placement-size.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang -S -fno-discard-value-names -emit-llvm -o - %s | FileCheck %s
+// Issue no: 41441
+#include <new>
+
+// CHECK: call void @llvm.memset.p0.i64(ptr align 1 %x, i8 0, i64 8, i1 false)
+// CHECK: call void @llvm.memset.p0.i64(ptr align 16 %x, i8 0, i64 32, i1 false)
+template <typename TYPE>
+void f()
+{
+    typedef TYPE TArray[8];
+
+    TArray x;
+    new(&x) TArray();
+}
+
+int main()
+{
+    f<char>();
+    f<int>();
+}