[clang] Fix static_cast to array of unknown bound (#96041)

Per P1975R0 an expression like static_cast<U[]>(...) defines the type of
the expression as U[1].

Fixes #62863

Author: Fznamznon

clang/docs/ReleaseNotes.rst

@@ -695,6 +695,8 @@ Bug Fixes in This Version
 - Correctly reject declarations where a statement is required in C.
   Fixes #GH92775
 
+- Fixed `static_cast` to array of unknown bound. Fixes (#GH62863).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaType.cpp

@@ -8771,6 +8771,20 @@ void Sema::completeExprArrayBound(Expr *E) {
       }
     }
   }
+  if (const auto CastE = dyn_cast<ExplicitCastExpr>(E)) {
+    QualType DestType = CastE->getTypeAsWritten();
+    if (const auto *IAT = Context.getAsIncompleteArrayType(DestType)) {
+      // C++20 [expr.static.cast]p.4: ... If T is array of unknown bound,
+      // this direct-initialization defines the type of the expression
+      // as U[1]
+      QualType ResultType = Context.getConstantArrayType(
+          IAT->getElementType(),
+          llvm::APInt(Context.getTypeSize(Context.getSizeType()), 1),
+          /*SizeExpr=*/nullptr, ArraySizeModifier::Normal,
+          /*IndexTypeQuals=*/0);
+      E->setType(ResultType);
+    }
+  }
 }
 
 QualType Sema::getCompletedType(Expr *E) {

clang/test/SemaCXX/paren-list-agg-init.cpp

@@ -313,3 +313,24 @@ namespace GH63903 {
   constexpr S s(0); // beforecxx20-warning {{aggregate initialization of type 'const S' from a parenthesized list of values is a C++20 extension}} \
                     // expected-error {{constexpr variable 's' must be initialized by a constant expression}}
 }
+
+
+namespace gh62863 {
+int (&&arr)[] = static_cast<int[]>(42);
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[1]' from a parenthesized list of values is a C++20 extension}}
+int (&&arr1)[1] = static_cast<int[]>(42);
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[1]' from a parenthesized list of values is a C++20 extension}}
+int (&&arr2)[2] = static_cast<int[]>(42); // expected-error {{reference to type 'int[2]' could not bind to an rvalue of type 'int[1]'}}
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[1]' from a parenthesized list of values is a C++20 extension}}
+int (&&arr3)[3] = static_cast<int[3]>(42);
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[3]' from a parenthesized list of values is a C++20 extension}}
+
+int (&&arr4)[] = (int[])(42);
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[1]' from a parenthesized list of values is a C++20 extension}}
+int (&&arr5)[1] = (int[])(42);
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[1]' from a parenthesized list of values is a C++20 extension}}
+int (&&arr6)[2] = (int[])(42); // expected-error {{reference to type 'int[2]' could not bind to an rvalue of type 'int[1]'}}
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[1]' from a parenthesized list of values is a C++20 extension}}
+int (&&arr7)[3] = (int[3])(42);
+// beforecxx20-warning@-1 {{aggregate initialization of type 'int[3]' from a parenthesized list of values is a C++20 extension}}
+}