[clang] Reject VLAs in __is_layout_compatible() (#87737)

This is a follow-up to #81506. Since `__is_layout_compatible()` is a C++
intrinsic
(https://github.com/llvm/llvm-project/blob/ff1e72d68d1224271801ff5192a8c14fbd3be83b/clang/include/clang/Basic/TokenKinds.def#L523),
I don't think we should define how it interacts with VLA extension
unless we have a compelling reason to.

Since #81506 was merged after 18 cut-off, we don't have to follow any
kind of deprecation process.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -165,7 +165,7 @@ def ext_vla_folded_to_constant : ExtWarn<
   "variable length array folded to constant array as an extension">,
   InGroup<GNUFoldingConstant>;
 def err_vla_unsupported : Error<
-  "variable length arrays are not supported for %select{the current target|'%1'}0">;
+  "variable length arrays are not supported %select{for the current target|in '%1'}0">;
 def err_vla_in_coroutine_unsupported : Error<
   "variable length arrays in a coroutine are not supported">;
 def note_vla_unsupported : Note<

clang/lib/Sema/SemaExprCXX.cpp

@@ -6026,6 +6026,9 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, QualType LhsT,
     return false;
   }
   case BTT_IsLayoutCompatible: {
+    if (LhsT->isVariableArrayType() || RhsT->isVariableArrayType())
+      Self.Diag(KeyLoc, diag::err_vla_unsupported)
+          << 1 << tok::kw___is_layout_compatible;
     return Self.IsLayoutCompatible(LhsT, RhsT);
   }
     default: llvm_unreachable("not a BTT");

clang/test/SemaCXX/type-traits.cpp

@@ -740,7 +740,7 @@ void is_bounded_array(int n) {
   static_assert(!__is_bounded_array(cvoid *));
 
   int t32[n];
-  (void)__is_bounded_array(decltype(t32)); // expected-error{{variable length arrays are not supported for '__is_bounded_array'}}
+  (void)__is_bounded_array(decltype(t32)); // expected-error{{variable length arrays are not supported in '__is_bounded_array'}}
 }
 
 void is_unbounded_array(int n) {
@@ -772,7 +772,7 @@ void is_unbounded_array(int n) {
   static_assert(!__is_unbounded_array(cvoid *));
 
   int t32[n];
-  (void)__is_unbounded_array(decltype(t32)); // expected-error{{variable length arrays are not supported for '__is_unbounded_array'}}
+  (void)__is_unbounded_array(decltype(t32)); // expected-error{{variable length arrays are not supported in '__is_unbounded_array'}}
 }
 
 void is_referenceable() {
@@ -1741,8 +1741,10 @@ void is_layout_compatible(int n)
   static_assert(!__is_layout_compatible(unsigned char, signed char));
   static_assert(__is_layout_compatible(int[], int[]));
   static_assert(__is_layout_compatible(int[2], int[2]));
-  static_assert(!__is_layout_compatible(int[n], int[2])); // FIXME: VLAs should be rejected
-  static_assert(!__is_layout_compatible(int[n], int[n])); // FIXME: VLAs should be rejected
+  static_assert(!__is_layout_compatible(int[n], int[2]));
+  // expected-error@-1 {{variable length arrays are not supported in '__is_layout_compatible'}}
+  static_assert(!__is_layout_compatible(int[n], int[n]));
+  // expected-error@-1 {{variable length arrays are not supported in '__is_layout_compatible'}}
   static_assert(__is_layout_compatible(int&, int&));
   static_assert(!__is_layout_compatible(int&, char&));
   static_assert(__is_layout_compatible(void(int), void(int)));