[clang] Reject incomplete types in __is_layout_compatible() (#87869)

This is a follow-up to #81506. As discussed in #87737, we're rejecting
incomplete types, save for exceptions listed in the C++ standard (`void`
and arrays of unknown bound). Note that arrays of unknown bound of
incomplete types are accepted.

Since we're happy with the current behavior of this intrinsic for
flexible array members
(#87737 (comment)),
I added a couple of tests for that as well.

clang/lib/Sema/SemaExprCXX.cpp

@@ -6025,6 +6025,11 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, QualType LhsT,
     return false;
   }
   case BTT_IsLayoutCompatible: {
+    if (!LhsT->isVoidType() && !LhsT->isIncompleteArrayType())
+      Self.RequireCompleteType(KeyLoc, LhsT, diag::err_incomplete_type);
+    if (!RhsT->isVoidType() && !RhsT->isIncompleteArrayType())
+      Self.RequireCompleteType(KeyLoc, RhsT, diag::err_incomplete_type);
+
     if (LhsT->isVariableArrayType() || RhsT->isVariableArrayType())
       Self.Diag(KeyLoc, diag::err_vla_unsupported)
           << 1 << tok::kw___is_layout_compatible;

clang/test/SemaCXX/type-traits.cpp

@@ -1622,7 +1622,7 @@ enum class EnumClassLayout {};
 enum EnumForward : int;
 enum class EnumClassForward;
 
-struct CStructIncomplete;
+struct CStructIncomplete; // #CStructIncomplete
 
 struct CStructNested {
   int a;
@@ -1719,6 +1719,20 @@ struct StructWithAnonUnion3 {
   } u;
 };
 
+struct CStructWithArrayAtTheEnd {
+  int a;
+  int b[4];
+};
+
+struct CStructWithFMA {
+  int c;
+  int d[];
+};
+
+struct CStructWithFMA2 {
+  int e;
+  int f[];
+};
 
 void is_layout_compatible(int n)
 {
@@ -1800,15 +1814,28 @@ void is_layout_compatible(int n)
   static_assert(__is_layout_compatible(EnumLayout, EnumClassLayout));
   static_assert(__is_layout_compatible(EnumForward, EnumForward));
   static_assert(__is_layout_compatible(EnumForward, EnumClassForward));
-  // Layout compatibility for enums might be relaxed in the future. See https://github.com/cplusplus/CWG/issues/39#issuecomment-1184791364
+  static_assert(__is_layout_compatible(CStructIncomplete, CStructIncomplete));
+  // expected-error@-1 {{incomplete type 'CStructIncomplete' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  // expected-error@-3 {{incomplete type 'CStructIncomplete' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  static_assert(!__is_layout_compatible(CStruct, CStructIncomplete));
+  // expected-error@-1 {{incomplete type 'CStructIncomplete' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  static_assert(__is_layout_compatible(CStructIncomplete[2], CStructIncomplete[2]));
+  // expected-error@-1 {{incomplete type 'CStructIncomplete[2]' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  // expected-error@-3 {{incomplete type 'CStructIncomplete[2]' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  static_assert(__is_layout_compatible(CStructIncomplete[], CStructIncomplete[]));
+  static_assert(!__is_layout_compatible(CStructWithArrayAtTheEnd, CStructWithFMA));
+  static_assert(__is_layout_compatible(CStructWithFMA, CStructWithFMA));
+  static_assert(__is_layout_compatible(CStructWithFMA, CStructWithFMA2));
+  // Layout compatibility rules for enums might be relaxed in the future. See https://github.com/cplusplus/CWG/issues/39#issuecomment-1184791364
   static_assert(!__is_layout_compatible(EnumLayout, int));
   static_assert(!__is_layout_compatible(EnumClassLayout, int));
   static_assert(!__is_layout_compatible(EnumForward, int));
   static_assert(!__is_layout_compatible(EnumClassForward, int));
-  // FIXME: the following should be rejected (array of unknown bound and void are the only allowed incomplete types)
-  static_assert(__is_layout_compatible(CStructIncomplete, CStructIncomplete)); 
-  static_assert(!__is_layout_compatible(CStruct, CStructIncomplete));
-  static_assert(__is_layout_compatible(CStructIncomplete[2], CStructIncomplete[2]));
 }
 
 void is_signed()