[clang] fd2ffc1 - [clang] Reject incomplete types in `__is_layout_compatible()` (#87869)

[via cfe-commits]

Author: Vlad Serebrennikov
Date: 2024-04-08T22:37:48+04:00
New Revision: fd2ffc1cbf51a75884369ec9253f47ae2fdba83c

URL: https://github.com/llvm/llvm-project/commit/fd2ffc1cbf51a75884369ec9253f47ae2fdba83c
DIFF: https://github.com/llvm/llvm-project/commit/fd2ffc1cbf51a75884369ec9253f47ae2fdba83c.diff

LOG: [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
(https://github.com/llvm/llvm-project/pull/87737#discussion_r1553652570),
I added a couple of tests for that as well.

Added: 
    

Modified: 
    clang/lib/Sema/SemaExprCXX.cpp
    clang/test/SemaCXX/type-traits.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index cfb5c6b6f28337..901c3c4d00cfb4 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/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;

diff  --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index e99ad11666e54c..e29763714341e7 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/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 at -1 {{incomplete type 'CStructIncomplete' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  // expected-error at -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 at -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 at -1 {{incomplete type 'CStructIncomplete[2]' where a complete type is required}}
+  //   expected-note@#CStructIncomplete {{forward declaration of 'CStructIncomplete'}}
+  // expected-error at -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()