[Clang] Fix __is_array returning true for zero-sized arrays (llvm#86652)

Fixes llvm#54705

Author: philnik777

clang/docs/ReleaseNotes.rst

@@ -586,6 +586,9 @@ Bug Fixes in This Version
 - Clang now correctly disallows VLA type compound literals, e.g. ``(int[size]){}``,
   as the C standard mandates. (#GH89835)
 
+- ``__is_array`` and ``__is_bounded_array`` no longer return ``true`` for
+  zero-sized arrays. Fixes (#GH54705).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaExprCXX.cpp

@@ -5217,10 +5217,18 @@ static bool EvaluateUnaryTypeTrait(Sema &Self, TypeTrait UTT,
   case UTT_IsFloatingPoint:
     return T->isFloatingType();
   case UTT_IsArray:
+    // Zero-sized arrays aren't considered arrays in partial specializations,
+    // so __is_array shouldn't consider them arrays either.
+    if (const auto *CAT = C.getAsConstantArrayType(T))
+      return CAT->getSize() != 0;
     return T->isArrayType();
   case UTT_IsBoundedArray:
     if (DiagnoseVLAInCXXTypeTrait(Self, TInfo, tok::kw___is_bounded_array))
       return false;
+    // Zero-sized arrays aren't considered arrays in partial specializations,
+    // so __is_bounded_array shouldn't consider them arrays either.
+    if (const auto *CAT = C.getAsConstantArrayType(T))
+      return CAT->getSize() != 0;
     return T->isArrayType() && !T->isIncompleteArrayType();
   case UTT_IsUnboundedArray:
     if (DiagnoseVLAInCXXTypeTrait(Self, TInfo, tok::kw___is_unbounded_array))

clang/test/SemaCXX/type-traits.cpp

@@ -25,6 +25,7 @@ typedef Empty EmptyArMB[1][2];
 typedef int Int;
 typedef Int IntAr[10];
 typedef Int IntArNB[];
+typedef Int IntArZero[0];
 class Statics { static int priv; static NonPOD np; };
 union EmptyUnion {};
 union IncompleteUnion; // expected-note {{forward declaration of 'IncompleteUnion'}}
@@ -685,6 +686,7 @@ void is_array()
 {
   static_assert(__is_array(IntAr));
   static_assert(__is_array(IntArNB));
+  static_assert(!__is_array(IntArZero));
   static_assert(__is_array(UnionAr));
 
   static_assert(!__is_array(void));
@@ -714,6 +716,7 @@ void is_array()
 void is_bounded_array(int n) {
   static_assert(__is_bounded_array(IntAr));
   static_assert(!__is_bounded_array(IntArNB));
+  static_assert(!__is_bounded_array(IntArZero));
   static_assert(__is_bounded_array(UnionAr));
 
   static_assert(!__is_bounded_array(void));
@@ -746,6 +749,7 @@ void is_bounded_array(int n) {
 void is_unbounded_array(int n) {
   static_assert(!__is_unbounded_array(IntAr));
   static_assert(__is_unbounded_array(IntArNB));
+  static_assert(!__is_unbounded_array(IntArZero));
   static_assert(!__is_unbounded_array(UnionAr));
 
   static_assert(!__is_unbounded_array(void));