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

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/86652

>From 90f88bdac3dc40635c58f3f67e29354e6a32896e Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Tue, 26 Mar 2024 12:23:24 +0100
Subject: [PATCH] [Clang] Fix __is_array returning true for zero-sized arrays

---
 clang/docs/ReleaseNotes.rst        | 2 ++
 clang/lib/Sema/SemaExprCXX.cpp     | 4 ++++
 clang/test/SemaCXX/type-traits.cpp | 4 +++-
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 7fbe2fec6ca06..e668202853710 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -345,6 +345,8 @@ Bug Fixes in This Version
 - Fixes an assertion failure on invalid code when trying to define member
   functions in lambdas.
 
+- ``__is_array`` no longer returns ``true`` for zero-sized arrays. Fixes (#GH54705).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index c34a40fa7c81a..93d2b4b259fbc 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -5143,6 +5143,10 @@ 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 (!T->isVariableArrayType()) {
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 14ec17989ec7c..49df4b668513c 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/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));
@@ -1804,7 +1806,7 @@ void is_layout_compatible(int n)
   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(CStructIncomplete, CStructIncomplete));
   static_assert(!__is_layout_compatible(CStruct, CStructIncomplete));
   static_assert(__is_layout_compatible(CStructIncomplete[2], CStructIncomplete[2]));
 }