[clang] [Sema] Fix __array_rank queried type at template instantiation (PR #124491)

[Robert Dazi via cfe-commits]

https://github.com/v01dXYZ created https://github.com/llvm/llvm-project/pull/124491

The type being queried was left as a template type parameter, making the whole expression as dependent and thus not eligible to static_assert.

Fixes #123498 

>From 8283e34bb0144604e52196e0a69a56355eb51e43 Mon Sep 17 00:00:00 2001
From: v01dxyz <v01dxyz at v01d.xyz>
Date: Sun, 26 Jan 2025 23:07:13 +0100
Subject: [PATCH] [Sema] Fix __array_rank queried type at template
 instantiation

The type being queried was left as a template type parameter, making
the whole expression as dependent and thus not eligible to
static_assert.
---
 clang/include/clang/AST/ExprCXX.h             |  4 +-
 clang/lib/Sema/TreeTransform.h                |  3 --
 .../array-type-trait-with-template.cpp        | 37 +++++++++++++++++++
 3 files changed, 39 insertions(+), 5 deletions(-)
 create mode 100644 clang/test/SemaCXX/array-type-trait-with-template.cpp

diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index aa10945addf78f..2a130bc6da79a0 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -2847,8 +2847,8 @@ class TypeTraitExpr final
 ///
 /// Example:
 /// \code
-///   __array_rank(int[10][20]) == 2
-///   __array_extent(int, 1)    == 20
+///   __array_rank(int[10][20])      == 2
+///   __array_extent(int[10][20], 1) == 20
 /// \endcode
 class ArrayTypeTraitExpr : public Expr {
   /// The trait. An ArrayTypeTrait enum in MSVC compat unsigned.
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 12680843a434a0..f04adf7fdf8ad2 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -14947,9 +14947,6 @@ TreeTransform<Derived>::TransformArrayTypeTraitExpr(ArrayTypeTraitExpr *E) {
     SubExpr = getDerived().TransformExpr(E->getDimensionExpression());
     if (SubExpr.isInvalid())
       return ExprError();
-
-    if (!getDerived().AlwaysRebuild() && SubExpr.get() == E->getDimensionExpression())
-      return E;
   }
 
   return getDerived().RebuildArrayTypeTrait(E->getTrait(), E->getBeginLoc(), T,
diff --git a/clang/test/SemaCXX/array-type-trait-with-template.cpp b/clang/test/SemaCXX/array-type-trait-with-template.cpp
new file mode 100644
index 00000000000000..b0cc9a12d6efb6
--- /dev/null
+++ b/clang/test/SemaCXX/array-type-trait-with-template.cpp
@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -fsyntax-only %s
+
+// When __array_rank is used with a template type parameter, this
+// test ensures clang considers the final expression as having an
+// integral type.
+//
+// Although array_extent was handled well, it is added here.
+template <typename T, int N>
+constexpr int array_rank(T (&lhs)[N]) {
+  return __array_rank(T[N]);
+}
+
+template <int I, typename T, int N>
+constexpr int array_extent(T (&lhs)[N]) {
+  return __array_extent(T[N], I);
+}
+
+int main() {
+  constexpr int vec[] = {0, 1, 2, 1};
+  constexpr int mat[4][4] = {
+    {1, 0, 0, 0},
+    {0, 1, 0, 0},
+    {0, 0, 1, 0},
+    {0, 0, 0, 1}
+  };
+
+  (void) (array_rank(vec) == 1);
+  (void) (array_rank(vec) == 2);
+
+  static_assert(array_rank(vec) == 1);
+  static_assert(array_rank(mat) == 2);
+
+  static_assert(array_extent<0>(vec) == 4);
+  static_assert(array_extent<0>(mat) == 4);
+  static_assert(array_extent<1>(mat) == 4);
+  static_assert(array_extent<1>(vec) == 0);
+}