[clang] [Clang] Implement CWG2517 Useless restriction on use of parameter in constraint-expression (PR #132919)

[Imad Aldij via cfe-commits]

https://github.com/imdj created https://github.com/llvm/llvm-project/pull/132919

Remove `[expr.prim.req.nested]` check which restrict that local parameters in constraint-expressions can only appear as unevaluated operands. This change makes the treatment of examples like `requires` expressions and other constant expression contexts uniform, consistent with the adoption of P2280.

References:
- https://cplusplus.github.io/CWG/issues/2517.html
- #132825

>From 0e8e663da86c8fe7007b3967d25081d68457f75a Mon Sep 17 00:00:00 2001
From: Imad Aldij <os at imadij.com>
Date: Tue, 25 Mar 2025 13:06:19 +0200
Subject: [PATCH] [Clang] Implement CWG2517 Useless restriction on use of
 parameter in constraint-expression

Remove the restriction that local parameters in constraint-expressions can only appear as unevaluated operands. This change makes the treatment of examples like `requires` expressions and other constant expression contexts uniform, consistent with the adoption of P2280.
---
 clang/lib/Sema/SemaExpr.cpp                        | 11 -----------
 clang/test/SemaCXX/constant-expression-p2280r4.cpp | 13 +++++++++++++
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 3af6d6c23438f..fa8eeb644c179 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -396,17 +396,6 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, ArrayRef<SourceLocation> Locs,
           targetDiag(*Locs.begin(), diag::err_thread_unsupported);
   }
 
-  if (isa<ParmVarDecl>(D) && isa<RequiresExprBodyDecl>(D->getDeclContext()) &&
-      !isUnevaluatedContext()) {
-    // C++ [expr.prim.req.nested] p3
-    //   A local parameter shall only appear as an unevaluated operand
-    //   (Clause 8) within the constraint-expression.
-    Diag(Loc, diag::err_requires_expr_parameter_referenced_in_evaluated_context)
-        << D;
-    Diag(D->getLocation(), diag::note_entity_declared_at) << D;
-    return true;
-  }
-
   return false;
 }
 
diff --git a/clang/test/SemaCXX/constant-expression-p2280r4.cpp b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
index f22430a0e88a2..9baedd9c6c263 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -155,6 +155,19 @@ int g() {
 }
 }
 
+namespace GH132825 {
+template<typename ArrayType> concept LargeArray =
+    requires (ArrayType my_array) { requires my_array.size() > 5; };
+
+struct Big {
+  constexpr int size() const { return 100; }
+};
+
+void g() {
+  static_assert(LargeArray<Big>);
+}
+}
+
 namespace GH128409 {
   int &ff();
   int &x = ff(); // nointerpreter-note {{declared here}}