From 6e973e87e3fec6f33e97edf8fce2fcd121e53961 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 4 Apr 2025 14:03:58 -0400
Subject: [PATCH] c++: constraint variable used in evaluated context [PR117849]

Here we wrongly reject the type-requirement at parse time due to its use
of the constraint variable 't' within a template argument (an evaluated
context).  Fix this simply by refining the "use of parameter outside
function body" error path to exclude constraint variables.

PR c++/104255 tracks the same issue for function parameters, but fixing
that would be more involved, requiring changes to the PARM_DECL case of
tsubst_expr.

	PR c++/117849

gcc/cp/ChangeLog:

	* semantics.cc (finish_id_expression_1): Allow use of constraint
	variable outside an unevaluated context.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires41.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
---
 gcc/cp/semantics.cc                           |  1 +
 .../g++.dg/cpp2a/concepts-requires41.C        | 25 +++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-requires41.C

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 7d8beb8833d..a10ef34383c 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -4755,6 +4755,7 @@ finish_id_expression_1 (tree id_expression,
 	 body, except inside an unevaluated context (i.e. decltype).  */
       if (TREE_CODE (decl) == PARM_DECL
 	  && DECL_CONTEXT (decl) == NULL_TREE
+	  && !CONSTRAINT_VAR_P (decl)
 	  && !cp_unevaluated_operand
 	  && !processing_contract_condition
 	  && !processing_omp_trait_property_expr)
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires41.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires41.C
new file mode 100644
index 00000000000..28c976116ca
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires41.C
@@ -0,0 +1,25 @@
+// PR c++/117849
+// { dg-do compile { target c++20 } }
+
+template<int N>
+struct array {
+  constexpr int size() const { return N; }
+};
+
+struct vector {
+  int _size = 3;
+  constexpr int size() const { return _size; }
+};
+
+template<int N>
+struct integral_constant {
+  constexpr operator int() const { return N; }
+};
+
+template<class T>
+concept StaticSize = requires (T& t) {
+  typename integral_constant<t.size()>;
+};
+
+static_assert(StaticSize<array<5>>);
+static_assert(!StaticSize<vector>);
-- 
2.47.2