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>

1 files changed, 25 insertions, 0 deletions

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 0000000..28c9761
--- /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>);