[PATCH] c++: std::is_constant_evaluated inside a constraint [PR97051]

[Patrick Palka via Gcc-patches]

According to [expr.const]/14, the result of substitution into an atomic
constraint is manifestly constant-evaluated; this patch adjusts the call
to maybe_constant_value in satisfy_atom to that effect.

Tested on x86_64-pc-linux-gnu, and also tested on the cmcstl2 and
range-v3 libraries.  Does this look OK to commit?

gcc/cp/ChangeLog:

        PR c++/97051
        * constraint.cc (satisfy_atom): Pass true as the
        manifestly_const_eval argument to maybe_constant_value.

gcc/testsuite/ChangeLog:

        PR c++/97051
        * g++.dg/cpp2a/is-constant-evaluated11.C: New test.
---
 gcc/cp/constraint.cc                             |  3 ++-
 .../g++.dg/cpp2a/is-constant-evaluated11.C       | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated11.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 0aab3073cc1..8137df59e37 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2643,7 +2643,8 @@ satisfy_atom (tree t, tree args, subst_info info)
     result = cxx_constant_value (result);
   else
     {
-      result = maybe_constant_value (result);
+      result = maybe_constant_value (result, NULL_TREE,
+                                    /*manifestly_const_eval=*/true);
       if (!TREE_CONSTANT (result))
        result = error_mark_node;
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated11.C 
b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated11.C
new file mode 100644
index 00000000000..a31867f74fb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated11.C
@@ -0,0 +1,16 @@
+// PR c++/97051
+// { dg-do compile { target c++20 } }
+
+namespace std {
+  constexpr inline bool
+  is_constant_evaluated () noexcept
+  {
+    return __builtin_is_constant_evaluated ();
+  }
+}
+
+template<typename>
+  requires (std::is_constant_evaluated())
+constexpr int a = 0;
+
+constexpr int b = a<int>;
-- 
2.28.0.497.g54e85e7af1