[PATCH] c++: Check constraints before instantiation from mark_used [PR95132]

[Patrick Palka]

This makes mark_used check constraints of a function _before_ calling
maybe_instantiate_decl, so that we don't try instantiating a function
(as part of return type deduction) with unsatisfied constraints.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk and perhaps the 10 branch?

gcc/cp/ChangeLog:

	PR c++/95132
	* decl2.c (mark_used): Move up the constraints_satisfied_p check
	so that it happens before calling maybe_instantiate_decl.

gcc/testsuite/ChangeLog:

	PR c++/95132
	* g++.dg/cpp2a/concepts-fn7.C: New test.
---
 gcc/cp/decl2.c                            | 30 +++++++++++------------
 gcc/testsuite/g++.dg/cpp2a/concepts-fn7.C | 11 +++++++++
 2 files changed, 26 insertions(+), 15 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-fn7.C

diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index 2f0d6370146..de2956aa5f0 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -5604,6 +5604,21 @@ mark_used (tree decl, tsubst_flags_t complain)
   if (DECL_ODR_USED (decl))
     return true;
 
+  if (flag_concepts && TREE_CODE (decl) == FUNCTION_DECL
+      && !constraints_satisfied_p (decl))
+    {
+      if (complain & tf_error)
+	{
+	  auto_diagnostic_group d;
+	  error ("use of function %qD with unsatisfied constraints",
+		 decl);
+	  location_t loc = DECL_SOURCE_LOCATION (decl);
+	  inform (loc, "declared here");
+	  diagnose_constraints (loc, decl, NULL_TREE);
+	}
+      return false;
+    }
+
   /* Normally, we can wait until instantiation-time to synthesize DECL.
      However, if DECL is a static data member initialized with a constant
      or a constexpr function, we need it right now because a reference to
@@ -5614,21 +5629,6 @@ mark_used (tree decl, tsubst_flags_t complain)
      directly.  */
   maybe_instantiate_decl (decl);
 
-  if (flag_concepts && TREE_CODE (decl) == FUNCTION_DECL
-      && !constraints_satisfied_p (decl))
-    {
-      if (complain & tf_error)
-	{
-	  auto_diagnostic_group d;
-	  error ("use of function %qD with unsatisfied constraints",
-		 decl);
-	  location_t loc = DECL_SOURCE_LOCATION (decl);
-	  inform (loc, "declared here");
-	  diagnose_constraints (loc, decl, NULL_TREE);
-	}
-      return false;
-    }
-
   if (processing_template_decl || in_template_function ())
     return true;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-fn7.C b/gcc/testsuite/g++.dg/cpp2a/concepts-fn7.C
new file mode 100644
index 00000000000..7fad6f374b7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-fn7.C
@@ -0,0 +1,11 @@
+// PR c++/95132
+// { dg-do compile { target c++20 } }
+
+template<typename T> struct A {
+  static auto f() requires false { return T::fail; }
+};
+
+template<typename T>
+constexpr bool v = requires { A<T>::f(); };
+
+static_assert(!v<int>);
-- 
2.29.0.rc0