c++: constraints and explicit instantiation [PR96164]

When considering to instantiate a member of a class template as part of
an explicit instantiation of the class template, we need to first check
the member's constraints before proceeding with the instantiation of the
member.

gcc/cp/ChangeLog:

	PR c++/96164
	* constraint.cc (constraints_satisfied_p): Return true if
	!flags_concepts.
	* pt.c (do_type_instantiation): Update a paragraph taken from
	[temp.explicit] to reflect the latest specification.  Don't
	instantiate a member with unsatisfied constraints.

gcc/testsuite/ChangeLog:

	PR c++/96164
	* g++.dg/cpp2a/concepts-explicit-inst5.C: New test.

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index d0da230..e4aace5 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2864,6 +2864,9 @@ constraint_satisfaction_value (tree t, tree args, tsubst_flags_t complain)
 bool
 constraints_satisfied_p (tree t)
 {
+  if (!flag_concepts)
+    return true;
+
   return constraint_satisfaction_value (t, tf_none) == boolean_true_node;
 }
 
@@ -2873,6 +2876,9 @@ constraints_satisfied_p (tree t)
 bool
 constraints_satisfied_p (tree t, tree args)
 {
+  if (!flag_concepts)
+    return true;
+
   return constraint_satisfaction_value (t, args, tf_none) == boolean_true_node;
 }