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.

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;
 }
 

gcc/cp/pt.c

@@ -24928,23 +24928,22 @@ do_type_instantiation (tree t, tree storage, tsubst_flags_t complain)
 
 	 [temp.explicit]
 
-	 The explicit instantiation of a class template specialization
-	 implies the instantiation of all of its members not
-	 previously explicitly specialized in the translation unit
-	 containing the explicit instantiation.
-
-     Of course, we can't instantiate member template classes, since we
-     don't have any arguments for them.  Note that the standard is
-     unclear on whether the instantiation of the members are
-     *explicit* instantiations or not.  However, the most natural
-     interpretation is that it should be an explicit
-     instantiation.  */
+	 An explicit instantiation that names a class template
+	 specialization is also an explicit instantiation of the same
+	 kind (declaration or definition) of each of its members (not
+	 including members inherited from base classes and members
+	 that are templates) that has not been previously explicitly
+	 specialized in the translation unit containing the explicit
+	 instantiation, provided that the associated constraints, if
+	 any, of that member are satisfied by the template arguments
+	 of the explicit instantiation.  */
   for (tree fld = TYPE_FIELDS (t); fld; fld = DECL_CHAIN (fld))
     if ((VAR_P (fld)
 	 || (TREE_CODE (fld) == FUNCTION_DECL
 	     && !static_p
 	     && user_provided_p (fld)))
-	&& DECL_TEMPLATE_INSTANTIATION (fld))
+	&& DECL_TEMPLATE_INSTANTIATION (fld)
+	&& constraints_satisfied_p (fld))
       {
 	mark_decl_instantiated (fld, extern_p);
 	if (! extern_p)

gcc/testsuite/g++.dg/cpp2a/concepts-explicit-inst5.C

@@ -0,0 +1,14 @@
+// PR c++/96164
+// { dg-do compile { target concepts } }
+
+template <int N>
+struct A {
+    void f() requires (N == 3) { static_assert(N == 3); }
+};
+template struct A<2>;
+
+template <int N>
+struct B {
+    void f() requires (N == 2) { static_assert(N == 3); } // { dg-error "assert" }
+};
+template struct B<2>;