c++: Fix ICE with constrained friend (PR93400).

Here, the problem was that tsubst_friend_function was modifying the
CONSTRAINT_INFO for the friend template to have the constraints for one
instantiation, which fell down when we went to adjust it for another
instantiation.  Fixed by deferring substitution of trailing requirements
until we try to check declaration matching.

	PR c++/93400 - ICE with constrained friend.
	* constraint.cc (maybe_substitute_reqs_for): New.
	* decl.c (function_requirements_equivalent_p): Call it.
	* pt.c (tsubst_friend_function): Only substitute
	TEMPLATE_PARMS_CONSTRAINTS.
	(tsubst_template_parms): Copy constraints.

1 files changed, 23 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 823604a..cda644e 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1189,6 +1189,29 @@ remove_constraints (tree t)
     decl_constraints->remove (t);
 }
 
+/* If DECL is a friend, substitute into REQS to produce requirements suitable
+   for declaration matching.  */
+
+tree
+maybe_substitute_reqs_for (tree reqs, const_tree decl_)
+{
+  if (reqs == NULL_TREE)
+    return NULL_TREE;
+  tree decl = CONST_CAST_TREE (decl_);
+  tree result = STRIP_TEMPLATE (decl);
+  if (DECL_FRIEND_P (result))
+    {
+      tree tmpl = decl == result ? DECL_TI_TEMPLATE (result) : decl;
+      tree gargs = generic_targs_for (tmpl);
+      processing_template_decl_sentinel s;
+      if (uses_template_parms (gargs))
+	++processing_template_decl;
+      reqs = tsubst_constraint (reqs, gargs,
+				tf_warning_or_error, NULL_TREE);
+    }
+  return reqs;
+}
+
 /* Returns the template-head requires clause for the template
    declaration T or NULL_TREE if none.  */