[pushed] c++: tweak deduction with auto template parms

[Jason Merrill <jason@redhat.com>]

While looking at PR105964 I noticed that we were unnecessarily repeating
the deduction loop because of seeing a non-type parameter with type 'auto'.
It is indeed dependent, but not on any other deductions.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog:

	* pt.cc (type_unification_real): An auto tparm can't
	be affected by other deductions.
---
 gcc/cp/pt.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 4d1c325432d..80d2bec2348 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -22982,6 +22982,7 @@ type_unification_real (tree tparms,
 	     deduced from a later argument than the one from which
 	     this parameter can be deduced.  */
 	  if (TREE_CODE (tparm) == PARM_DECL
+	      && !is_auto (TREE_TYPE (tparm))
 	      && uses_template_parms (TREE_TYPE (tparm))
 	      && saw_undeduced < 2)
 	    {
@@ -23042,6 +23043,7 @@ type_unification_real (tree tparms,
 
 	  if (saw_undeduced == 1
 	      && TREE_CODE (parm) == PARM_DECL
+	      && !is_auto (TREE_TYPE (parm))
 	      && uses_template_parms (TREE_TYPE (parm)))
 	    {
 	      /* The type of this non-type parameter depends on undeduced

base-commit: 349a39f061407ce2339d2ba25da97005f2030c88
prerequisite-patch-id: d5622f522428b601200c8c234ddb5e58c91dcb06
-- 
2.27.0