[gcc r12-9443] c++: default template arg, partial ordering [PR105481]

[Jason Merrill]

https://gcc.gnu.org/g:ab7c0a44ca9e913af654ae82963565d61d8bb39f

commit r12-9443-gab7c0a44ca9e913af654ae82963565d61d8bb39f
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Mar 23 18:20:52 2023 -0400

    c++: default template arg, partial ordering [PR105481]
    
    The default argument code in type_unification_real was assuming that all
    targs we've deduced by that point are non-dependent, but that's not the case
    for partial ordering.
    
            PR c++/105481
    
    gcc/cp/ChangeLog:
    
            * pt.cc (type_unification_real): Adjust for partial ordering.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/fntmpdefarg-partial1.C: New test.

Diff:
---
 gcc/cp/pt.cc                                      | 18 ++++++++++++++----
 gcc/testsuite/g++.dg/cpp0x/fntmpdefarg-partial1.C |  8 ++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index a638d30cc7e..b5fabe4341e 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23072,14 +23072,24 @@ type_unification_real (tree tparms,
 	  return unify_parameter_deduction_failure (explain_p, tparm);
 	}
 
+      /* During partial ordering, we deduce dependent template args.  */
+      bool any_dependent_targs = false;
+
       /* Now substitute into the default template arguments.  */
       for (i = 0; i < ntparms; i++)
 	{
 	  tree targ = TREE_VEC_ELT (targs, i);
 	  tree tparm = TREE_VEC_ELT (tparms, i);
 
-	  if (targ || tparm == error_mark_node)
+	  if (targ)
+	    {
+	      if (!any_dependent_targs && dependent_template_arg_p (targ))
+		any_dependent_targs = true;
+	      continue;
+	    }
+	  if (tparm == error_mark_node)
 	    continue;
+
 	  tree parm = TREE_VALUE (tparm);
 	  tree arg = TREE_PURPOSE (tparm);
 	  reopen_deferring_access_checks (*checks);
@@ -23114,9 +23124,9 @@ type_unification_real (tree tparms,
 		 do this substitution without processing_template_decl.  This
 		 is important if the default argument contains something that
 		 might be instantiation-dependent like access (87480).  */
-	      processing_template_decl_sentinel s;
+	      processing_template_decl_sentinel s (!any_dependent_targs);
 	      tree substed = NULL_TREE;
-	      if (saw_undeduced == 1)
+	      if (saw_undeduced == 1 && !any_dependent_targs)
 		{
 		  /* First instatiate in template context, in case we still
 		     depend on undeduced template parameters.  */
@@ -23139,7 +23149,7 @@ type_unification_real (tree tparms,
 						 complain, i, NULL_TREE);
 	      else if (saw_undeduced == 1)
 		arg = NULL_TREE;
-	      else
+	      else if (!any_dependent_targs)
 		arg = error_mark_node;
 	    }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/fntmpdefarg-partial1.C b/gcc/testsuite/g++.dg/cpp0x/fntmpdefarg-partial1.C
new file mode 100644
index 00000000000..2a6783e566b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/fntmpdefarg-partial1.C
@@ -0,0 +1,8 @@
+// PR c++/105481
+// { dg-do compile { target c++11 } }
+
+template<unsigned> struct uint;
+template<unsigned N> uint<N> f(const uint<N> &);
+template<unsigned N, typename T, typename = uint<N>> uint<N> f(T);
+using X = uint<1>;
+X (*fp)(X const &) = f;