C++ PATCH for c++/84489, dependent default template argument

[Jason Merrill]

The logic in type_unification_real for handling template parms that
depend on earlier template parms is a bit complicated.  It already
recognizes when the type of the parm depends on something not
available yet, and it dealt with the case where substituting partial
args left some template parm uses behind, but it didn't handle the
case where substituting partial args just failed.

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

commit dade5ae09222e696b2aaa05d730e2414b3568bd4
Author: Jason Merrill <ja...@redhat.com>
Date:   Mon Feb 26 23:38:10 2018 -0500

            PR c++/84489 - dependent default template argument
    
            * pt.c (type_unification_real): Handle early substitution failure.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 40c897aadc9..2a64fa6d9ad 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -19831,21 +19831,28 @@ type_unification_real (tree tparms,
            continue;
          tree parm = TREE_VALUE (tparm);
 
-         if (TREE_CODE (parm) == PARM_DECL
-             && uses_template_parms (TREE_TYPE (parm))
-             && saw_undeduced < 2)
-           continue;
+         tsubst_flags_t fcomplain = complain;
+         if (saw_undeduced == 1)
+           {
+             /* When saw_undeduced == 1, substitution into parm and arg might
+                fail or not replace all template parameters, and that's
+                fine.  */
+             fcomplain = tf_none;
+             if (TREE_CODE (parm) == PARM_DECL
+                 && uses_template_parms (TREE_TYPE (parm)))
+               continue;
+           }
 
          tree arg = TREE_PURPOSE (tparm);
          reopen_deferring_access_checks (*checks);
          location_t save_loc = input_location;
          if (DECL_P (parm))
            input_location = DECL_SOURCE_LOCATION (parm);
-         arg = tsubst_template_arg (arg, full_targs, complain, NULL_TREE);
-         if (!uses_template_parms (arg))
+         arg = tsubst_template_arg (arg, full_targs, fcomplain, NULL_TREE);
+         if (arg != error_mark_node && !uses_template_parms (arg))
            arg = convert_template_argument (parm, arg, full_targs, complain,
                                             i, NULL_TREE);
-         else if (saw_undeduced < 2)
+         else if (saw_undeduced == 1)
            arg = NULL_TREE;
          else
            arg = error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp0x/fntmpdefarg7.C 
b/gcc/testsuite/g++.dg/cpp0x/fntmpdefarg7.C
new file mode 100644
index 00000000000..636bf1afd88
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/fntmpdefarg7.C
@@ -0,0 +1,10 @@
+// PR c++/84489
+// { dg-do compile { target c++11 } }
+
+template <class T = int, T N = T(), bool B = (N >> 1)>
+T f1() {return 0;}
+
+int main()
+{
+  f1(); // Bug here
+}