Improve deduction failure diagnostics.

	* pt.c (unify): Don't diagnose no common base if we already have
	the same template.
	(do_auto_deduction): Explain deduction failure.

From-SVN: r229627

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index a995242..592eb5d 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,9 @@
 2015-10-31  Jason Merrill  <jason@redhat.com>
 
+       * pt.c (unify): Don't diagnose no common base if we already have
+       the same template.
+       (do_auto_deduction): Explain deduction failure.
+
        * pt.c (hash_tmpl_and_args): Use iterative_hash_object on template
        uid.
 

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 243464d..c6ef2f5 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -19633,7 +19633,16 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
                                         explain_p, &t);
 
                  if (!t)
-                   return unify_no_common_base (explain_p, r, parm, arg);
+                   {
+                     /* Don't give the derived diagnostic if we're
+                        already dealing with the same template.  */
+                     bool same_template
+                       = (CLASSTYPE_TEMPLATE_INFO (arg)
+                          && (CLASSTYPE_TI_TEMPLATE (parm)
+                              == CLASSTYPE_TI_TEMPLATE (arg)));
+                     return unify_no_common_base (explain_p && !same_template,
+                                                  r, parm, arg);
+                   }
                }
            }
          else if (CLASSTYPE_TEMPLATE_INFO (arg)
@@ -23500,6 +23509,9 @@ do_auto_deduction (tree type, tree init, tree auto_node,
                error ("unable to deduce lambda return type from %qE", init);
              else
                error ("unable to deduce %qT from %qE", type, init);
+             type_unification_real (tparms, targs, parms, &init, 1, 0,
+                                    DEDUCE_CALL, LOOKUP_NORMAL,
+                                    NULL, /*explain_p=*/true);
            }
          return error_mark_node;
        }

diff --git a/gcc/testsuite/g++.old-deja/g++.pt/explicit77.C b/gcc/testsuite/g++.old-deja/g++.pt/explicit77.C
index b97c1cd..d79ee7f 100644 (file)
--- a/gcc/testsuite/g++.old-deja/g++.pt/explicit77.C
+++ b/gcc/testsuite/g++.old-deja/g++.pt/explicit77.C
@@ -15,5 +15,5 @@ void g() {
   
   f<0>(s0, s2);
   f(s0, s2); // { dg-error "" } no matching function
-  // { dg-message "(candidate|deduced conflicting types|ambiguous base class)" "candidate note" { target *-*-* } 17 }
+  // { dg-message "(candidate|deduced conflicting|ambiguous base class)" "candidate note" { target *-*-* } 17 }
 }