c++: fix template instantiation comparison in redeclarations

This change fixes a primordial c++11 frontend defect where function template
redeclarations with trailing return types that used dependent
sizeof/alignof/noexcept expressions in template value arguments failed to
compare as equivalent to the identical primary template declaration. By
forcing structural AST comparison of the template arguments, we no longer
require TYPE_CANONICAL to match in this case. The new canon-type-{15..18}.C
tests failed with all prior GCC versions, where the redeclarations were
incorrectly reported as ambiguous overloads. The new dependent-name{15,16}.C
tests are regression tests for sneaky problems encountered during
development of this fix. Note that this fix does not address the use of parm
objects' constexpr members as template arguments within a declaration (a
superficially similar longstanding defect).

gcc/cp/ChangeLog:

	* pt.c (find_parm_usage_r): New walk_tree callback to find func
	parms.
	(any_template_arguments_need_structural_equality_p): New special
	case.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-52830.C: Remove unwanted dg-ice.
	* g++.dg/template/canon-type-15.C: New test.
	* g++.dg/template/canon-type-16.C: New test.
	* g++.dg/template/canon-type-17.C: New test.
	* g++.dg/template/canon-type-18.C: New test.
	* g++.dg/template/dependent-name15.C: New regression test.
	* g++.dg/template/dependent-name16.C: New regression test.

1 files changed, 21 insertions, 0 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 4d42899..048bbc4 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -27813,6 +27813,20 @@ dependent_template_arg_p (tree arg)
     return value_dependent_expression_p (arg);
 }
 
+/* Identify any expressions that use function parms.  */
+
+static tree
+find_parm_usage_r (tree *tp, int *walk_subtrees, void*)
+{
+  tree t = *tp;
+  if (TREE_CODE (t) == PARM_DECL)
+    {
+      *walk_subtrees = 0;
+      return t;
+    }
+  return NULL_TREE;
+}
+
 /* Returns true if ARGS (a collection of template arguments) contains
    any types that require structural equality testing.  */
 
@@ -27857,6 +27871,13 @@ any_template_arguments_need_structural_equality_p (tree args)
 	      else if (!TYPE_P (arg) && TREE_TYPE (arg)
 		       && TYPE_STRUCTURAL_EQUALITY_P (TREE_TYPE (arg)))
 		return true;
+	      /* Checking current_function_decl because this structural
+		 comparison is only necessary for redeclaration.  */
+	      else if (!current_function_decl
+		       && dependent_template_arg_p (arg)
+		       && (cp_walk_tree_without_duplicates
+			   (&arg, find_parm_usage_r, NULL)))
+		return true;
 	    }
 	}
     }