c++: always check arity before deduction

This simple patch extends the r12-3271-gf1e73199569287 optimization
to happen for deduction without explicit template arguments as well.
The motivation for this is to accept testcases such as conv20.C and
ttp40.C below, which don't use explicit template arguments but for
which unnecessary template instantiation during deduction could be
avoided if we uniformly pruned overloads according to arity early.
This incidentally causes us to accept one reduced testcase from
PR c++/84075, but the underlying issue there remains at large.

As a nice side effect, this change causes the "candidate expects N
argument(s)" note during overload resolution failure to point to the
template candidate instead of the call site, which seems like an
improvement along the lines of r14-309-g14e881eb030509.

gcc/cp/ChangeLog:

	* call.cc (add_template_candidate_real): Check arity even
	when there are no explicit template arguments.  Combine the
	two adjacent '!obj' tests into one.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/vt-57397-1.C: Expect "candidate expects ... N
	argument(s)" at the declaration site instead of the call site.
	* g++.dg/cpp0x/vt-57397-2.C: Likewise.
	* g++.dg/overload/template5.C: Likewise.
	* g++.dg/template/local6.C: Likewise.
	* g++.dg/template/conv20.C: New test.
	* g++.dg/template/ttp40.C: New test.

gcc/cp/call.cc

@@ -3535,13 +3535,13 @@ add_template_candidate_real (struct z_candidate **candidates, tree tmpl,
     }
   gcc_assert (ia == nargs_without_in_chrg);
 
-  if (!obj && explicit_targs)
+  if (!obj)
     {
       /* Check that there's no obvious arity mismatch before proceeding with
 	 deduction.  This avoids substituting explicit template arguments
-	 into the template (which could result in an error outside the
-	 immediate context) when the resulting candidate would be unviable
-	 anyway.  */
+	 into the template or e.g. derived-to-base parm/arg unification
+	 (which could result in an error outside the immediate context) when
+	 the resulting candidate would be unviable anyway.  */
       int min_arity = 0, max_arity = 0;
       tree parms = TYPE_ARG_TYPES (TREE_TYPE (tmpl));
       parms = skip_artificial_parms_for (tmpl, parms);
@@ -3571,11 +3571,7 @@ add_template_candidate_real (struct z_candidate **candidates, tree tmpl,
 	  reason = arity_rejection (NULL_TREE, max_arity, ia);
 	  goto fail;
 	}
-    }
 
-  errs = errorcount+sorrycount;
-  if (!obj)
-    {
       convs = alloc_conversions (nargs);
 
       if (shortcut_bad_convs
@@ -3602,6 +3598,8 @@ add_template_candidate_real (struct z_candidate **candidates, tree tmpl,
 	    }
 	}
     }
+
+  errs = errorcount+sorrycount;
   fn = fn_type_unification (tmpl, explicit_targs, targs,
 			    args_without_in_chrg,
 			    nargs_without_in_chrg,

gcc/testsuite/g++.dg/cpp0x/vt-57397-1.C

@@ -3,20 +3,20 @@
 
 template<class T1, class... Tn>
 void foo(T1, Tn...);
+// { dg-message "candidate expects at least 1 argument, 0 provided" "" { target *-*-* } .-1 }
 
 template<class T1, class T2, class... Tn>
 void bar(T1, T2, Tn...);
+// { dg-message "candidate expects at least 2 arguments, 0 provided" "" { target *-*-* } .-1 }
+// { dg-message "candidate expects at least 2 arguments, 1 provided" "" { target *-*-* } .-2 }
 
 int main()
 {
   foo();   // { dg-error "no matching" }
-  // { dg-message "candidate expects at least 1 argument, 0 provided" "" { target *-*-* } .-1 }
   foo(1);
   foo(1, 2);
   bar();   // { dg-error "no matching" }
-  // { dg-message "candidate expects at least 2 arguments, 0 provided" "" { target *-*-* } .-1 }
   bar(1);  // { dg-error "no matching" }
-  // { dg-message "candidate expects at least 2 arguments, 1 provided" "" { target *-*-* } .-1 }
   bar(1, 2);
   bar(1, 2, 3);
 }

gcc/testsuite/g++.dg/cpp0x/vt-57397-2.C

@@ -3,21 +3,21 @@
 
 template<class T1, class... Tn, class... Tm>
 void foo(T1, Tn..., Tm...);
+// { dg-message "candidate expects at least 1 argument, 0 provided" "" { target *-*-* } .-1 }
 
 template<class T1, class T2, class... Tn, class... Tm>
 void bar(T1, T2, Tn..., Tm...);
+// { dg-message "candidate expects at least 2 arguments, 0 provided" "" { target *-*-* } .-1 }
+// { dg-message "candidate expects at least 2 arguments, 1 provided" "" { target *-*-* } .-2 }
 
 int main()
 {
   foo();   // { dg-error "no matching" }
-  // { dg-message "candidate expects at least 1 argument, 0 provided" "" { target *-*-* } .-1 }
   foo(1);
   foo(1, 2);
   foo(1, 2, 3);
   bar();   // { dg-error "no matching" }
-  // { dg-message "candidate expects at least 2 arguments, 0 provided" "" { target *-*-* } .-1 }
   bar(1);  // { dg-error "no matching" }
-  // { dg-message "candidate expects at least 2 arguments, 1 provided" "" { target *-*-* } .-1 }
   bar(1, 2);
   bar(1, 2, 3);
   bar(1, 2, 3, 4);

gcc/testsuite/g++.dg/overload/template5.C

@@ -2,14 +2,14 @@
 
 template<typename T>
 int low(T a, T b, T c) { return a + b + c; } // { dg-message "template" }
+// { dg-message "(candidate|3 arguments, 2 provided)" "" { target *-*-* } .-1 }
 
 template<typename T>
 int high(T a, T b, T c) { return a + b + c; } // { dg-message "template" }
+// { dg-message "(candidate|3 arguments, 4 provided)" "" { target *-*-* } .-1 }
 
 void test (void)
 {
   low (5, 6);			// { dg-error "no matching function" }
-  // { dg-message "(candidate|3 arguments, 2 provided)" "" { target *-*-* } .-1 }
   high (5, 6, 7, 8);		// { dg-error "no matching function" }
-  // { dg-message "(candidate|3 arguments, 4 provided)" "" { target *-*-* } .-1 }
 }

gcc/testsuite/g++.dg/template/conv20.C

@@ -0,0 +1,17 @@
+// Verify we check arity early before deduction without explicit
+// template arguments.
+
+template<class T>
+struct A;
+
+template<class T>
+struct B : A<T> { };
+
+template<class T> void f(A<T>&, int); // #1
+template<class T> void f(B<T>&);      // #2
+
+int main() {
+  extern B<int> b;
+  ::f(b); // OK, deduction for #1 short-circuited and B<int> not instantiated,
+	  // which would have resulted in a hard error
+}

gcc/testsuite/g++.dg/template/local6.C

@@ -1,11 +1,11 @@
 template <class T> struct PCVector2 // { dg-message "note" }
 {
-    template <class T2> PCVector2(const PCVector2<T> &cv) ; // { dg-message "note" }
+    template <class T2> PCVector2(const PCVector2<T> &cv) ; // { dg-message "candidate:" }
+    // { dg-message "(candidate|expects 1 argument, 2 provided|cannot convert)" "candidate note" { target *-*-* } .-1 }
 
     PCVector2<T> operator- (const PCVector2<T> &ov) const 
 	{ 
 	  return PCVector2<T>(ov.xFIELD, ov.yFIELD); // { dg-error "matching" }
-	  // { dg-message "(candidate|expects 1 argument, 2 provided|cannot convert)" "candidate note" { target *-*-* } .-1 }
 	}
 
     T xFIELD, yFIELD;

gcc/testsuite/g++.dg/template/ttp40.C

@@ -0,0 +1,25 @@
+// Verify we check arity early before deduction without explicit
+// template arguments.
+// PR c++/84075
+
+template<class T>
+struct trait {
+  static const int value = T::value; // { dg-bogus "not a member of 'B'" }
+};
+
+template<class T, int N = trait<T>::value>
+struct A { };
+
+template<class T>
+void f(A<T, 42>, int); // #1
+
+struct B { };
+
+template<template<class> class TT>
+void f(TT<B>); // #2
+
+int main() {
+  A<int, 42> a;
+  f(a, 0); // OK, deduction for #2 short-circuited and A<B> not specialized,
+	   // which would have resulted in a hard error
+}