C++: special-case single non-viable candidate (more PR c++/85110)

I broke out the "no viable candidates" case in build_new_method_call_1
into a subroutine, and added special-case handling for when there's
a single non-viable candidate where there's an argument conversion
error.  I turned the error-handling from convert_for_assignment into
a subroutine, calling it from this new special-case.

This converts:

demo.cc: In function 'int test_4(int, const char*, float)':
demo.cc:5:44: error: no matching function for call to 's4::member_1(int&, const char*&, float&)'
5 |   return s4::member_1 (first, second, third);
  |                                            ^
demo.cc:1:24: note: candidate: 'static int s4::member_1(int, const char**, float)'
1 | struct s4 { static int member_1 (int one, const char **two, float three); };
  |                        ^~~~~~~~
demo.cc:1:56: note:   no known conversion for argument 2 from 'const char*' to 'const char**'
1 | struct s4 { static int member_1 (int one, const char **two, float three); };
  |                                           ~~~~~~~~~~~~~^~~

to:

demo.cc: In function 'int test_4(int, const char*, float)':
demo.cc:5:31: error: cannot convert 'const char*' to 'const char**'
5 |   return s4::member_1 (first, second, third);
  |                               ^~~~~~
  |                               |
  |                               const char*
demo.cc:1:56: note:   initializing argument 2 of 'static int s4::member_1(int, const char**, float)'
1 | struct s4 { static int member_1 (int one, const char **two, float three); };
  |                                           ~~~~~~~~~~~~~^~~

thus highlighting the problematic argument at the callsite (and its type).

gcc/cp/ChangeLog:
	PR c++/85110
	* call.c (struct conversion_info): Add "loc" field.
	(arg_conversion_rejection): Add "loc" param, using it to
	initialize the new field.
	(bad_arg_conversion_rejection): Likewise.
	(explicit_conversion_rejection): Initialize the new field to
	UNKNOWN_LOCATION.
	(template_conversion_rejection): Likewise.
	(add_function_candidate): Pass on the argument location to the new
	param of arg_conversion_rejection.
	(add_conv_candidate): Likewise.
	(build_builtin_candidate): Likewise.
	(build_user_type_conversion_1): Likewise.
	(single_z_candidate): New function.
	(maybe_get_bad_conversion_for_unmatched_call): New function.
	(complain_about_bad_argument): New function, based on part of
	convert_for_assignment.
	(build_new_method_call_1): Split out handling of the "no viable
	candidates" case into...
	(complain_about_no_candidates_for_method_call): ...this new
	function, and use the new functions above to special-case the
	handling of a single non-viable candidate due to a bad argument.
	* cp-tree.h (complain_about_bad_argument): New decl.
	* typeck.c (convert_for_assignment): Split out one error-handling
	case into complain_about_bad_argument.

gcc/testsuite/ChangeLog:
	PR c++/85110
	* g++.dg/cpp0x/explicit4.C: Update expected output to reflect
	special-casing of diagnostic for a single non-viable candidate due
	to a bad argument.
	* g++.dg/diagnostic/param-type-mismatch-2.C: Likewise.
	Add test coverage for an unmatched overloaded operator.
	* g++.dg/expr/pmf-1.C: Likewise.
	* g++.old-deja/g++.bugs/900330_02.C: Likewise.
	* g++.old-deja/g++.jason/conversion11.C: Likewise.
	* g++.old-deja/g++.law/arg11.C: Likewise.
	* g++.old-deja/g++.law/arm9.C: Likewise.
	* g++.old-deja/g++.robertl/eb131.C: Likewise.

From-SVN: r264250

4 files changed, 188 insertions, 55 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index e4d635d..9201d67 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,31 @@
+2018-09-12  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/85110
+	* call.c (struct conversion_info): Add "loc" field.
+	(arg_conversion_rejection): Add "loc" param, using it to
+	initialize the new field.
+	(bad_arg_conversion_rejection): Likewise.
+	(explicit_conversion_rejection): Initialize the new field to
+	UNKNOWN_LOCATION.
+	(template_conversion_rejection): Likewise.
+	(add_function_candidate): Pass on the argument location to the new
+	param of arg_conversion_rejection.
+	(add_conv_candidate): Likewise.
+	(build_builtin_candidate): Likewise.
+	(build_user_type_conversion_1): Likewise.
+	(single_z_candidate): New function.
+	(maybe_get_bad_conversion_for_unmatched_call): New function.
+	(complain_about_bad_argument): New function, based on part of
+	convert_for_assignment.
+	(build_new_method_call_1): Split out handling of the "no viable
+	candidates" case into...
+	(complain_about_no_candidates_for_method_call): ...this new
+	function, and use the new functions above to special-case the
+	handling of a single non-viable candidate due to a bad argument.
+	* cp-tree.h (complain_about_bad_argument): New decl.
+	* typeck.c (convert_for_assignment): Split out one error-handling
+	case into complain_about_bad_argument.
+
 2018-09-09  Cesar Philippidis  <cesar@codesourcery.com>
             Julian Brown  <julian@codesourcery.com>
 
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 03b4c5a..69503ca 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -436,6 +436,8 @@ struct conversion_info {
   tree from;
   /* The type of the parameter.  */
   tree to_type;
+  /* The location of the argument.  */
+  location_t loc;
 };
   
 struct rejection_reason {
@@ -627,24 +629,28 @@ arity_rejection (tree first_arg, int expected, int actual)
 }
 
 static struct rejection_reason *
-arg_conversion_rejection (tree first_arg, int n_arg, tree from, tree to)
+arg_conversion_rejection (tree first_arg, int n_arg, tree from, tree to,
+			  location_t loc)
 {
   struct rejection_reason *r = alloc_rejection (rr_arg_conversion);
   int adjust = first_arg != NULL_TREE;
   r->u.conversion.n_arg = n_arg - adjust;
   r->u.conversion.from = from;
   r->u.conversion.to_type = to;
+  r->u.conversion.loc = loc;
   return r;
 }
 
 static struct rejection_reason *
-bad_arg_conversion_rejection (tree first_arg, int n_arg, tree from, tree to)
+bad_arg_conversion_rejection (tree first_arg, int n_arg, tree from, tree to,
+			      location_t loc)
 {
   struct rejection_reason *r = alloc_rejection (rr_bad_arg_conversion);
   int adjust = first_arg != NULL_TREE;
   r->u.bad_conversion.n_arg = n_arg - adjust;
   r->u.bad_conversion.from = from;
   r->u.bad_conversion.to_type = to;
+  r->u.bad_conversion.loc = loc;
   return r;
 }
 
@@ -655,6 +661,7 @@ explicit_conversion_rejection (tree from, tree to)
   r->u.conversion.n_arg = 0;
   r->u.conversion.from = from;
   r->u.conversion.to_type = to;
+  r->u.conversion.loc = UNKNOWN_LOCATION;
   return r;
 }
 
@@ -665,6 +672,7 @@ template_conversion_rejection (tree from, tree to)
   r->u.conversion.n_arg = 0;
   r->u.conversion.from = from;
   r->u.conversion.to_type = to;
+  r->u.conversion.loc = UNKNOWN_LOCATION;
   return r;
 }
 
@@ -2257,14 +2265,17 @@ add_function_candidate (struct z_candidate **candidates,
       if (! t)
 	{
 	  viable = 0;
-	  reason = arg_conversion_rejection (first_arg, i, argtype, to_type);
+	  reason = arg_conversion_rejection (first_arg, i, argtype, to_type,
+					     EXPR_LOCATION (arg));
 	  break;
 	}
 
       if (t->bad_p)
 	{
 	  viable = -1;
-	  reason = bad_arg_conversion_rejection (first_arg, i, arg, to_type);
+	  reason = bad_arg_conversion_rejection (first_arg, i, arg, to_type,
+						 EXPR_LOCATION (arg));
+
 	}
     }
 
@@ -2353,7 +2364,8 @@ add_conv_candidate (struct z_candidate **candidates, tree fn, tree obj,
       if (t->bad_p)
 	{
 	  viable = -1;
-	  reason = bad_arg_conversion_rejection (NULL_TREE, i, arg, convert_type);
+	  reason = bad_arg_conversion_rejection (NULL_TREE, i, arg, convert_type,
+						 EXPR_LOCATION (arg));
 	}
 
       if (i == 0)
@@ -2414,13 +2426,14 @@ build_builtin_candidate (struct z_candidate **candidates, tree fnname,
 	  /* We need something for printing the candidate.  */
 	  t = build_identity_conv (types[i], NULL_TREE);
 	  reason = arg_conversion_rejection (NULL_TREE, i, argtypes[i],
-					     types[i]);
+					     types[i], EXPR_LOCATION (args[i]));
 	}
       else if (t->bad_p)
 	{
 	  viable = 0;
 	  reason = bad_arg_conversion_rejection (NULL_TREE, i, args[i],
-						 types[i]);
+						 types[i],
+						 EXPR_LOCATION (args[i]));
 	}
       convs[i] = t;
     }
@@ -2439,7 +2452,8 @@ build_builtin_candidate (struct z_candidate **candidates, tree fnname,
 	{
 	  viable = 0;
 	  reason = arg_conversion_rejection (NULL_TREE, 0, argtypes[2],
-					     boolean_type_node);
+					     boolean_type_node,
+					     EXPR_LOCATION (args[2]));
 	}
     }
 
@@ -3930,7 +3944,8 @@ build_user_type_conversion_1 (tree totype, tree expr, int flags,
 	    {
 	      cand->viable = 0;
 	      cand->reason = arg_conversion_rejection (NULL_TREE, -2,
-						       rettype, totype);
+						       rettype, totype,
+						       EXPR_LOCATION (expr));
 	    }
 	  else if (DECL_NONCONVERTING_P (cand->fn)
 		   && ics->rank > cr_exact)
@@ -3950,7 +3965,8 @@ build_user_type_conversion_1 (tree totype, tree expr, int flags,
 	      cand->viable = -1;
 	      cand->reason
 		= bad_arg_conversion_rejection (NULL_TREE, -2,
-						rettype, totype);
+						rettype, totype,
+						EXPR_LOCATION (expr));
 	    }
 	  else if (primary_template_specialization_p (cand->fn)
 		   && ics->rank > cr_exact)
@@ -9165,6 +9181,129 @@ name_as_c_string (tree name, tree type, bool *free_p)
   return CONST_CAST (char *, pretty_name);
 }
 
+/* If CANDIDATES contains exactly one candidate, return it, otherwise
+   return NULL.  */
+
+static z_candidate *
+single_z_candidate (z_candidate *candidates)
+{
+  if (candidates == NULL)
+    return NULL;
+
+  if (candidates->next)
+    return NULL;
+
+  return candidates;
+}
+
+/* If CANDIDATE is invalid due to a bad argument type, return the
+   pertinent conversion_info.
+
+   Otherwise, return NULL.  */
+
+static const conversion_info *
+maybe_get_bad_conversion_for_unmatched_call (const z_candidate *candidate)
+{
+  /* Must be an rr_arg_conversion or rr_bad_arg_conversion.  */
+  rejection_reason *r = candidate->reason;
+
+  if (r == NULL)
+    return NULL;
+
+  switch (r->code)
+    {
+    default:
+      return NULL;
+
+    case rr_arg_conversion:
+      return &r->u.conversion;
+
+    case rr_bad_arg_conversion:
+      return &r->u.bad_conversion;
+    }
+}
+
+/* Issue an error and note complaining about a bad argument type at a
+   callsite with a single candidate FNDECL.
+
+   ARG_LOC is the location of the argument (or UNKNOWN_LOCATION, in which
+   case input_location is used).
+   FROM_TYPE is the type of the actual argument; TO_TYPE is the type of
+   the formal parameter.  */
+
+void
+complain_about_bad_argument (location_t arg_loc,
+			     tree from_type, tree to_type,
+			     tree fndecl, int parmnum)
+{
+  auto_diagnostic_group d;
+  range_label_for_type_mismatch rhs_label (from_type, to_type);
+  range_label *label = &rhs_label;
+  if (arg_loc == UNKNOWN_LOCATION)
+    {
+      arg_loc = input_location;
+      label = NULL;
+    }
+  gcc_rich_location richloc (arg_loc, label);
+  error_at (&richloc,
+	    "cannot convert %qH to %qI",
+	    from_type, to_type);
+  inform (get_fndecl_argument_location (fndecl, parmnum),
+	  "  initializing argument %P of %qD", parmnum, fndecl);
+}
+
+/* Subroutine of build_new_method_call_1, for where there are no viable
+   candidates for the call.  */
+
+static void
+complain_about_no_candidates_for_method_call (tree instance,
+					      z_candidate *candidates,
+					      tree explicit_targs,
+					      tree basetype,
+					      tree optype, tree name,
+					      bool skip_first_for_error,
+					      vec<tree, va_gc> *user_args)
+{
+  auto_diagnostic_group d;
+  if (!COMPLETE_OR_OPEN_TYPE_P (basetype))
+    cxx_incomplete_type_error (instance, basetype);
+  else if (optype)
+    error ("no matching function for call to %<%T::operator %T(%A)%#V%>",
+	   basetype, optype, build_tree_list_vec (user_args),
+	   TREE_TYPE (instance));
+  else
+    {
+      /* Special-case for when there's a single candidate that's failing
+	 due to a bad argument type.  */
+      if (z_candidate *candidate = single_z_candidate (candidates))
+	  if (const conversion_info *conv
+		= maybe_get_bad_conversion_for_unmatched_call (candidate))
+	    {
+	      complain_about_bad_argument (conv->loc,
+					   conv->from, conv->to_type,
+					   candidate->fn, conv->n_arg);
+	      return;
+	    }
+
+      tree arglist = build_tree_list_vec (user_args);
+      tree errname = name;
+      bool twiddle = false;
+      if (IDENTIFIER_CDTOR_P (errname))
+	{
+	  twiddle = IDENTIFIER_DTOR_P (errname);
+	  errname = constructor_name (basetype);
+	}
+      if (explicit_targs)
+	errname = lookup_template_function (errname, explicit_targs);
+      if (skip_first_for_error)
+	arglist = TREE_CHAIN (arglist);
+      error ("no matching function for call to %<%T::%s%E(%A)%#V%>",
+	     basetype, &"~"[!twiddle], errname, arglist,
+	     TREE_TYPE (instance));
+    }
+  print_z_candidates (location_of (name), candidates);
+}
+
 /* Build a call to "INSTANCE.FN (ARGS)".  If FN_P is non-NULL, it will
    be set, upon return, to the function called.  ARGS may be NULL.
    This may change ARGS.  */
@@ -9382,34 +9521,11 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
   if (!any_viable_p)
     {
       if (complain & tf_error)
-	{
-	  auto_diagnostic_group d;
-	  if (!COMPLETE_OR_OPEN_TYPE_P (basetype))
-	    cxx_incomplete_type_error (instance, basetype);
-	  else if (optype)
-	    error ("no matching function for call to %<%T::operator %T(%A)%#V%>",
-		   basetype, optype, build_tree_list_vec (user_args),
-		   TREE_TYPE (instance));
-	  else
-	    {
-	      tree arglist = build_tree_list_vec (user_args);
-	      tree errname = name;
-	      bool twiddle = false;
-	      if (IDENTIFIER_CDTOR_P (errname))
-		{
-		  twiddle = IDENTIFIER_DTOR_P (errname);
-		  errname = constructor_name (basetype);
-		}
-	      if (explicit_targs)
-		errname = lookup_template_function (errname, explicit_targs);
-	      if (skip_first_for_error)
-		arglist = TREE_CHAIN (arglist);
-	      error ("no matching function for call to %<%T::%s%E(%A)%#V%>",
-		     basetype, &"~"[!twiddle], errname, arglist,
-		     TREE_TYPE (instance));
-	    }
-	  print_z_candidates (location_of (name), candidates);
-	}
+	complain_about_no_candidates_for_method_call (instance, candidates,
+						      explicit_targs, basetype,
+						      optype, name,
+						      skip_first_for_error,
+						      user_args);
       call = error_mark_node;
     }
   else
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b78e9eb..6cd6e5f 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6088,6 +6088,9 @@ extern bool can_convert_arg_bad			(tree, tree, tree, int,
 extern int conv_flags				(int, int, tree, tree, int);
 extern struct conversion * good_conversion	(tree, tree, tree, int, tsubst_flags_t);
 extern location_t get_fndecl_argument_location  (tree, int);
+extern void complain_about_bad_argument	(location_t arg_loc,
+						 tree from_type, tree to_type,
+						 tree fndecl, int parmnum);
 
 
 /* A class for recording information about access failures (e.g. private
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 9fa4c16..e993220 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -8820,23 +8820,9 @@ convert_for_assignment (tree type, tree rhs,
 						   parmnum, complain, flags);
 		}
 	      else if (fndecl)
-		{
-		  auto_diagnostic_group d;
-		  location_t loc = cp_expr_location (rhs);
-		  range_label_for_type_mismatch rhs_label (rhstype, type);
-		  range_label *label = &rhs_label;
-		  if (loc == UNKNOWN_LOCATION)
-		    {
-		      loc = input_location;
-		      label = NULL;
-		    }
-		  gcc_rich_location richloc (loc, label);
-		  error_at (&richloc,
-			    "cannot convert %qH to %qI",
-			    rhstype, type);
-		  inform (get_fndecl_argument_location (fndecl, parmnum),
-			  "  initializing argument %P of %qD", parmnum, fndecl);
-		}
+		complain_about_bad_argument (cp_expr_location (rhs),
+					     rhstype, type,
+					     fndecl, parmnum);
 	      else
 		switch (errtype)
 		  {