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