c++: tweaks for explicit conversion fns diagnostic

1) When saying that a conversion is erroneous because it would use
an explicit constructor, it might be nice to show where exactly
the explicit constructor is located.  For example, with this patch:

[...]
explicit.C:4:12: note: 'S::S(int)' declared here
    4 |   explicit S(int) { }
      |            ^

2) When a conversion doesn't work out merely because the conversion
function necessary to do the conversion couldn't be used because
it was marked explicit, it would be useful to the user to say so,
rather than just saying "cannot convert".  For example, with this patch:

explicit.C:13:12: error: cannot convert 'S' to 'bool' in initialization
   13 |   bool b = S{1};
      |            ^~~~
      |            |
      |            S
explicit.C:5:12: note: explicit conversion function was not considered
    5 |   explicit operator bool() const { return true; }
      |            ^~~~~~~~

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal): Show where the conversion function
	was declared.
	(maybe_show_nonconverting_candidate): New.
	* cp-tree.h (maybe_show_nonconverting_candidate): Declare.
	* typeck.cc (convert_for_assignment): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/explicit.C: New test.

0 files changed, 0 insertions, 0 deletions