[C++, diagnostic Patch] PR 58362

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi all, hi Gaby,

thus a more sensible try at fixing this issue: 
function.c:do_warn_unused_parameter uses "%q+D" for the error call. That 
means that cp/error.c:cp_printer does:

  if (set_locus && t != NULL)
    *text->locus = location_of (t);

and it's important that location_of (t) is correct. However, for 
PARM_DECLs, location_of (still) does:

  if (TREE_CODE (t) == PARM_DECL && DECL_CONTEXT (t))
    t = DECL_CONTEXT (t);
  else if ...
  ...

which means that for those we end up with the location of a 
FUNCTION_DECL or anyway not exactly the location of the PARM_DECL 
itself. I went through the C++ front-end and currently there are very 
few cases if any where location_of may be fed a PARM_DECL, and the very 
few seem Ok to me. The testsuite passes on x86_64-linux of course, thus 
I'm proposing the below. IMHO now that we are in Stage 1 the risk of 
breaking locations somewhere else is low enough. By the way, as 
discussed today elsewhere, the C front-end is already Ok.

Thanks!
Paolo.

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