C++: missing result_decl loc of (member) functions

[Bernhard Reutner-Fischer]

Hi!

I'm having a hard time to set the correct location of the result decl of
member functions in C++. Any hint on where to best fix the missing
locus?
TIA for any hint (or a fix :).

Long version:
Neither the C nor the C++ frontend seem to have a working/proper
location for the RESULT_DECL of functions. This makes it impossible to
have diagnostics point at the return type of a function and hence it is
impossible to have fixit-replace hints for the return types. [There is
a gfortran bug that asks to use narrower return types for functions
in the FE and i meant to get me a C++ warning that would point out all
affected spots in the fortran FE..]

Adding the correct location to the result decl of a function in the C
frontend was easy, see attached. Same for the C++ FE for normal
functions. These are covered in the attached sample-testcase
return-narrow.cc ¹) and work well. I'm attaching a single patch for the
C and C++ FE, knowing that they are to be sent separately upon
real submission and probably only once stage1 reopens.

But i cannot seem to find a proper spot to handle member functions (if
that's the correct term for them).
These are preparsed functions AFAICS and i'd need a place where i can
get at the declspecs[ds_type_spec] and the function is parsed "enough"
that the return type is determined, i think.

I'm attaching the sample-testcase return-narrow-2.cc which, with a
pristine compiler, gives no location for the result decl at all:

---8<---
$ XXX=1 ../gcc/xg++ -B../gcc -c -o return-narrow.o return-narrow-2.cc -fdiagnostics-color=always 
return-narrow-2.cc: In member function ‘long unsigned int __mini_vector< <template-parameter-1-1> >::_M_space_left()’:
return-narrow-2.cc:7:3: warning: result decl locus sample
    7 |   { return _M_finish != 0; }
      |   ^
      |   the return type
return-narrow-2.cc: In instantiation of ‘long unsigned int __mini_vector< <template-parameter-1-1> >::_M_space_left() [with <template-parameter-1-1> = std::pair<long int, long int>]’:
return-narrow-2.cc:9:17:   required from here
return-narrow-2.cc:6:3: warning: result decl locus sample
    6 |   _M_space_left()
      |   ^~~~~~~~~~~~~
      |   the return type
return-narrow-2.cc: In instantiation of ‘long unsigned int __mini_vector< <template-parameter-1-1> >::_M_space_left() [with <template-parameter-1-1> = int]’:
return-narrow-2.cc:10:17:   required from here
return-narrow-2.cc:6:3: warning: result decl locus sample
    6 |   _M_space_left()
      |   ^~~~~~~~~~~~~
      |   the return type
---8<---

I'd like to have the result_decl of _M_space_left point to the 
type unsigned long on line 5, which i think would be the correct type
of that memberfn:
 1	namespace std { template < typename, typename > struct pair; }
 2	template < typename > struct __mini_vector
 3	{
 4	  int _M_finish;
 5	  unsigned long
 6	  _M_space_left()
 7	  { return _M_finish != 0; }
 8	};
 9	 template class __mini_vector< std::pair< long, long > >;
10	 template class __mini_vector< int >;

help? :) TIA!

¹) C functions work out fine once fixed to have the correct loc.
   C++ functions worked fine once, but i now see that apparently
   "int my_main" on line 42 regressed again in the meantime:


$ XXX=1 ../gcc/xg++ -B../gcc -c -o return-narrow.o return-narrow.cc -fdiagnostics-color=always 
return-narrow.cc: In function ‘int xyz(int, int, int)’:
return-narrow.cc:1:1: warning: result decl locus sample
    1 | int xyz (int param1, int param2, int param3)
      | ^~~
      | the return type
return-narrow.cc: In function ‘int abc(int, int, int)’:
return-narrow.cc:11:1: warning: result decl locus sample
   11 | int abc (int param1, int param2, int param3)
      | ^~~
      | the return type
return-narrow.cc: In function ‘const void* const* pointer_thingie(void*)’:
return-narrow.cc:21:7: warning: result decl locus sample
   21 | const void *const * pointer_thingie (void *i)
      |       ^~~~
      |       the return type
return-narrow.cc: In function ‘int comparer(int, int, int)’:
return-narrow.cc:25:1: warning: result decl locus sample
   25 | int comparer (int param1, int param2, int param3)
      | ^~~
      | the return type
return-narrow.cc: In function ‘int main()’:
return-narrow.cc:35:1: warning: result decl locus sample
   35 | int main (void) // dg-bogus "Function .main. could return .bool."
      | ^~~
      | the return type
return-narrow.cc: In function ‘int my_main(int, char**)’:
return-narrow.cc:42:25: warning: result decl locus sample
   42 | int my_main (int, char**) { return 0; }
      |                         ^
      |                         the return type
return-narrow.cc: In function ‘int hidden_main(int, char**)’:
return-narrow.cc:43:1: warning: result decl locus sample
   43 | int hidden_main (int, char**) { return 1; }
      | ^~~
      | the return type
return-narrow.cc: In function ‘int i64c(int)’:
return-narrow.cc:46:1: warning: result decl locus sample
   46 | int i64c(int i)
      | ^~~
      | the return type

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