Re: [PATCH 2/5] c++: Set the locus of the function result decl

[Jason Merrill <jason@redhat.com>]

On 11/18/22 05:49, Bernhard Reutner-Fischer wrote:
> On Thu, 17 Nov 2022 18:52:36 -0500
> Jason Merrill <jason@redhat.com> wrote:
> 
>> On 11/17/22 14:02, Bernhard Reutner-Fischer wrote:
>>> On Thu, 17 Nov 2022 09:53:32 -0500
>>> Jason Merrill <jason@redhat.com> wrote:
> 
>>>> Instead, you want to copy the location for instantiations, i.e. check
>>>> DECL_TEMPLATE_INSTANTIATION instead of !DECL_USE_TEMPLATE.
>>>
>>> No, that makes no difference.
>>
>> Hmm, when I stop there when processing the instantiation the template's
>> DECL_RESULT has the right location information, e.g. for
>>
>> template <class T> int f() { return 42; }
>>
>> int main()
>> {
>>     f<int>();
>> }
>>
>> #1  0x0000000000f950e8 in instantiate_body (pattern=<template_decl
>> 0x7ffff7ff5080 f>, args=<tree_vec 0x7fffe9712ae0>, d=<function_decl
>> 0x7fffe971e600 f>, nested_p=false) at /home/jason/gt/gcc/cp/pt.cc:26470
>> #0  start_preparsed_function (decl1=<function_decl 0x7fffe971e600 f>,
>> attrs=<tree 0x0>, flags=1) at /home/jason/gt/gcc/cp/decl.cc:17252
>> (gdb) p expand_location (input_location)
>> $13 = {file = 0x4962370 "wa.C", line = 1, column = 24, data = 0x0, sysp
>> = false}
>> (gdb) p expand_location (DECL_SOURCE_LOCATION (DECL_RESULT
>> (DECL_TEMPLATE_RESULT (DECL_TI_TEMPLATE (decl1)))))
>> $14 = {file = 0x4962370 "wa.C", line = 1, column = 20, data = 0x0, sysp
>> = false}
> 
> Yes, that works. Sorry if i was not clear: The thing in the cover
> letter in this series does not work, the mini_vector reduced testcase
> from the libstdc++-v3/include/ext/bitmap_allocator.h.
> class template member function return type location, would that be it?
> 
> AFAIR the problem was that that these member functions get their result
> decl late. When they get them, there are no
> declspecs->locations[ds_type_spec] around anywhere to tuck that on the
> resdecl. While the result decl is clear, there is no obvious way where
> to store the ds_type_spec (somewhere in the template, as you told me).
> 
> Back then I tried moving the resdecl building from
> start_preparsed_function to grokfndecl but that did not work out easily
> IIRC and i ultimately gave up to move stuff around rather blindly.
> I also tried to find a spot where i could store the ds_type_spec locus
> somewhere in grokmethod, but i think the problem was the same, i had
> just the type where i cannot store a locus and did not find a place
> where i could smuggle the locus along.

Ah, so the problem is deferred parsing of methods, rather than 
templates.  Building the DECL_RESULT sooner does seem like the right 
approach to handling that, whether that's in grokfndecl or grokmethod.

> So, to make that clear. Your template function (?) works:
> 
> $ XXX=1 ./xg++ -B. -S -o /dev/null ../tmp4/return-narrow-2j.cc
> ../tmp4/return-narrow-2j.cc: In function ‘int f()’:
> ../tmp4/return-narrow-2j.cc:1:20: warning: result decl locus sample
>      1 | template <class T> int f() { return 42; }
>        |                    ^~~
>        |                    the return type
> ../tmp4/return-narrow-2j.cc: In function ‘int main()’:
> ../tmp4/return-narrow-2j.cc:3:1: warning: result decl locus sample
>      3 | int main()
>        | ^~~
>        | the return type
> ../tmp4/return-narrow-2j.cc: In instantiation of ‘int f() [with T = int]’:
> ../tmp4/return-narrow-2j.cc:5:10:   required from here
> ../tmp4/return-narrow-2j.cc:1:20: warning: result decl locus sample
>      1 | template <class T> int f() { return 42; }
>        |                    ^~~
>        |                    the return type
> 
> 
> The class member fn not so much (IMHO, see attached):
> 
> $ XXX=1 ./xg++ -B. -S -o /dev/null ../tmp4/return-narrow-2.cc
> ../tmp4/return-narrow-2.cc: In member function ‘const long unsigned int __mini_vector< <template-parameter-1-1> >::_M_space_left()’:
> ../tmp4/return-narrow-2.cc:9:3: warning: result decl locus sample
>      9 |   { return _M_finish != 0; }
>        |   ^
>        |   the return type
> ../tmp4/return-narrow-2.cc: In instantiation of ‘const long unsigned int __mini_vector< <template-parameter-1-1> >::_M_space_left() [with <template-parameter-1-1> = std::pair<long int, long int>]’:
> ../tmp4/return-narrow-2.cc:11:17:   required from here
> ../tmp4/return-narrow-2.cc:9:3: warning: result decl locus sample
>      9 |   { return _M_finish != 0; }
>        |   ^
>        |   the return type
> ../tmp4/return-narrow-2.cc: In instantiation of ‘const long unsigned int __mini_vector< <template-parameter-1-1> >::_M_space_left() [with <template-parameter-1-1> = int]’:
> ../tmp4/return-narrow-2.cc:12:17:   required from here
> ../tmp4/return-narrow-2.cc:9:3: warning: result decl locus sample
>      9 |   { return _M_finish != 0; }
>        |   ^
>        |   the return type
> 
> 
>>
>>> But really I'm not interested in the template case, i only mentioned
>>> them because they don't work and in case somebody wanted to have correct
>>> locations.
>>> I remember just frustration when i looked at those a year ago.
>>
>> I'd like to get the template case right while we're looking at it.  I
>> guess I can add that myself if you're done trying.
>>
>>> Is the hunk for normal functions OK for trunk?
>>
>> You also need a testcase for the desired behavior, with e.g.
>> { dg-error "23:" }
> 
> I'd have to think about how to test that with trunk, yes.
> There are no existing warnings that want to point to the return type,
> are there?

Good point.  Do any of your later patches add such a warning?

> Maybe a g++.dg/plugin/result_decl_plugin.c then.
> 
> set plugin_test_list [list
> hmz. That strikes me as not all that flexible.
> We could glob *_plugin.[cC][c]*, and have foo_plugin.lst contain it's
> files. Whatever.
> 
> thanks,