[Bug tree-optimization/105423] New: Bogus -Werror=maybe-uninitialized with definitely initialized variable

["byteslice at airmail dot cc" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105423

            Bug ID: 105423
           Summary: Bogus -Werror=maybe-uninitialized with definitely
                    initialized variable
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: byteslice at airmail dot cc
  Target Milestone: ---

Created attachment 52896
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52896&action=edit
Cvised example

On gcc 12.0.1 20220413 (Fedora 36 Beta), with c++ -O1
-Werror=maybe-uninitialized, the attachment fails to compile, with the
following message:

In member function 'void std::Trans_NS___cxx11_basic_string<_CharT,
<template-parameter-1-2>, <template-parameter-1-3> >::_S_copy(_CharT*) [with
_CharT = char; _Traits = std::char_traits; <template-parameter-1-3> = char]',
    inlined from 'constexpr std::Trans_NS___cxx11_basic_string<_CharT, _Traits,
_Alloc>& std::Trans_NS___cxx11_basic_string<_CharT, <template-parameter-1-2>,
<template-parameter-1-3> >::_M_replace(const _CharT*, long int) [with _CharT =
char; _Traits = std::char_traits; <template-parameter-1-3> = char]' at
<source>:49:12,
    inlined from 'void std::Trans_NS___cxx11_basic_string<_CharT,
<template-parameter-1-2>, <template-parameter-1-3> >::operator=(const _CharT*)
[with _CharT = char; _Traits = std::char_traits; <template-parameter-1-3> =
char]' at <source>:40:15,
    inlined from 'void Shuffle()' at <source>:55:8:
<source>:36:46: error: '((char*)((char*)&mask +
offsetof(std::Trans_NS___cxx11_basic_string<char, std::char_traits,
char>,std::Trans_NS___cxx11_basic_string<char, std::char_traits,
char>::<unnamed>)))[2]' may be used uninitialized [-Werror=maybe-uninitialized]
   36 |   void _S_copy(_CharT *__s) { _Traits::assign(_S_copy___d, *__s); }
      |                               ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
<source>: In function 'void Shuffle()':
<source>:54:44: note: 'mask' declared here
   54 |   std::Trans_NS___cxx11_basic_string<char> mask{};
      |                                            ^~~~
In member function 'void std::Trans_NS___cxx11_basic_string<_CharT,
<template-parameter-1-2>, <template-parameter-1-3> >::_S_copy(_CharT*) [with
_CharT = char; _Traits = std::char_traits; <template-parameter-1-3> = char]',
    inlined from 'constexpr std::Trans_NS___cxx11_basic_string<_CharT, _Traits,
_Alloc>& std::Trans_NS___cxx11_basic_string<_CharT, <template-parameter-1-2>,
<template-parameter-1-3> >::_M_replace(const _CharT*, long int) [with _CharT =
char; _Traits = std::char_traits; <template-parameter-1-3> = char]' at
<source>:49:12,
    inlined from 'void std::Trans_NS___cxx11_basic_string<_CharT,
<template-parameter-1-2>, <template-parameter-1-3> >::operator=(const _CharT*)
[with _CharT = char; _Traits = std::char_traits; <template-parameter-1-3> =
char]' at <source>:40:15,
    inlined from 'void Shuffle()' at <source>:55:8:
<source>:36:47: error:
'mask.std::Trans_NS___cxx11_basic_string<char>::_S_copy___d' may be used
uninitialized [-Werror=maybe-uninitialized]
   36 |   void _S_copy(_CharT *__s) { _Traits::assign(_S_copy___d, *__s); }
      |                                               ^~~~~~~~~~~
<source>: In function 'void Shuffle()':
<source>:54:44: note: 'mask' declared here
   54 |   std::Trans_NS___cxx11_basic_string<char> mask{};
      |                                            ^~~~
cc1plus: some warnings being treated as errors
Compiler returned: 1

This warning is bogus because the variable mask is default brace-initialized
and thus its member _S_copy___d is also initialized. The bogus warning does not
appear in older versions of GCC.

Adding -fno-inline to options allows compilation to succeed.