[Bug c++/101831] New: Spurious maybe-uninitialized warning on std::array::size

[m101010a at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

            Bug ID: 101831
           Summary: Spurious maybe-uninitialized warning on
                    std::array::size
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m101010a at gmail dot com
  Target Milestone: ---

When calling std::array::size on an uninitialized array, gcc emits a warning:

$ cat x.cpp
#include <array>
int f() {
        std::array<char, 1> a;
        return a.size();
}
$ g++ -v -S -o /dev/null -Wmaybe-uninitialized x.cpp
Using built-in specs.
COLLECT_GCC=g++
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.1.0 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-S' '-o' '/dev/null' '-Wmaybe-uninitialized'
'-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' '/dev/'
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/cc1plus -quiet -v -D_GNU_SOURCE x.cpp
-quiet -dumpdir /dev/ -dumpbase x.cpp -dumpbase-ext .cpp -mtune=generic
-march=x86-64 -Wmaybe-uninitialized -version -o /dev/null
GNU C++17 (GCC) version 11.1.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 11.1.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

warning: MPFR header version 4.1.0 differs from library version 4.1.0-p13.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0

/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0/x86_64-pc-linux-gnu

/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0/backward
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/include
 /usr/local/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/include-fixed
 /usr/include
End of search list.
GNU C++17 (GCC) version 11.1.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 11.1.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

warning: MPFR header version 4.1.0 differs from library version 4.1.0-p13.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 45eb9a71cc15bfdc579557bea4b77e51
x.cpp: In function ‘int f()’:
x.cpp:4:22: warning: ‘a’ may be used uninitialized [-Wmaybe-uninitialized]
    4 |         return a.size();
      |                ~~~~~~^~
In file included from x.cpp:1:
/usr/include/c++/11.1.0/array:176:7: note: by argument 1 of type ‘const
std::array<char, 1>*’ to ‘constexpr std::array<_Tp, _Nm>::size_type
std::array<_Tp, _Nm>::size() const [with _Tp = char; long unsigned int _Nm =
1]’ declared here
  176 |       size() const noexcept { return _Nm; }
      |       ^~~~
x.cpp:3:29: note: ‘a’ declared here
    3 |         std::array<char, 1> a;
      |                             ^
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-S' '-o' '/dev/null' '-Wmaybe-uninitialized'
'-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' '/dev/'

It also happens on the following reduced test case:

struct array {
  int d;
  int e() const { return 1; }
};
int f() {
  array a;
  return a.e();
}

This does not happen when optimization is enabled, and does not happen if
"-fsyntax-only" is used instead of "-S -o /dev/null".  This also happens with
GCC 11.2, but does not happen with GCC 10.