[Bug c++/96917] New: decltype(auto) variable initialized with lambda by-reference capture has incorrect type

[andrey.vihrov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96917

            Bug ID: 96917
           Summary: decltype(auto) variable initialized with lambda
                    by-reference capture has incorrect type
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrey.vihrov at gmail dot com
  Target Milestone: ---

Consider

    #include <type_traits>

    int main()
    {
        int x = 5;

        [&]() {
            decltype(auto) b = x;
            static_assert(std::is_same<decltype(b), int>::value, "types should
match");
        };
    }

When compiled simply with "g++ x.cpp", this gives

x.cpp: In lambda function:
x.cpp:9:59: error: static assertion failed: types should match
    9 |             static_assert(std::is_same<decltype(b), int>::value, "types
should match");
      |                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Note that replacing decltype(auto) with decltype(x) fixes the error. Clang
accepts both assertions.

Although it is not specified explicitly, from
http://eel.is/c++draft/expr.prim.lambda.capture it may seem that the variable
captured by reference has the original type and not the reference type inside
the lambda. Please correct me if this is wrong.

gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
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 10.2.0 (GCC)