[Bug c++/87327] New: Calling member functions on captured constexpr variables "is not a constant expression"

[jbassett271 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87327

            Bug ID: 87327
           Summary: Calling member functions on captured constexpr
                    variables "is not a constant expression"
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jbassett271 at gmail dot com
  Target Milestone: ---

The following code fails to compile on GCC 8.1.0 and above, but compiles fine
on GCC 7.3 and Clang (https://godbolt.org/z/ACYBBt). This bug doesn't happen if
`size` is a member variable instead of a member function, and it doesn't happen
when using an `int` for `a` and just writing `Foo<a>` instead of
`Foo<a.size()>`.

Command: g++ -std=c++17 bug.cpp


template <int N>
struct Foo {
    constexpr auto size() const {
        return N;
    }
};

constexpr void foo() {
    constexpr auto a = Foo<5>{};

    [&] {
        Foo<a.size()> it = {};

        return it;
    }();
}


Error message:

bug.cpp: In lambda function:
bug.cpp:12:21: error: ‘__closure’ is not a constant expression
         Foo<a.size()> it = {};
                     ^
bug.cpp:12:19: note: in template argument for type ‘int’
         Foo<a.size()> it = {};
             ~~~~~~^~


System info (g++ -v):

Using built-in specs.
COLLECT_GCC=g++-8
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
8.1.0-5ubuntu1~16.04' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-8
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--with-target-system-zlib --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 8.1.0 (Ubuntu 8.1.0-5ubuntu1~16.04)