[llvm-bugs] [Bug 50904] New: Clang++ fails to instantiate std::optional if type has a enum class field

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50904

            Bug ID: 50904
           Summary: Clang++ fails to instantiate std::optional if type has
                    a enum class field
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: florin at signbit.net
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Using clang trunk:

   Debian clang version 13.0.0-++20210619101720+e1adf90826a5-1~exp1
   Target: x86_64-pc-linux-gnu
   Thread model: posix
   InstalledDir: /usr/bin
   Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
   Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
   Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
   Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
   Candidate multilib: .;@m64
   Selected multilib: .;@m64

The following program fails to compile:

#include <optional>

class Bar
{
public:
   struct Foo
   {
#if WANT_TO_SEE_IT_FAIL
      enum class Toast
      {
         Alpha,
         Beta,
         Gamma
      };

      Toast aToast = Toast::Beta;
#endif

      int someInt;
   };

   std::optional<Foo> theFoo;
};

int main()
{
   Bar aBar;

   aBar.theFoo = std::make_optional<Bar::Foo>();

   return 0;
}

$ clang++-13 -std=c++17 -DWANT_TO_SEE_IT_FAIL test_std_optional_fail.cpp      
(main)clang_limitations
test_std_optional_fail.cpp:29:18: error: no matching function for call to
'make_optional'
   aBar.theFoo = std::make_optional<Bar::Foo>();
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/optional:1269:5:
note: candidate template ignored: requirement 'is_constructible_v<Bar::Foo>'
was not satisfied [with _Tp = Bar::Foo, _Args = <>]
    make_optional(_Args&&... __args)
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/optional:1261:5:
note: candidate function template not viable: requires single argument '__t',
but no arguments were provided
    make_optional(_Tp&& __t)
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/optional:1277:5:
note: candidate function template not viable: requires at least argument
'__il', but no arguments were provided
    make_optional(initializer_list<_Up> __il, _Args&&... __args)
    ^
1 error generated.

If we don't define WANT_TO_SEE_IT_FAIL then the program compiles.

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