[clang] "incomplete type 'const A' is not a literal type" for a static constexpr data member

[mmatrosov]

Not sure if this is a bug or not, bug gcc compiles this code just fine:

template<typename Base>
struct Mixin {
  static constexpr Base base{};
};

struct A {
    Mixin<A> m;
};

while clang argues:

<source>:3:25: error: constexpr variable cannot have non-literal type 'const A'
    3 |   static constexpr Base base{};
      |                         ^
<source>:7:14: note: in instantiation of template class 'Mixin<A>' requested here
    7 |     Mixin<A> m;
      |              ^
<source>:3:25: note: incomplete type 'const A' is not a literal type
    3 |   static constexpr Base base{};
      |                         ^
<source>:6:8: note: definition of 'A' is not complete until the closing '}'
    6 | struct A {
      |        ^

https://godbolt.org/z/YbvW4Gq3n

[keinflue]

A corresponding GCC bug report seems to be https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107945.

As argued there in the comments by Brian Bi, and I think the same applies here, whether or not Clang's behavior is correct depends on whether or not the instantiation of the declaration of a static data member, which happens implicitly with the instantiation of Mixin<A> before A is completed, requires its type to be complete, or whether that only applies to the instantiation of the definition of the static data member, which is delayed until it is needed.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mikhail Matrosov (mmatrosov)

Not sure if this is a bug or not, bug gcc compiles this code just fine:

template<typename Base>
struct Mixin {
  static constexpr Base base{};
};

struct A {
    Mixin<A> m;
};

while clang argues:

<source>:3:25: error: constexpr variable cannot have non-literal type 'const A'
    3 |   static constexpr Base base{};
      |                         ^
<source>:7:14: note: in instantiation of template class 'Mixin<A>' requested here
    7 |     Mixin<A> m;
      |              ^
<source>:3:25: note: incomplete type 'const A' is not a literal type
    3 |   static constexpr Base base{};
      |                         ^
<source>:6:8: note: definition of 'A' is not complete until the closing '}'
    6 | struct A {
      |        ^

https://godbolt.org/z/YbvW4Gq3n

[mmatrosov]

Here is a link from the GCC bug report with a pretty convincing explanation why this code indeed should not compile, meaning that clang is right and gcc is wrong: https://stackoverflow.com/a/74642460/261217