[clang][rejects-valid] not finding partial specialization of nested class template

[ericniebler]

the following valid code does not compile on any version of clang i have tested, including trunk:

template <class I>
struct Outer
{
  template <I X, I Y>
  struct Inner;
};

Outer<int> e2; // <== need to instantiate the outer template here

template <class I> 
template <I Y>
struct Outer<I>::Inner<0, Y> {};

constexpr Outer<int>::Inner<0, 4> e{}; // <== type incomplete here

the error is:

<source>:15:35: error: constexpr variable cannot have non-literal type 'const Outer<int>::Inner<0, 4>'
constexpr Outer<int>::Inner<0, 4> e{};
                                  ^
<source>:15:35: error: implicit instantiation of undefined template 'Outer<int>::Inner<0, 4>'
<source>:6:10: note: template is declared here
  struct Inner;
         ^
2 errors generated.
Compiler returned: 1

demo:
https://godbolt.org/z/8rvKqqWhM

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eric Niebler (ericniebler)

the following valid code does not compile on any version of clang i have tested, including trunk:

template <class I>
struct Outer
{
  template <I X, I Y>
  struct Inner;
};

Outer<int> e2; // <== need to instantiate the outer template here

template <class I> 
template <I Y>
struct Outer<I>::Inner<0, Y> {};

constexpr Outer<int>::Inner<0, 4> e{}; // <== type incomplete here

the error is:

<source>:15:35: error: constexpr variable cannot have non-literal type 'const Outer<int>::Inner<0, 4>'
constexpr Outer<int>::Inner<0, 4> e{};
                                  ^
<source>:15:35: error: implicit instantiation of undefined template 'Outer<int>::Inner<0, 4>'
<source>:6:10: note: template is declared here
  struct Inner;
         ^
2 errors generated.
Compiler returned: 1

demo:
https://godbolt.org/z/8rvKqqWhM

[shafik]

Bote both edg/MSVC reject as well: https://godbolt.org/z/6v9hTaEP3

If we comment out the definition of e2 then only edg rejects: https://godbolt.org/z/KcKfvo7qK

[shafik]

CC @erichkeane

[ericniebler]

i discussed this issue on the wg21 core reflector. it seems that clang's behavior is conforming. closing.