[Clang][Windows] Microsoft mangling cannot handle lambda in decltype in return type

[MagentaTreehouse]

Given this code:

template <class>
struct B {};

struct A {
    template <class T>
    auto f(T) -> B<decltype([] (T) {})> {
        return {};
    }
};

int main() {
    A{}.f(0);
}

Clang on Windows error:

<source>(6,5): error: cannot mangle this template specialization type yet
    6 |     auto f(T) -> B<decltype([] (T) {})> {
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7 |         return {};
      |         ~~~~~~~~~~
    8 |     }
      |     ~

Similar error happens on simpler code:

template <class T> // must be a function template
auto f(T) -> decltype([] {}) {
    return {};
}
int main() { f(0)(); }

On other platforms with Itanium mangling, or if using MSVC, it can be compiled.

See https://compiler-explorer.com/z/vPz6E5ve7. (Compiler explorer seems to only have version 18 for clang-cl)

I can reproduce this using the main branch.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (MagentaTreehouse)

Given this code: ```c++ template <class> struct B {};

struct A {
template <class T>
auto f(T) -> B<decltype([] (T) {})> {
return {};
}
};

int main() {
A{}.f(0);
}

Clang on Windows error:
```console
<source>(6,5): error: cannot mangle this template specialization type yet
    6 |     auto f(T) -> B<decltype([] (T) {})> {
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7 |         return {};
      |         ~~~~~~~~~~
    8 |     }
      |     ~

Similar error happens on simpler code:

template <class T> // must be a function template
auto f(T) -> decltype([] {}) {
    return {};
}
int main() { f(0)(); }

On other platforms with Itanium mangling, or if using MSVC, it can be compiled.

See https://compiler-explorer.com/z/vPz6E5ve7. (Compiler explorer seems to only have version 18 for clang-cl)

I can reproduce this using the main branch.

[shafik]

CC @AaronBallman

[zyn0217]

@cor3ntin another case of unevaluated lambdas in MS ABI