Incorrect "invalid conversion" error with decltype(<lambda>) in default template arguments

[ilya-biryukov]

The following code fails to compile (see https://godbolt.org/z/zTf49f9oe):

template <typename MakeValue = decltype([] { return 1; })>
class Example {
   public:
    using Result = decltype(MakeValue()());
    explicit Example(MakeValue&& make_value = {})
        : make_value_(make_value) {}

    Result Make() { return make_value_(); }

   private:
     MakeValue make_value_;
};

template <typename CastTo = int>
CastTo CastResultTo() {
    auto example = Example();
    return example.Make(); // error: 'Cannot initialize object parameter'
}

int main() { return CastResultTo(); }

Adding an empty template argument list makes the error go away (see https://godbolt.org/z/dz7eMaM54):

    auto example = Example<>();

So this seems to be related to deduction guides somehow. Could it be that we synthesize a new lambda type for CTAD and happen to mix it with types that we also creates during substitution?

cc @hokein who recently worked on CTAD, in case he has any ideas.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Ilya Biryukov (ilya-biryukov)

The following code fails to compile (see https://godbolt.org/z/zTf49f9oe): ```cpp template <typename MakeValue = decltype([] { return 1; })> class Example { public: using Result = decltype(MakeValue()()); explicit Example(MakeValue&& make_value = {}) : make_value_(make_value) {}

Result Make() { return make_value_(); }

private:
MakeValue make_value_;
};

template <typename CastTo = int>
CastTo CastResultTo() {
auto example = Example();
return example.Make(); // error: 'Cannot initialize object parameter'
}

int main() { return CastResultTo(); }

Adding an empty template argument list makes the error go away (see https://godbolt.org/z/dz7eMaM54):
```cpp
    auto example = Example<>();

So this seems to be related to deduction guides somehow. Could it be that we synthesize a new lambda type for CTAD and happen to mix it with types that we also creates during substitution?

cc @hokein who recently worked on CTAD, in case he has any ideas.