Clang error when using a constexpr variable as NTTP default value in lambda expression.

[ldalessa]

void foo()
{
    constexpr int x{};
    auto a = []<int=x>{};
}

Looks like a regression from 16 to 17, persists in 18 and current trunk.

https://godbolt.org/z/TErqq79ao

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Luke D'Alessandro (ldalessa)

```c++ void foo() { constexpr int x{}; auto a = []<int=x>{}; } ```

Looks like a regression from 16 to 17, persists in 18 and current trunk.

https://godbolt.org/z/TErqq79ao

[zyn0217]

Now only EDG accepts it. https://godbolt.org/z/h58Ko1rce

[zyn0217]

Probably, this has been turned into ill-formed according to P2036R3:

Otherwise (if P either precedes E’s function parameter scope or is in E’s parameter-declaration-clause), the program is ill-formed.

And clues from P2579R0 could also prove this point:

Template Parameters

  int x;
  [x]<auto N = x> {}

This is made ill-formed by P2036 and the proposed mitigation. [...]

But what confuses me is that we don't appear to diagnose such lambdas containing explicit captures; instead, we just give errors on lambda with no captures. (Which is OP's example)

@cor3ntin Could you confirm?

[ldalessa]

I want to mention that the real use case I have is in a decltype() for the lambda. I’m not sure if this changes any of the standardese associated with it. I can post a more sophisticated use when I get back to my computer.