Clang rejects templated consteval lambda within template

[dsacre]

Consider this piece of code:

template <int...>
consteval void foo()
{
  constexpr auto bar = []<int...>() consteval {
    return [](int* p) { *p = 42; };
  };
  (void) bar();
}

int main()
{
  foo();
}

This compiles with Clang 18 and earlier, as well as with GCC. Later versions of Clang give this error:

<source>:5:26: error: variable 'p' cannot be implicitly captured in a lambda with no capture-default specified
    5 |     return [](int* p) { *p = 42; };

https://godbolt.org/z/dzqj9Pq9c

Removing consteval from the lambda gets the code to compile, as does converting either foo() or the lambda into a non-template.

I managed to bisect this down to commit 5e767bd.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Dominic Sacré (dsacre)

Consider this piece of code:

template <int...>
consteval void foo()
{
  constexpr auto bar = []<int...>() consteval {
    return [](int* p) { *p = 42; };
  };
  (void) bar();
}

This compiles with Clang 18 and earlier, as well as with GCC. Later versions of Clang give this error:

<source>:5:26: error: variable 'p' cannot be implicitly captured in a lambda with no capture-default specified
    5 |     return [](int* p) { *p = 42; };

https://godbolt.org/z/dzqj9Pq9c

Removing consteval from the lambda gets the code to compile, as does converting either foo() or the lambda into a non-template.

I managed to bisect this down to commit 5e767bd.

[shafik]

The code does not match the code in your godbolt, I am not sure if you were trying to demonstrate something else in your summary.

Everyone is going to be at the standard meeting this week either in person or remote, so I would not expect an answer soon.

CC @katzdm @cor3ntin

[dsacre]

The code does not match the code in your godbolt, I am not sure if you were trying to demonstrate something else in your summary.

I've now edited the description to include the instantiation of foo(), without which the error is not triggered. It should now be a complete MRE, and match exactly what's linked in Compiler Explorer.