capture local constexpr variable

[ordinary-github-user]

int main()
{
    constexpr int a{};
    []{return a;}();//compile yes
    []{a;}();//compile no
}

Diagnostic:

<source>:5:8: error: variable 'a' cannot be implicitly captured in a lambda with no capture-default specified
    5 |     []{a;}();//compile no
      |        ^
<source>:3:19: note: 'a' declared here
    3 |     constexpr int a{};
      |                   ^
<source>:5:5: note: lambda expression begins here
    5 |     []{a;}();//compile no
      |     ^
<source>:5:6: note: capture 'a' by value
    5 |     []{a;}();//compile no
      |      ^
      |      a
<source>:5:6: note: capture 'a' by reference
    5 |     []{a;}();//compile no
      |      ^
      |      &a
<source>:5:6: note: default capture by value
    5 |     []{a;}();//compile no
      |      ^
      |      =
<source>:5:6: note: default capture by reference
    5 |     []{a;}();//compile no
      |      ^
      |      &
<source>:5:8: warning: expression result unused [-Wunused-value]
    5 |     []{a;}();//compile no
      |        ^
1 warning and 1 error generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

I believe this is a clang bug, a is implicitly captured and not odr-used as far as I can tell from basic.def.odr p5 because it is a discarded-value expression.

CC @cor3ntin

[shafik]

May be related to: #55617

[shafik]

This looks like a duplicate of: #34603

The second test case from David is basically identical.

[shafik]

Also see: #26001