[clang++] Can not capture member of structured binding to lambda in constant evaluation.

[MiroPalmu]

Clang rejects following code while gcc compiles it.

https://godbolt.org/z/E3TcTq1hx

struct bar {
    int x;
};

consteval int foo() {
    const auto [a] = bar{};

    return [=] {
        return a;
    }();
}

int main() {
    return foo();
}

Error message from clang:

<source>:14:12: error: call to consteval function 'foo' is not a constant expression
   14 |     return foo();
      |            ^
<source>:9:16: note: read of non-constexpr variable '[a]' is not allowed in a constant expression
    9 |         return a;
      |                ^
<source>:8:12: note: in call to '[=] {
    return a;
}.operator()()'
    8 |     return [=] {
      |            ^~~~~
    9 |         return a;
      |         ~~~~~~~~~
   10 |     }();
      |     ~~~
<source>:14:12: note: in call to 'foo()'
   14 |     return foo();
      |            ^~~~~
<source>:6:16: note: declared here
    6 |     const auto [a] = bar{};
      |                ^
1 error generated.

[shafik]

I believe this should be valid, we accept this: https://godbolt.org/z/4Pv5so3qs

consteval int foo() {
    int  x = 1;
    return [=] {
        return x;
    }();
}

int main() {
    return foo();
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Miro Palmu (MiroPalmu)

Clang rejects following code while gcc compiles it.

https://godbolt.org/z/E3TcTq1hx

struct bar {
    int x;
};

consteval int foo() {
    const auto [a] = bar{};

    return [=] {
        return a;
    }();
}

int main() {
    return foo();
}

Error message from clang:

<source>:14:12: error: call to consteval function 'foo' is not a constant expression
   14 |     return foo();
      |            ^
<source>:9:16: note: read of non-constexpr variable '[a]' is not allowed in a constant expression
    9 |         return a;
      |                ^
<source>:8:12: note: in call to '[=] {
    return a;
}.operator()()'
    8 |     return [=] {
      |            ^~~~~
    9 |         return a;
      |         ~~~~~~~~~
   10 |     }();
      |     ~~~
<source>:14:12: note: in call to 'foo()'
   14 |     return foo();
      |            ^~~~~
<source>:6:16: note: declared here
    6 |     const auto [a] = bar{};
      |                ^
1 error generated.

[zyn0217]

Is this https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2686r5.pdf? @cor3ntin

[frederick-vs-ja]

Is this https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2686r5.pdf? @cor3ntin

I don't think so. This seems to be a bug only in the implicit capture mechanisms.

If one of the following (IMO, conceptually equivalent) variants is used, Clang would accept the program.

consteval int foo() {
    const auto [a] = bar{};

    return [a = a] { // note that this also reads the structured binding a
        return a;
    }();
}

consteval int foo() {
    const auto e = bar{};
    auto&& a = e.x;

    return [=] {
        return a;
    }();
}