Error on use of parameter in lambda outside function body

[Eczbek]

Compiler Explorer link: https://godbolt.org/z/118sGzbbM

void foo(auto x) noexcept(noexcept([&x] { x; })) {}

int main() {
	foo(3);
}

<source>:1:43: error: variable 'x' cannot be implicitly captured in a lambda with no capture-default specified
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                           ^
<source>:1:41: note: while substituting into a lambda expression here
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                         ^
<source>:4:2: note: in instantiation of exception specification for 'foo<int>' requested here
    4 |         foo(3);
      |         ^
<source>:1:15: note: 'x' declared here
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |               ^
<source>:1:36: note: lambda expression begins here
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                    ^
<source>:1:39: note: capture 'x' by value
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                       ^
      |                                       x
<source>:1:39: note: capture 'x' by reference
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                       ^
      |                                       &x
<source>:1:37: note: default capture by value
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                     ^
      |                                     =
<source>:1:37: note: default capture by reference
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                     ^
      |                                     &
<source>:1:43: warning: expression result unused [-Wunused-value]
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                           ^
<source>:4:2: note: in instantiation of exception specification for 'foo<int>' requested here
    4 |         foo(3);
      |         ^
1 warning and 1 error generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Eczbek)

Compiler Explorer link: https://godbolt.org/z/118sGzbbM

void foo(auto x) noexcept(noexcept([&x] { x; })) {}

int main() {
	foo(3);
}

<source>:1:43: error: variable 'x' cannot be implicitly captured in a lambda with no capture-default specified
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                           ^
<source>:1:41: note: while substituting into a lambda expression here
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                         ^
<source>:4:2: note: in instantiation of exception specification for 'foo<int>' requested here
    4 |         foo(3);
      |         ^
<source>:1:15: note: 'x' declared here
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |               ^
<source>:1:36: note: lambda expression begins here
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                    ^
<source>:1:39: note: capture 'x' by value
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                       ^
      |                                       x
<source>:1:39: note: capture 'x' by reference
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                       ^
      |                                       &x
<source>:1:37: note: default capture by value
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                     ^
      |                                     =
<source>:1:37: note: default capture by reference
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                     ^
      |                                     &
<source>:1:43: warning: expression result unused [-Wunused-value]
    1 | void foo(auto x) noexcept(noexcept([&x] { x; })) {}
      |                                           ^
<source>:4:2: note: in instantiation of exception specification for 'foo<int>' requested here
    4 |         foo(3);
      |         ^
1 warning and 1 error generated.
Compiler returned: 1

[zyn0217]

@mizvekov you probably want to test your patch with this one

[mizvekov]

#107942 has no effect yet on this one, but it probably should.
I'll put this on my list to double check later.

[Eczbek]

This might be invalid because lambdas can only have captures in block scope, and the noexcept specifier/requires clause are outside it :(

https://eel.is/c++draft/expr.prim.lambda.capture#3
https://eel.is/c++draft/dcl.fct.def#nt:function-body
https://eel.is/c++draft/basic.scope.block#1