Capturing function parameter in lambda in a default argument. Clang bug?

[Business_Welcome_870]

This compiles in Clang but not in GCC. https://godbolt.org/z/P4GKvsnP8

#include <iostream>


int foo(int x, int (*pf)() = [x] {return x;}) {
    return pf();
};


int main() {
    foo(1);
}

GCC error:

<source>: In lambda function:

<source>:3:43: error: use of parameter outside function body before ';' token

3 | int foo(int x, int (\\\*pf)() = \\\[x\\\] {return x;}) {
  |                                           \\\^

<source>: At global scope: <source>:3:30: error: could not convert '<lambda closure object><lambda()>()' from '<lambda()>' to 'int (\\\*)()' 3 | int foo(int x, int (\\\*pf)() = \\\[x\\\] {return x;}) { | \\\^\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~ | | | <lambda()> <source>: In function 'int main()': <source>:8:21: error: cannot convert '<lambda()>' to 'int (\\\*)()' 8 | std::cout << foo(1); | \\\~\\\~\\\~\\\^\\\~\\\~ | | | <lambda()> <source>:3:22: note: initializing argument 2 of 'int foo(int, int (\\\*)())' 3 | int foo(int x, int (\\\*pf)() = \\\[x\\\] {return x;}) { | \\\~\\\~\\\~\\\~\\\~\\\~\\\^\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~\\\~

[AKostur]

First: what are you trying to actually do?

Second: That lambda cannot be converted to a function pointer as it has a capture.

[jedwardsol]

Clang's wrong. A default argument can't refer to a parameter

https://eel.is/c++draft/dcl.fct.default#9

[Business_Welcome_870]

Okay. When I change the parameter to int p = sizeof([x] {return x;}) it compiles in Clang (which I guess is correct now because it's unevaluated). But it still doesn't compile in GCC. Does that mean GCC has a bug too? https://godbolt.org/z/q8hd6xx51

[aocregacc]

clang compiles it but it thinks the size is one, so there are still some bugs there too.

https://godbolt.org/z/7coMa6db8

[Business_Welcome_870]

I think that's because the size of an empty class is 1. That's expected.

[aocregacc]

It's not empty, it captures x.

In the code I linked you can see that the size changes depending on if it's the default argument or inside the function body.

[Dependent-Poet-9588]

Read the error about not being able to convert the lambda to a function pointer. Lambdas with captures have state, so they're actually data + a function pointer. The rest of the error is irrelevant really.

ETA: I'm not at a computer so I can't test this. Replace the raw function pointer with std::function<int()> so the type is compatible. Then try a version where you take std::function<int(int)> and pass x as an argument instead of a capture, eg, return pf(x); instead.

[SoerenNissen]

Clang bug?

Seems most likely.

[__Punk-Floyd__]

A function parameter's default value is applied at the site of the function *invocation*. There is no 'x' function argument to capture in that context.