[clang-tidy] `misc-const-correctness` false positive when calling a function with default lambda argument

[BobIsOnFire]

Hi! Here's minimal repro:

class A {};
void poison(void(*f)() = []{}) {}
void foo() {
    A a;
    a = {};
    poison();
}

I get following warning (godbolt: https://godbolt.org/z/cedY7o5ev):

<source>:4:5: warning: variable 'a' of type 'A' can be declared 'const' [misc-const-correctness]
    4 |     A a;
      |     ^
      |       const 
1 warning generated.

Even though I assign to a on the next line. Something is wrong with calling a function with a function pointer type (or std::function) that defaults to a lambda, it completely messes up the check -- note that it does not even use a in any way. Following changes would remove the false-positive:

• Removing the argument from poison function: https://godbolt.org/z/qvcz4jqE1
• Replacing lambda with nullptr: https://godbolt.org/z/cTTW84x15
• Explicitly passing a lambda to the call: https://godbolt.org/z/YEaWjY15r

Thanks in advance! Let me know if any other information is needed.

[pkaff]

The compiler used in the godbolt link, clang 18.1, does not seem to give me the warning currently. Is this still reproducible in godbolt? As mentioned in #127494 I specifically encountered this on clang for MacOS.

[BobIsOnFire]

@pkaff still reproduces locally for me on clang 19.1.7. It doesn't reproduce on godbolt anymore because godbolt always uses trunk version of clang-tidy, regardless of compiler selected.

I've bisected history between 19.1.7 and trunk, and found that my issue was fixed in #117474 (which will be included in version 20). Cherry-picked changes locally on top of 19.1.7, and the warning is now gone.