[clang-tidy] False positive bugprone-exception-escape for throwing lambda in noexcept function

[Eisenwave]

https://godbolt.org/z/1sf8Gne35

void awoo() noexcept {
    [] { throw 0; };
}

This outputs

<source>:1:6: warning: an exception may be thrown in function 'awoo' which should not throw exceptions [bugprone-exception-escape]
    1 | void awoo() noexcept {
      |      ^

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Jan Schultke (Eisenwave)

https://godbolt.org/z/1sf8Gne35 ```cpp void awoo() noexcept { [] { throw 0; }; } ``` This outputs ``` <source>:1:6: warning: an exception may be thrown in function 'awoo' which should not throw exceptions [bugprone-exception-escape] 1 | void awoo() noexcept { | ^ ```

[denzor200]

I tried to reproduce the issue in a simplified clang-query request. Does anybody can explain why the simple matcher functionDecl(isNoThrow()) triggers on lambda like []() noexcept(false) { throw 0; }; ? While it doesn't trigger on a regular function with noexcept(false)

https://godbolt.org/z/j3ac791qo

Is it a bug on the AST side?

[vbvictor]

I tried to reproduce the issue in a simplified clang-query request. Does anybody can explain why the simple matcher functionDecl(isNoThrow()) triggers on lambda like []() noexcept(false) { throw 0; }; ? While it doesn't trigger on a regular function with noexcept(false)

In your example, only CXXConversionDecl and CXXDestructorDecl are matched as noexcept functions- https://godbolt.org/z/aefG99dYv. The main operator() with throw expression is not matched.

Your misunderstanding happens because ctor/dtor/operator() of lambda points to one location.

[vbvictor]

Note: code example in issue is only false-positive when lambda is not assigned to a variable.

void awoo() noexcept { // no warning - lambda assigned to a variable and never called - GOOD
    auto l = [] { throw 0; };
}

void awoo() noexcept { // warning - lambda is called - GOOD
    auto l = [] { throw 0; };
    l();
}

void awoo() noexcept { // warning - lambda is not assigned to a variable and never called - BAD
    [] { throw 0; };
}