`bugprone-exception-escape` for `std::string` member in default copy constructor invocation reported with libc++

[firewave]

#include <string>

struct CallBase {
    std::string callId;
};

void f() noexcept
{
    CallBase cb;
    CallBase cb1 = cb;
}

<source>:7:6: warning: an exception may be thrown in function 'f' which should not throw exceptions [bugprone-exception-escape]
    7 | void f() noexcept
      |      ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/stdexcept:243:3: note: frame #0: unhandled exception of type 'std::length_error' may be thrown in function '__throw_length_error' here
  243 |   throw length_error(__msg);
      |   ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/string:2477:5: note: frame #1: function '__throw_length_error' calls function '__throw_length_error' here
 2477 |     std::__throw_length_error("basic_string");
      |     ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/string:2583:7: note: frame #2: function '__init_copy_ctor_external' calls function '__throw_length_error' here
 2583 |       this->__throw_length_error();
      |       ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/string:997:7: note: frame #3: function 'basic_string' calls function '__init_copy_ctor_external' here
  997 |       __init_copy_ctor_external(std::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
      |       ^
<source>:3:8: note: frame #4: function 'CallBase' calls function 'basic_string' here
    3 | struct CallBase {
      |        ^
<source>:10:20: note: frame #5: function 'f' calls function 'CallBase' here
   10 |     CallBase cb1 = cb;
      |                    ^

https://godbolt.org/z/G75sj5fYz

This is only reported with libc++.

Somewhat related to #109587.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Oliver Stöneberg (firewave)

```cpp #include <string>

struct CallBase {
std::string callId;
};

void f() noexcept
{
CallBase cb;
CallBase cb1 = cb;
}

<source>:7:6: warning: an exception may be thrown in function 'f' which should not throw exceptions [bugprone-exception-escape]
7 | void f() noexcept
| ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/stdexcept:243:3: note: frame #0: unhandled exception of type 'std::length_error' may be thrown in function '__throw_length_error' here
243 | throw length_error(__msg);
| ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/string:2477:5: note: frame #1: function '__throw_length_error' calls function '__throw_length_error' here
2477 | std::__throw_length_error("basic_string");
| ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/string:2583:7: note: frame #2: function '__init_copy_ctor_external' calls function '__throw_length_error' here
2583 | this->__throw_length_error();
| ^
/opt/compiler-explorer/clang-trunk-20250804/bin/../include/c++/v1/string:997:7: note: frame #3: function 'basic_string' calls function '__init_copy_ctor_external' here
997 | __init_copy_ctor_external(std::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
| ^
<source>:3:8: note: frame #4: function 'CallBase' calls function 'basic_string' here
3 | struct CallBase {
| ^
<source>:10:20: note: frame #5: function 'f' calls function 'CallBase' here
10 | CallBase cb1 = cb;
| ^

https://godbolt.org/z/G75sj5fYz

This is only reported with libc++.

Somewhat related to #<!-- -->109587.
</details>

[vbvictor]

Is there anything we can do on clang-tidy part?
From std::basic_string cppreference, 11th constructor (copy ctor) is not marked noexcept and clang-tidy finds an exception path so it's a valid finding...?

[firewave]

That is actually fine. But this is more about it only being reported with libc++.