Right now, the clang plugin complains when a class is "complex" and its constructor is defined in the header. However, if the constructor is a constexpr constructor, it needs to be defined in the header.

The only way around the chromium-style error seems to be to further declare the constructor as an "inline" constructor, and define it outside the class in the header, see [1] for an example.

I believe a more sensible fix for this would be to exempt constexpr constructors from the clang plugin rule.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1169179/22/base/task/task_traits_extension.h