Improve error message for constexpr constructors of virtual base classes

[Eisenwave]

Code to Reproduce (https://godbolt.org/z/6EhPfTTfE)

struct Base {
  constexpr Base() = default;
};
struct Derived : virtual Base {
  constexpr Derived() = default;
};

Clang diagnostic:

<source>:5:3: error: defaulted definition of default constructor is not constexpr
    5 |   constexpr Derived() = default;
      |   ^

GCC diagnostic:

<source>:5:13: error: explicitly defaulted function 'constexpr Derived::Derived()' cannot be declared 'constexpr' because the implicit declaration is not 'constexpr':
    5 |   constexpr Derived() = default;
      |             ^~~~~~~

MSVC diagnostic:

<source>(5): error C2468: constructor cannot be 'constexpr' in a class or struct with virtual base classes
<source>(6): note: see reference to function 'Derived::Derived(void)'

Suggested Resolution

Out of the three, the MSVC diagnostic seems best. It would be nice if clang could output something similar.

[shafik]

Confirmed

[NoumanAmir657]

I would like to work on this issue.
Since, I am new to this can someone give me a brief overview of what to do.
I am assuming I have to find the diagnostics file where this error is generated and change the error string to MSVC one.

[NoumanAmir657]

https://reviews.llvm.org/D158540