[clang] [clang][C23] N3006 Underspecified object declarations (PR #79845)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! I think there's something here worth considering: how an implementation handles use of underspecified declarations is left implementation-defined. I think we want to diagnose many of the situations it covers, but I think we already have most of those diagnostics covered. 

The one thing that's not covered is declaring or defining new identifiers as part of an underspecified declaration. However, we might want to accept that as an extension rather than follow GCC's footsteps. C users are used to the fact that you can define a type basically anywhere you can spell a type name. Is it confusing? Absolutely. But I think it's just as confusing to allow users to define a type in *some* places but not others, along fairly arbitrary lines. In other words, it's weird to me that we would allow:
```
void func() {
  int x = (struct Foo { int x; }){ 0 }.x;
}
```
but would reject:
```
void func() {
  constexpr int x = (struct Foo { int x; }){ 0 }.x;
}
```
So support for this paper might entail adding documentation for what our implementation-defined behavior is when we accept code + test coverage, rather than rejecting code because GCC rejects it.

CC @zygoloid @jyknight @jrtc27 @nickdesaulniers for some more opinions on this.

https://github.com/llvm/llvm-project/pull/79845