PR44406: Follow behavior of array bound constant folding in more recent versions of GCC.

[rsmith]

Summary

Old GCC used to aggressively fold VLAs to constant-bound arrays at block
scope in GNU mode. That's non-conforming, and more modern versions of
GCC only do this at file scope. Update Clang to do the same.

Also promote the warning for this from off-by-default to on-by-default
in all cases; more recent versions of GCC likewise warn on this by
default.

This is still slightly more permissive than GCC, as pointed out in
PR44406, as we still fold VLAs to constant arrays in structs, but that
seems justifiable given that we don't support VLA-in-struct (and don't
intend to ever support it), but GCC does.

[efriedma]

Also update the documentation? See https://clang.llvm.org/docs/UsersManual.html#differences-between-various-standard-modes .

clang/lib/Sema/SemaDecl.cpp

5939

It's a little surprising to me that you need new code here... was this case getting picked up by the SemaType code?

clang/lib/Sema/SemaType.cpp

2278

We probably want some note to explain why OpenCL is special here... if you can't figure out easily, please leave a FIXME.

[efriedma]

clang/lib/Sema/SemaType.cpp

2278

Oh, I see you posted a separate patch for that.

[rsmith]

clang/lib/Sema/SemaDecl.cpp

5939

Yes, and it previously didn't work outside GNU mode: https://godbolt.org/z/ncqKvo

I suppose that means that it probably isn't important to handle this, but we have a test for the multi-level VLA -> constant array case and it seemed easy enough to make it work.

clang/lib/Sema/SemaType.cpp

2278

For posterity, that's D89520.

[rsmith]

• Update User's Manual to more accurately describe GNU mode and

[rsmith]

• Update User's Manual to more accurately describe GNU mode and

[rsmith]

In D89523#2333939, @efriedma wrote:

Also update the documentation? See https://clang.llvm.org/docs/UsersManual.html#differences-between-various-standard-modes .

Done.

[efriedma]

LGTM