[clang] [clang][Parser] Warn when the body of expansion statement is not a compound statement (PR #209229)

[Yihan Wang via cfe-commits]

yronglin wrote:

> Let's make it a pedantic warning for now. If WG21 changes the design we can adjust.

A pedantic warning sounds good to me. I also can not figure out why this is specified to require a compound-statement rather than just a statement. The standard seem to have no reason to refuse a single statement.

https://github.com/llvm/llvm-project/pull/209229