Please be more careful when changing the language behavior

[Johan]

Hi all,
   Please be more careful when changing the language behavior.

The change made by https://github.com/dlang/dmd/pull/9289 
silently breaks code in ways that can be very hard to detect. We 
were lucky to detect one case. There is no warning/deprecation 
whatsoever to warn me about any other potential breakages in a 
heavily templated 400kloc codebase with 212 instances of alias 
this.

The breakage introduced was very obvious, and warning about it is 
not difficult either. We need to raise the bar for accepting 
changes like this. The code on buildkite project tester is 
(fortunately) not the only D code in the world.

Again, please be more careful when changing language behavior.

Thanks,
   Johan