Rollup merge of #111449 - compiler-errors:recover-impl-generics-correctly, r=Nilstrieb

Recover `impl<T ?Sized>` correctly

Fixes #111327

r? ````@Nilstrieb```` but you can re-roll

Alternatively, happy to close this if we're okay with just saying "sorry #111327 is just a poor side-effect of parser ambiguity" 🤷

Author: Noratrieb