Sema: fix invalid coercion `*[n:x]T` -> `*[m]T` for `n != m`

The change in `Sema.coerceExtra` is just to avoid an unhelpful error
message, covered by the added test case.

Resolves: #22373
5333d2443amlugg committed on 1/1/2025, 4:20:40 PMparentd02c2c7
2 files changedLine totals unavailable