Commit 426b0ddc authored by Committed by Marge Bot
Don't skip validity checks for built-in classes (#17355)
Issue #17355 occurred because the control flow for `TcValidity.check_valid_inst_head` was structured in such a way that whenever it checked a special, built-in class (like `Generic` or `HasField`), it would skip the most important check of all: `checkValidTypePats`, which rejects nonsense like this: ```hs instance Generic (forall a. a) ``` This fixes the issue by carving out `checkValidTypePats` from `check_valid_inst_head` so that `checkValidTypePats` is always invoked. `check_valid_inst_head` has also been renamed to `check_special_inst_head` to reflect its new purpose of _only_ checking for instances headed by special classes. Fixes #17355.
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