Commit 71006532 authored by Committed by Marge Bot
Reject nested foralls/contexts in instance types more consistently
GHC is very wishy-washy about rejecting instance declarations with nested `forall`s or contexts that are surrounded by outermost parentheses. This can even lead to some strange interactions with `ScopedTypeVariables`, as demonstrated in #18240. This patch makes GHC more consistently reject instance types with nested `forall`s/contexts so as to prevent these strange interactions. On the implementation side, this patch tweaks `splitLHsInstDeclTy` and `getLHsInstDeclHead` to not look through parentheses, which can be semantically significant. I've added a `Note [No nested foralls or contexts in instance types]` in `GHC.Hs.Type` to explain why. This also introduces a `no_nested_foralls_contexts_err` function in `GHC.Rename.HsType` to catch nested `forall`s/contexts in instance types. This function is now used in `rnClsInstDecl` (for ordinary instance declarations) and `rnSrcDerivDecl` (for standalone `deriving` declarations), the latter of which fixes #18271. On the documentation side, this adds a new "Formal syntax for instance declaration types" section to the GHC User's Guide that presents a BNF-style grammar for what is and isn't allowed in instance types. Fixes #18240. Fixes #18271.
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