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  • Ryan Scott's avatar
    Control validity-checking of type synonym applications more carefully · 9dc56b61
    Ryan Scott authored
    Trac #16059 shows that when validity checking applications of type
    synonyms, GHC sometimes wasn't checking the expanded type enough.
    We must be careful, however, since checking both the expanded type as
    well as the arguments to the type synonym can lead to exponential
    blowup (see https://ghc.haskell.org/trac/ghc/ticket/16059#comment:4).
    Nor can we omit checking either the expanded type or the argument for
    correctness reasons.
    
    The solution here is to introduce a new `ExpandMode` data type that
    is plumbed through all of the type-validity-checking functions in
    `TcValidity`. `ExpandMode` dictates whether we only check the
    expanded type (`Expand`), only check the arguments (`NoExpand), or
    both (`Both`). Importantly, if we check `Both` in the function for
    validity checking type synonym applications, then we switch to
    `NoExpand` when checking the arguments so as to avoid exponential
    blowup. See `Note [Correctness and performance of type synonym validity
    checking]` for the full story.
    9dc56b61