• Ryan Scott's avatar
    Tweak error messages for narrowly-kinded assoc default decls · 33b0a291
    Ryan Scott authored
    This program, from #13971, currently has a rather confusing error
    message:
    
    ```hs
    class C a where
      type T a :: k
      type T a = Int
    ```
    ```
        • Kind mis-match on LHS of default declaration for ‘T’
        • In the default type instance declaration for ‘T’
          In the class declaration for ‘C’
    ```
    
    It's not at all obvious why GHC is complaining about the LHS until
    you realize that the default, when printed with
    `-fprint-explicit-kinds`, is actually `type T @{k} @* a = Int`.
    That is to say, the kind of `a` is being instantiated to `Type`,
    whereas it ought to be a kind variable. The primary thrust of this
    patch is to weak the error message to make this connection
    more obvious:
    
    ```
        • Illegal argument ‘*’ in:
            ‘type T @{k} @* a = Int’
            The arguments to ‘T’ must all be type variables
        • In the default type instance declaration for ‘T’
          In the class declaration for ‘C’
    ```
    
    Along the way, I performed some code cleanup suggested by @rae in
    ghc/ghc#13971 (comment 191287). Before,
    we were creating a substitution from the default declaration's type
    variables to the type family tycon's type variables by way of
    `tcMatchTys`. But this is overkill, since we already know (from the
    aforementioned validity checking) that all the arguments in a default
    declaration must be type variables anyway. Therefore, creating the
    substitution is as simple as using `zipTvSubst`. I took the
    opportunity to perform this refactoring while I was in town.
    
    Fixes #13971.
    33b0a291
Code owners : Simon Peyton Jones
TcTyClsDecls.hs 166 KB