Type casts cause confusing error

Try this

instance Eq (Either a)

GHC HEAD gives

Foo.hs:9:10: error:
    • Illegal instance declaration for ‘Eq (Either a)’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘Eq (Either a)’

Foo.hs:9:14: error:
    • Expecting one more argument to ‘Either a’
      Expected a type, but ‘Either a’ has kind ‘* -> *’
    • In the first argument of ‘Eq’, namely ‘Either a’
      In the instance declaration for ‘Eq (Either a)’

The second is right but the first is outright misleading.

Reason: we elaborate to

Eq (Either a |> g)

where g is a type-level kind cast; it's insoluble and gives rise to the second error. But the elaborated type confuses checkValidInstHead.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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changed weight to 5

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mentioned in commit b2e6350f

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Trac field	Value
Test case	- → typecheck/should_fail/T11464

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closed

Merged to ghc-8.0 in 76d8549d.

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Trac field	Value
Resolution	Unresolved → ResolvedFixed

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One could theoretically want tcSplitCastedTyConApp_maybe which is somewhat less crude, but I agree that this works.

added Pnormal label