Unused type variables with non-linear patterns

[Jan Stolarek]

An intersting thing came up when implementing warnings for unused type variables in type family 
patterns (#10982 / D1576):

type family X a b where
  X a a = Int 

Should we emit a warning that `a` is unused? It does not appear in the RHS but it is actually used 
because of non-linear pattern. We can fix this by prefixing both appearances of `a` with an 
underscore - this silences the warning and maintains semantics. Nevertheless, if a variable is 
unused it should be perfectly valid to replace it with an underscore - that is not the case in 
this example. Thoughts?

Janek

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Politechnika Łódzka
Lodz University of Technology

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