Type error in one function causes wrong type error report in another function in the presence of functionally dependent types

When trying to reduce my problem in #5970 (closed) to a simple program I encountered an even more obvious bug. The problem is essentially as follows: I have

correct :: T
correct = ...

wrong :: T
wrong = f correct

where 'wrong' has a type error and 'correct' is type correct. If I outcomment 'wrong' then GHC correctly confirms type-correctness of 'correct', but if 'wrong' is enabled then GHC claims a type error in both 'wrong' and 'correct'. To me it looks like the type-checker is trying to unify types across function boundaries. This would explain both non-linear growth of type-checking time and the observed effect of incorrect type errors. See attached program for a working example.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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