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We only do newtype unwrapping for Coercible constraints if
the newtype's data constructor is in scope. We were trying to
record the fact that the data constructor was thereby 'used', so
that an import statement would not be flagged as unnecsssary
(by -Wunused-imports).
But the code was simply wrong. It was wrong because it assumed
that only one level of unwrapping happened, whereas
tcTopNormaliseNewTypeTF_maybe actually unwraps multiple layers.
So we need to return a /list/ of data constructors that are used.
This entailed a bit of refactoring, as usual.
Fixes Trac #12067
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