Stop misusing EWildPat in pattern match coverage checking
Stop misusing EWildPat in pattern match coverage checking

EWildPat is a constructor of HsExpr used in the parser to represent
wildcards in ambiguous positions:

* in expression context, EWildPat is turned into hsHoleExpr (see rnExpr)
* in pattern context, EWildPat is turned into WildPat (see checkPattern)

Since EWildPat exists solely for the needs of the parser, we could
remove it by improving the parser.

However, EWildPat has also been used for a different purpose since
8a50610: to represent patterns that the coverage checker cannot handle.
Not only this is a misuse of EWildPat, it also stymies the removal of
EWildPat.

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