Fix Trac #3126: matching overloaded literals
Claus Reinke uncovered a long-standing bug in GHC, whereby we were combining the pattern-match on overloaded literals, missing the fact that an intervening pattern (for a different literal) might also match. (If someone had a very odd implementation of fromInteger!) See Note [Grouping overloaded literal patterns] in Match.lhs If this merges smoothly to 6.10, go for it, but it's very much a corner case. Thank you Claus!
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