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ghc generates seemingly incorrect code

GHC generates a wrong code for the case-match on GADT with polymorphic type,

here is a minimal example attached, expected results are 'A' in all test cases (this is case for ghc-HEAD), however in: test0 - result is missing as matches are removed from code (according to core) test1 - returns A as expected test2 - output 'o_O' without any checks

Trac metadata
Trac field Value
Version 7.8.3
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
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Blocking
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Operating system
Architecture
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