ASSERT failed compiler/typecheck/TcMType.lhs line 809

With current master (9c2dade9) I get

[1 of 1] Compiling ShouldCompile    ( ../ghc/gadt6.hs, ../ghc/gadt6.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20131121 for x86_64-unknown-linux):
	ASSERT failed!
    file compiler/typecheck/TcMType.lhs line 809
    [W] cobox_aBx{v} [lid] :: t_aBv{tv} [tau[0]]
                              ghc-prim:GHC.Types.~{(w) tc 31Q} t_aBu{tv} [tau[0]] (CIrredEvCan)

(And it took me a while to figure out that it was not my code that was introducing the bug.)

According to git log -L 809,809:compiler/typecheck/TcMType.lhs (a useful feature) the history of the ASSERT is:

• SPJ, “Accommodate Derived constraints in two places”, Sep 17, 8cfbdccb
• SPJ, “Fix an ASSERT”, Oct 1 2012, adb6bcb7
• SPJ, “Tidy up and simplify TcMType.zonkFlats”, Sep 17, 2012, bd6b183e

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture