Regression: Implicitly kind variables not in scope

Our new head.hackage CI infrastructure has caught its first regression, it seems. A number of packages now fail to build due to out-of-scope kind variables. For instance, singleton-bool:

[1 of 1] Compiling Data.Singletons.Bool ( src/Data/Singletons/Bool.hs, dist/build/Data/Singletons/Bool.o )
*** Parser [Data.Singletons.Bool]:
Parser [Data.Singletons.Bool]: alloc=4264704 time=6.901
*** Renamer/typechecker [Data.Singletons.Bool]:
Renamer/typechecker [Data.Singletons.Bool]: alloc=17554720 time=92.794

src/Data/Singletons/Bool.hs:119:29: error:
    Not in scope: type variable ‘k’
    |
119 | sboolEqRefl :: forall (a :: k) (b :: k). SBoolI (a == b) => Maybe (a :~: b)
    |                             ^

src/Data/Singletons/Bool.hs:119:38: error:
    Not in scope: type variable ‘k’
    |
119 | sboolEqRefl :: forall (a :: k) (b :: k). SBoolI (a == b) => Maybe (a :~: b)
    |                                      ^

Trac metadata

Trac field	Value
Version	8.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Architecture