ScopedTypeVariables could allow more programs

Consider

data P a = P
data T1 a where
  MkT1 :: forall a.              P a               -> T1 a
  MkT2 :: forall a.              P a               -> T1 (a,a)
  MkT3 :: forall a b. b ~ Int => P a -> P b        -> T1 a
  MkT4 :: forall a b.            P a -> P b        -> T1 a
  MkT5 :: forall a b c. b ~ c => P a -> P b -> P c -> T1 a

I can write this function

foo :: T1 (Int, Int) -> ()
foo (MkT1 (P::P (Int,Int)))    = ()
foo (MkT2 (P::P x))            = (() :: x ~ Int => ())
foo (MkT3 P (P::P Int))        = ()
foo (MkT4 P (P::P b))          = ()
foo (MkT5 P (P::P b) (P::P b)) = ()

but this these two equations fail

foo (MkT1 (P::P (Int,x)))      = (() :: x ~ Int => ())
foo (MkT1 (P::P x))            = (() :: x ~ (Int,Int) => ())

I am especially surprised by the second one, given that the very similar equation with MkT2 works.

Trac metadata

Trac field	Value
Version	8.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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