Type synonym unused binds no warning?

I was taken aback this was accepted

type Silly a b = Maybe b

x :: Silly Int Bool
x = Just True
y :: Silly Char Bool
y = Just False

z = x == y                    -- returns False

What's with the a in Silly's decl? That's not a phantom type. It's ignored and thrown away. So x and y are the same type, and can be compare for equality.

I expected a rule: all tyvars in the type's head must appear on rhs. Or at least a warning there was something silly. I tried -Wall, -fwarn-unused-binds, -Wunused-type-patterns.

I was just checking up on a remark that type synonyms are at the type level like implicit bidirectional pattern synonyms. For those, all vars must appear on both sides.

Trac metadata

Trac field	Value
Version	8.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
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Operating system	Windows
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