There are two patches here
One fixes #19768 (closed). There was nothing difficult here, although it touches a lot of files.
The second fixes #18481 (closed) and #19775 (closed), plus refactors HsConLikeOut so that it doesn't pollute Language.Haskell.Syntax.
Allow visible type application for levity-poly data cons
This patch was driven by #18481, to allow visible type application
for levity-polymorphic newtypes. As so often, it started simple
but grew:
* Significant refactor: I removed HsConLikeOut from the
client-independent Language.Haskell.Syntax.Expr, and put it where it
belongs, as a new constructor `ConLikeTc` in the GHC-specific extension
data type for expressions, `GHC.Hs.Expr.XXExprGhcTc`.
That changed touched a lot of files in a very superficial way.
* Note [Typechecking data constructors] explains the main payload.
The eta-expansion part is no longer done by the typechecker, but
instead deferred to the desugarer, via `ConLikeTc`
* A little side benefit is that I was able to restore VTA for
data types with a "stupid theta": #19775. Not very important,
but the code in GHC.Tc.Gen.Head.tcInferDataCon is is much, much
more elegant now.
* I had to refactor the levity-polymorphism checking code in
GHC.HsToCore.Expr, see
Note [Checking for levity-polymorphic functions]
Note [Checking levity-polymorphic data constructors]