GHC often depicts H98 datatypes as GADTs when it shouldn't

(Spun off from D3880.)

There are at least a couple of points in the GHC codebase where poor decisions are made regarding whether to depict a datatype using H98 syntax or GADT syntax. In particular:

1. When reifying a datatype in Template Haskell, some complicated heuristics are used to determine whether a datatype is reified with (Rec)GadtC or not (see Note [Reifying GADT data constructors] in TcSplice for the horrifying details). But these heuristics are far from perfect, as they will cause some vanilla datatypes to be reified as GADTs (e.g., data T a = (a ~ Int) => MkT).
2. When pretty-printing a datatype (for instance, via the :info command in GHCi), GHC also chooses to sometimes depict vanilla datatypes as GADTs. For instance:

λ> :set -XExistentialQuantification 
λ> data Foo a = Show a => Foo
λ> :i Foo
type role Foo nominal
data Foo a where
  Foo :: Show a => Foo a

Foo is depicted as a GADT, despite the fact that we didn't use GADT syntax.

There might be other places in the code that exhibit these symptoms too—we should take a closer look.

I use the phrase "poor decisions" above since GHC really does have the right info it needs to make the correct decisions. That info is in the form of the isGadtSyntaxTyCon, which conveniently recalls whether a datatype was declared with GADT syntax or not. With this, we should be able to eliminate the flawed heuristics that GHC is currently using and accurately depict datatypes the way users wrote them.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug TemplateHaskell Trac import labels

GHC really does have the right info it needs to make the correct decisions.

Yes indeed. Go for it.

Trac metadata

Trac field	Value
Differential revisions	- → D3901

mentioned in commit 8e4229ab

closed

changed milestone to %8.4.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added GADTs label

added Pnormal label