In TcEnv.hs there is a note AFamDataCon: not promoting data family constructors. It states that we can't use a promoted data family instance constructor because we would have to interleave the checking of instances and data types. But with the fix of #11348 (closed), we now do exactly this. In the example from the note

data family T a
data instance T Int = MkT
data Proxy (a :: k)
data S = MkS (Proxy 'MkT)

-ddump-rn-trace shows these groups

rnTycl dependency analysis made groups
 [[data family T a_apG]
 []
 [data instance T Int = MkT],
 [data Proxy (a_apF :: k_apE)]
 []
 [],
 [data S = MkS (Proxy MkT)]
 []
 []]

That's to say, the instance T Int will in fact be checked before S. So let's remove this restriction.

See also

• #11348 (closed)
• #16693
• #16410 (closed)