Empty Haddock comments no longer occur in the AST as `HsDoc`

Summary

Consider the following two type signatures.

foo :: {- |-} A -> B
bar :: {- | -} A -> B

Comparing the AST (with -haddock) of foo and bar, note that foo does not contain a HsDoc (searchf or WithHsDocIdentifiers), but bar does:

foo

bar

(L (SrcSpanAnn (EpAnn (Anchor { <interactive>:1:1-20 } (UnchangedAnchor)) (AnnListItem []) (EpaComments [])) { <interactive>:1:1-20 }) (SigD (NoExtField) (TypeSig (EpAnn (Anchor { <interactive>:1:1-3 } (UnchangedAnchor)) (AnnSig (AddEpAnn AnnDcolon (EpaSpan { <interactive>:1:5-6 })) []) (EpaComments [])) [(L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:1-3 }) (Unqual {OccName: foo}))] (HsWC (NoExtField) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15-20 }) (HsSig (NoExtField) (HsOuterImplicit (NoExtField)) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15-20 }) (HsFunTy (EpAnn (Anchor { <interactive>:1:15 } (UnchangedAnchor)) (NoEpAnns) (EpaComments [])) (HsUnrestrictedArrow (L (TokenLoc (EpaSpan { <interactive>:1:17-18 })) (HsNormalTok))) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15 }) (HsTyVar (EpAnn (Anchor { <interactive>:1:15 } (UnchangedAnchor)) [] (EpaComments [])) (NotPromoted) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15 }) (Unqual {OccName: A})))) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:20 }) (HsTyVar (EpAnn (Anchor { <interactive>:1:20 } (UnchangedAnchor)) [] (EpaComments [])) (NotPromoted) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:20 }) (Unqual {OccName: B}))))))))))))

(L (SrcSpanAnn (EpAnn (Anchor { <interactive>:1:1-21 } (UnchangedAnchor)) (AnnListItem []) (EpaComments [])) { <interactive>:1:1-21 }) (SigD (NoExtField) (TypeSig (EpAnn (Anchor { <interactive>:1:1-3 } (UnchangedAnchor)) (AnnSig (AddEpAnn AnnDcolon (EpaSpan { <interactive>:1:5-6 })) []) (EpaComments [])) [(L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:1-3 }) (Unqual {OccName: bar}))] (HsWC (NoExtField) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16-21 }) (HsSig (NoExtField) (HsOuterImplicit (NoExtField)) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16-21 }) (HsFunTy (EpAnn (Anchor { <interactive>:1:16 } (UnchangedAnchor)) (NoEpAnns) (EpaComments [])) (HsUnrestrictedArrow (L (TokenLoc (EpaSpan { <interactive>:1:18-19 })) (HsNormalTok))) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16 }) (HsDocTy (EpAnnNotUsed) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16 }) (HsTyVar (EpAnn (Anchor { <interactive>:1:16 } (UnchangedAnchor)) [] (EpaComments [])) (NotPromoted) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16 }) (Unqual {OccName: A})))) (L { <interactive>:1:8-14 } (WithHsDocIdentifiers (NestedDocString (HsDocStringNext) (L { <interactive>:1:8-14 } (HsDocStringChunk " "))) [])))) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:21 }) (HsTyVar (EpAnn (Anchor { <interactive>:1:21 } (UnchangedAnchor)) [] (EpaComments [])) (NotPromoted) (L (SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:21 }) (Unqual {OccName: B}))))))))))))

Is there a particular reason for this? In GHC 8.10, the AST contained Haddock comments in both cases.

Concrete effects of this behavior:

• It makes the job of formatters like Ormolu (see issues 1068, 1065, 726) that check of AST discrepancies automatically harder than necessary, as eg a natural rewrite from

  foo ::
    -- |
    --
    A ->
    B

  to

  foo ::
    -- |
    A -> 
    B

  contains a Haddock comment in the AST in the first snippet, but not in the second.
• A nice Haddock trick by @tomjaguarpaw1 (blog post, search for "Forced type signatures to wrap") does no longer work.

Ideally, the behavior would be changed as it was in 8.10; I could try to do that in case this behavior is not intentional.

Environment

• GHC version used: Any GHC since 9.0 (I think this change is due to !2377 (closed))