Fix documentation on type families not being extracted

It looks like the location of the Names used for CoAxioms on type
families are now located at their type constructors. Previously, Docs.hs
thought the Names were located in the RHS, so the RealSrcSpan in the
instanceMap and getInstLoc didn't match up. Fixes #18241

3 files changed, 18 insertions, 12 deletions

diff --git a/compiler/GHC/HsToCore/Docs.hs b/compiler/GHC/HsToCore/Docs.hs
index 1aee1fbd68..0f6d8ba9e4 100644
--- a/compiler/GHC/HsToCore/Docs.hs
+++ b/compiler/GHC/HsToCore/Docs.hs
@@ -97,13 +97,7 @@ mkMaps instances decls =
     instanceMap = M.fromList [(l, n) | n <- instances, RealSrcSpan l _ <- [getSrcSpan n] ]
 
     names :: RealSrcSpan -> HsDecl GhcRn -> [Name]
-    names l (InstD _ d) = maybeToList $ -- See Note [1].
-      case d of
-              TyFamInstD _ _ -> M.lookup l instanceMap
-                -- The CoAx's loc is the whole line, but only
-                -- for TFs
-              _ -> lookupSrcSpan (getInstLoc d) instanceMap
-
+    names _ (InstD _ d) = maybeToList $ lookupSrcSpan (getInstLoc d) instanceMap
     names l (DerivD {}) = maybeToList (M.lookup l instanceMap) -- See Note [1].
     names _ decl = getMainDeclBinder decl
 
@@ -145,14 +139,16 @@ sigNameNoLoc _                             = []
 getInstLoc :: InstDecl (GhcPass p) -> SrcSpan
 getInstLoc = \case
   ClsInstD _ (ClsInstDecl { cid_poly_ty = ty }) -> getLoc (hsSigType ty)
+  -- The Names of data and type family instances have their SrcSpan's attached
+  -- to the *type constructor*. For example, the Name "D:R:Foo:Int" would have
+  -- its SrcSpan attached here:
+  --   type family Foo a
+  --   type instance Foo Int = Bool
+  --                 ^^^
   DataFamInstD _ (DataFamInstDecl
     { dfid_eqn = HsIB { hsib_body = FamEqn { feqn_tycon = L l _ }}}) -> l
   TyFamInstD _ (TyFamInstDecl
-    -- Since CoAxioms' Names refer to the whole line for type family instances
-    -- in particular, we need to dig a bit deeper to pull out the entire
-    -- equation. This does not happen for data family instances, for some
-    -- reason.
-    { tfid_eqn = HsIB { hsib_body = FamEqn { feqn_rhs = L l _ }}}) -> l
+    { tfid_eqn = HsIB { hsib_body = FamEqn { feqn_tycon = L l _ }}}) -> l
 
 -- | Get all subordinate declarations inside a declaration, and their docs.
 -- A subordinate declaration is something like the associate type or data
diff --git a/testsuite/tests/showIface/DocsInHiFile.hs b/testsuite/tests/showIface/DocsInHiFile.hs
index 26156722ac..e753eb5e42 100644
--- a/testsuite/tests/showIface/DocsInHiFile.hs
+++ b/testsuite/tests/showIface/DocsInHiFile.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE TypeFamilies #-}
 {-| `elem`, 'print',
 `Unknown',
 '<>', ':=:', 'Bool'
@@ -35,3 +36,8 @@ class P f where
 -- | Another datatype...
 data D'
 -- ^ ...with two docstrings.
+
+-- | A type family
+type family F a
+-- | A type family instance
+type instance F Int = Bool
diff --git a/testsuite/tests/showIface/DocsInHiFile1.stdout b/testsuite/tests/showIface/DocsInHiFile1.stdout
index 83d21f1953..fa642627d6 100644
--- a/testsuite/tests/showIface/DocsInHiFile1.stdout
+++ b/testsuite/tests/showIface/DocsInHiFile1.stdout
@@ -22,6 +22,10 @@ declaration docs:
     " Another datatype...
 
  ...with two docstrings."
+  D:R:FInt:
+    " A type family instance"
+  F:
+    " A type family"
 arg docs:
   add:
     0: