Add missing Note [Improvement from Ground Wanteds]

Closes #17659.

1 files changed, 13 insertions, 0 deletions

diff --git a/compiler/typecheck/TcCanonical.hs b/compiler/typecheck/TcCanonical.hs
index f8bd367252..a53cdac72e 100644
--- a/compiler/typecheck/TcCanonical.hs
+++ b/compiler/typecheck/TcCanonical.hs
@@ -543,6 +543,19 @@ mk_strict_superclasses rec_clss ev tvs theta cls tys
       = do { sc_ev <- newDerivedNC loc sc_pred
            ; mk_superclasses rec_clss sc_ev [] [] sc_pred }
 
+{- Note [Improvement from Ground Wanteds]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose class C b a => D a b
+and consider
+  [W] D Int Bool
+Is there any point in emitting [D] C Bool Int?  No!  The only point of
+emitting superclass constraints for W/D constraints is to get
+improvement, extra unifications that result from functional
+dependencies.  See Note [Why adding superclasses can help] above.
+
+But no variables means no improvement; case closed.
+-}
+
 mk_superclasses :: NameSet -> CtEvidence
                 -> [TyVar] -> ThetaType -> PredType -> TcS [Ct]
 -- Return this constraint, plus its superclasses, if any