Phantom Note [Improvement from Ground Wanteds]
TcCanonical contains
mk_strict_superclasses rec_clss ev tvs theta cls tys
| all noFreeVarsOfType tys
= return [] -- Wanteds with no variables yield no deriveds.
-- See Note [Improvement from Ground Wanteds]
But there is no Note [Improvement from Ground Wanteds]
in sight. The most recent commit that includes the string Improvement from
is 27310213, which introduces this comment, but no Note.
Task: figure out what is going on, see if the behavior is correct, and then either write the note or delete the reference.