diff options
Diffstat (limited to 'compiler/GHC/Tc/Solver.hs')
| -rw-r--r-- | compiler/GHC/Tc/Solver.hs | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/compiler/GHC/Tc/Solver.hs b/compiler/GHC/Tc/Solver.hs index eaa62e44ea..9c371af463 100644 --- a/compiler/GHC/Tc/Solver.hs +++ b/compiler/GHC/Tc/Solver.hs @@ -3088,6 +3088,18 @@ neededEvVars implic@(Implic { ic_given = givens ------------------------------------------------- simplifyDelayedErrors :: Bag DelayedError -> TcS (Bag DelayedError) +-- Simplify any delayed errors: e.g. type and term holes +-- NB: At this point we have finished with all the simple +-- constraints; they are in wc_simple, not in the inert set. +-- So those Wanteds will not rewrite these delayed errors. +-- That's probably no bad thing. +-- +-- However if we have [W] alpha ~ Maybe a, [W] alpha ~ Int +-- and _ : alpha, then we'll /unify/ alpha with the first of +-- the Wanteds we get, and thereby report (_ : Maybe a) or +-- (_ : Int) unpredictably, depending on which we happen to see +-- first. Doesn't matter much; there is a type error anyhow. +-- T17139 is a case in point. simplifyDelayedErrors = mapBagM simpl_err where simpl_err :: DelayedError -> TcS DelayedError @@ -3104,6 +3116,7 @@ simplifyDelayedErrors = mapBagM simpl_err -- code, because we have all the givens already set up simpl_hole h@(Hole { hole_ty = ty, hole_loc = loc }) = do { ty' <- rewriteType loc ty + ; traceTcS "simpl_hole" (ppr ty $$ ppr ty') ; return (h { hole_ty = ty' }) } {- Note [Delete dead Given evidence bindings] |