Commit e365d496 authored by Simon Peyton Jones's avatar Simon Peyton Jones

Improve the insolubility check when quantifying

See Note [Quantification with errors]
Fixes Trac #8262
parent dc8b06e8
......@@ -247,8 +247,8 @@ simplifyInfer _top_lvl apply_mr name_taus wanteds
-- constraint.
; ev_binds_var <- newTcEvBinds
; wanted_transformed <- solveWantedsTcMWithEvBinds ev_binds_var wanteds $
solve_wanteds_and_drop
; wanted_transformed_incl_derivs
<- solveWantedsTcMWithEvBinds ev_binds_var wanteds solve_wanteds
-- Post: wanted_transformed are zonked
-- Step 4) Candidates for quantification are an approximation of wanted_transformed
......@@ -263,9 +263,11 @@ simplifyInfer _top_lvl apply_mr name_taus wanteds
; tc_lcl_env <- TcRnMonad.getLclEnv
; let untch = tcl_untch tc_lcl_env
wanted_transformed = dropDerivedWC wanted_transformed_incl_derivs
; quant_pred_candidates -- Fully zonked
<- if insolubleWC wanted_transformed
<- if insolubleWC wanted_transformed_incl_derivs
then return [] -- See Note [Quantification with errors]
-- NB: must include derived errors
else do { gbl_tvs <- tcGetGlobalTyVars
; let quant_cand = approximateWC wanted_transformed
meta_tvs = filter isMetaTyVar (varSetElems (tyVarsOfCts quant_cand))
......@@ -391,7 +393,11 @@ over, and instead make the function fully-polymorphic in whatever
type we have found. For two reasons
a) Minimise downstream errors
b) Avoid spurious errors from this function
But NB that we must include *derived* errors in the check. Example:
(a::*) ~ Int#
We get an insoluble derived error *~#, and we don't want to discard
it before doing the isInsolubleWC test! (Trac #8262)
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
......@@ -607,7 +613,7 @@ solveWantedsTcM wanted
solve_wanteds_and_drop :: WantedConstraints -> TcS (WantedConstraints)
-- Since solve_wanteds returns the residual WantedConstraints,
-- it should alway be called within a runTcS or something similar,
-- it should always be called within a runTcS or something similar,
solve_wanteds_and_drop wanted = do { wc <- solve_wanteds wanted
; return (dropDerivedWC wc) }
......
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