Relax "suppressing errors" assert in reportWanteds

The assertion in reportWanteds that we aren't suppressing all the
Wanted constraints was too strong: it might be the case that we are
inside an implication, and have already reported an unsolved Wanted
from outside the implication. It is possible that all Wanteds inside
the implication have been rewritten by the outer Wanted, so we shouldn't
throw an assertion failure in that case.

Fixes #21405

1 files changed, 9 insertions, 4 deletions

diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index f662495f2c..bbdda9c731 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -519,10 +519,15 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics
 
        -- This check makes sure that we aren't suppressing the only error that will
        -- actually stop compilation
-       ; massert $
-           null simples ||                  -- no errors to report here
-           any ignoreConstraint simples ||  -- one error is ignorable (is reported elsewhere)
-           not (all ei_suppress tidy_items) -- not all error are suppressed
+       ; assertPprM
+           ( do { errs_already <- ifErrsM (return True) (return False)
+                ; return $
+                    errs_already ||                  -- we already reported an error (perhaps from an outer implication)
+                    null simples ||                  -- no errors to report here
+                    any ignoreConstraint simples ||  -- one error is ignorable (is reported elsewhere)
+                    not (all ei_suppress tidy_items) -- not all errors are suppressed
+                } )
+           (vcat [text "reportWanteds is suppressing all errors"])
 
          -- First, deal with any out-of-scope errors:
        ; let (out_of_scope, other_holes) = partition isOutOfScopeHole tidy_holes