Move zonkWC to the right place in simplfyInfer

runTcSWithEvBinds does some unification, so the zonkWC
must be after, not before!  Yikes.  An outright bug.

This fixes Trac #14715.

compiler/typecheck/TcSimplify.hs

@@ -649,9 +649,7 @@ simplifyInfer rhs_tclvl infer_mode sigs name_taus wanteds
                         psig_givens = mkGivens loc psig_theta_vars
                   ; _ <- solveSimpleGivens psig_givens
                          -- See Note [Add signature contexts as givens]
-                  ; wanteds' <- solveWanteds wanteds
-                  ; TcS.zonkWC wanteds' }
-
+                  ; solveWanteds wanteds }
 
        -- Find quant_pred_candidates, the predicates that
        -- we'll consider quantifying over
@@ -659,6 +657,7 @@ simplifyInfer rhs_tclvl infer_mode sigs name_taus wanteds
        --      the psig_theta; it's just the extra bit
        -- NB2: We do not do any defaulting when inferring a type, this can lead
        --      to less polymorphic types, see Note [Default while Inferring]
+       ; wanted_transformed_incl_derivs <- TcM.zonkWC wanted_transformed_incl_derivs
        ; let definite_error = insolubleWC wanted_transformed_incl_derivs
                               -- See Note [Quantification with errors]
                               -- NB: must include derived errors in this test,

testsuite/tests/partial-sigs/should_compile/T14715.hs

+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE PartialTypeSignatures #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T14715 (bench_mulPublic) where
+
+data Cyc r
+data CT zp r'q
+class Reduce a b
+type family LiftOf b
+
+bench_mulPublic :: forall z zp zq . (z ~ LiftOf zq, _) => Cyc zp -> Cyc z -> IO (zp,zq)
+bench_mulPublic pt sk = do
+  ct :: CT zp (Cyc zq) <- encrypt sk pt
+  undefined ct
+
+encrypt :: forall z zp zq. Reduce z zq => Cyc z -> Cyc zp -> IO (CT zp (Cyc zq))
+encrypt = undefined

testsuite/tests/partial-sigs/should_compile/T14715.stderr

+ 
\ No newline at end of file

testsuite/tests/partial-sigs/should_compile/all.T

@@ -73,4 +73,5 @@ test('T13482', normal, compile, [''])
 test('T14217', normal, compile_fail, [''])
 test('T14643', normal, compile, [''])
 test('T14643a', normal, compile, [''])
+test('T14715', normal, compile, [''])