diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index f7c03dd7a538800e51b05881db3a736034391f00..63a63308c3eee0730eca7021b0ce161f62a6a247 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -2380,9 +2380,12 @@ checkValidClass cls
(_,theta2,_) = tcSplitSigmaTy tau1
check_constraint :: TcPredType -> TcM ()
- check_constraint pred
- = when (tyCoVarsOfType pred `subVarSet` cls_tv_set)
+ check_constraint pred -- See Note [Class method constraints]
+ = when (not (isEmptyVarSet pred_tvs) &&
+ pred_tvs `subVarSet` cls_tv_set)
(addErrTc (badMethPred sel_id pred))
+ where
+ pred_tvs = tyCoVarsOfType pred
check_at (ATI fam_tc m_dflt_rhs)
= do { checkTc (cls_arity == 0 || any (`elemVarSet` cls_tv_set) fam_tvs)
@@ -2421,7 +2424,21 @@ checkFamFlag tc_name
err_msg = hang (text "Illegal family declaration for" <+> quotes (ppr tc_name))
2 (text "Use TypeFamilies to allow indexed type families")
-{-
+{- Note [Class method constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Haskell 2010 is supposed to reject
+ class C a where
+ op :: Eq a => a -> a
+where the method type costrains only the class variable(s). (The extension
+-XConstrainedClassMethods switches off this check.) But regardless
+we should not reject
+ class C a where
+ op :: (?x::Int) => a -> a
+as pointed out in Trac #11793. So the test here rejects the program if
+ * -XConstrainedClassMethods is off
+ * the tyvars of the constraint are non-empty
+ * all the tyvars are class tyvars, none are locally quantified
+
Note [Abort when superclass cycle is detected]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We must avoid doing the ambiguity check for the methods (in
diff --git a/testsuite/tests/typecheck/should_compile/T11793.hs b/testsuite/tests/typecheck/should_compile/T11793.hs
new file mode 100644
index 0000000000000000000000000000000000000000..f42a6230b3d88f2af54125f317b3de67bd01dd50
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T11793.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE ImplicitParams #-}
+
+module T11793 where
+
+class C a where
+ op :: (?x::Int) => a -> a
+
+-- Should be OK even without ConstrainedClassMethods
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index c03773a7ac079211665efff5bab559fdfa2bfd74..f6c9d2ee5ca1f8d1dccb404cc3b68a1b53d86417 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -510,3 +510,4 @@ test('T11699', normal, compile, [''])
test('T11512', normal, compile, [''])
test('T11754', normal, compile, [''])
test('T11811', normal, compile, [''])
+test('T11793', normal, compile, [''])