diff --git a/src/Init/Lean/Meta/Basic.lean b/src/Init/Lean/Meta/Basic.lean
index 4b5664efcd..5363761b4c 100644
--- a/src/Init/Lean/Meta/Basic.lean
+++ b/src/Init/Lean/Meta/Basic.lean
@@ -168,16 +168,16 @@ IO.mkRef $ fun _ _ => throw $ Exception.other "isDefEq implementation was not se
 
 @[init mkIsExprDefEqAuxRef] def isExprDefEqAuxRef : IO.Ref (Expr → Expr → MetaM Bool) := arbitrary _
 
-def mkSynthPendingRef : IO (IO.Ref (Expr → MetaM Bool)) :=
+def mkSynthPendingRef : IO (IO.Ref (MVarId → MetaM Bool)) :=
 IO.mkRef $ fun _ => pure false
 
-@[init mkSynthPendingRef] def synthPendingRef : IO.Ref (Expr → MetaM Bool) := arbitrary _
+@[init mkSynthPendingRef] def synthPendingRef : IO.Ref (MVarId → MetaM Bool) := arbitrary _
 
 structure MetaExtState :=
 (whnf         : Expr → MetaM Expr)
 (inferType    : Expr → MetaM Expr)
 (isDefEqAux   : Expr → Expr → MetaM Bool)
-(synthPending : Expr → MetaM Bool)
+(synthPending : MVarId → MetaM Bool)
 
 instance MetaExtState.inhabited : Inhabited MetaExtState :=
 ⟨{ whnf := arbitrary _, inferType := arbitrary _, isDefEqAux := arbitrary _, synthPending := arbitrary _ }⟩
@@ -212,10 +212,10 @@ withIncRecDepth $ do
   env ← getEnv;
   (metaExt.getState env).isDefEqAux t s
 
-def synthPending (e : Expr) : MetaM Bool :=
+def synthPending (mvarId : MVarId) : MetaM Bool :=
 withIncRecDepth $ do
   env ← getEnv;
-  (metaExt.getState env).synthPending e
+  (metaExt.getState env).synthPending mvarId
 
 def mkFreshId : MetaM Name := do
 s ← get;
diff --git a/src/Init/Lean/Meta/ExprDefEq.lean b/src/Init/Lean/Meta/ExprDefEq.lean
index e860144e5a..e447f51ddb 100644
--- a/src/Init/Lean/Meta/ExprDefEq.lean
+++ b/src/Init/Lean/Meta/ExprDefEq.lean
@@ -110,6 +110,12 @@ private partial def isDefEqArgsAux (args₁ args₂ : Array Expr) (h : args₁.s
   else
     pure true
 
+@[specialize] private def trySynthPending (e : Expr) : MetaM Bool := do
+mvarId? ← getStuckMVar? e;
+match mvarId? with
+| some mvarId => synthPending mvarId
+| none        => pure false
+
 private def isDefEqArgs (f : Expr) (args₁ args₂ : Array Expr) : MetaM Bool :=
 if h : args₁.size = args₂.size then do
   finfo ← getFunInfoNArgs f args₁.size;
@@ -124,8 +130,8 @@ if h : args₁.size = args₂.size then do
     let a₂   := args₂.get! i;
     let info := finfo.paramInfo.get! i;
     when info.instImplicit $ do {
-      synthPending a₁;
-      synthPending a₂;
+      trySynthPending a₁;
+      trySynthPending a₂;
       pure ()
     };
     withAtLeastTransparency TransparencyMode.default $ isExprDefEqAux a₁ a₂)
@@ -892,8 +898,8 @@ private partial def isDefEqQuick : Expr → Expr → MetaM LBool
   condM (isAssigned sFn) (do s ← instantiateMVars s; isDefEqQuick t s) $
   condM (isDelayedAssignedHead tFn t) (do t ← instantiateMVars t; isDefEqQuick t s) $
   condM (isDelayedAssignedHead sFn s) (do s ← instantiateMVars s; isDefEqQuick t s) $
-  condM (isSynthetic tFn <&&> synthPending tFn) (do t ← instantiateMVars t; isDefEqQuick t s) $
-  condM (isSynthetic sFn <&&> synthPending sFn) (do s ← instantiateMVars s; isDefEqQuick t s) $ do
+  condM (isSynthetic tFn <&&> trySynthPending tFn) (do t ← instantiateMVars t; isDefEqQuick t s) $
+  condM (isSynthetic sFn <&&> trySynthPending sFn) (do s ← instantiateMVars s; isDefEqQuick t s) $ do
   tAssign? ← isAssignable tFn;
   sAssign? ← isAssignable sFn;
   trace! `Meta.isDefEq
@@ -957,10 +963,10 @@ else
 @[specialize] private def unstuckMVar
     (e : Expr)
     (successK : Expr → MetaM Bool) (failK : MetaM Bool): MetaM Bool := do
-s? ← WHNF.getStuckMVar getConst whnf e;
-match s? with
-| some s =>
-  condM (synthPending s)
+mvarId? ← getStuckMVar? e;
+match mvarId? with
+| some mvarId =>
+  condM (synthPending mvarId)
     (do e ← instantiateMVars e; successK e)
     failK
 | none   => failK
diff --git a/src/Init/Lean/Meta/SynthInstance.lean b/src/Init/Lean/Meta/SynthInstance.lean
index 8b61f7e32d..dd2cda5b5f 100644
--- a/src/Init/Lean/Meta/SynthInstance.lean
+++ b/src/Init/Lean/Meta/SynthInstance.lean
@@ -9,6 +9,7 @@ import Init.Lean.Meta.Basic
 import Init.Lean.Meta.Instances
 import Init.Lean.Meta.LevelDefEq
 import Init.Lean.Meta.AbstractMVars
+import Init.Lean.Meta.WHNF
 
 namespace Lean
 namespace Meta
@@ -594,6 +595,26 @@ match result? with
 | some result => pure result
 | none        => throwEx $ Exception.synthInstance type
 
+def synthPendingImp (mvarId : MVarId) : MetaM Bool := do
+mvarDecl ← getMVarDecl mvarId;
+match mvarDecl.kind with
+| MetavarKind.synthetic => do
+  c? ← isClass mvarDecl.type;
+  match c? with
+  | none   => pure false
+  | some _ => do
+    val? ← synthInstance? mvarDecl.type;
+    match val? with
+    | none     => pure false
+    | some val =>
+      condM (isExprMVarAssigned mvarId) (pure false) $ do
+        assignExprMVar mvarId val;
+        pure true
+| _ => pure false
+
+@[init] def setSynthPendingRef : IO Unit :=
+synthPendingRef.set synthPendingImp
+
 @[init] private def regTraceClasses : IO Unit := do
 registerTraceClass `Meta.synthInstance;
 registerTraceClass `Meta.synthInstance.globalInstances;
diff --git a/src/Init/Lean/Meta/WHNF.lean b/src/Init/Lean/Meta/WHNF.lean
index 84ce45baec..61eff6c4f4 100644
--- a/src/Init/Lean/Meta/WHNF.lean
+++ b/src/Init/Lean/Meta/WHNF.lean
@@ -66,5 +66,8 @@ whnfHeadPredAux pred e
 def whnfUntil (e : Expr) (declName : Name) : MetaM Expr :=
 whnfHeadPredAux (fun e => pure $ e.isAppOf declName) e
 
+def getStuckMVar? (e : Expr) : MetaM (Option MVarId) :=
+WHNF.getStuckMVar? getConst whnf e
+
 end Meta
 end Lean
diff --git a/src/Init/Lean/Util/WHNF.lean b/src/Init/Lean/Util/WHNF.lean
index 9a3ad4c88c..1f2b7c000f 100644
--- a/src/Init/Lean/Util/WHNF.lean
+++ b/src/Init/Lean/Util/WHNF.lean
@@ -121,10 +121,10 @@ if h : majorIdx < recArgs.size then do
 else
   failK ()
 
-@[specialize] def isRecStuck {m : Type → Type} [Monad m]
-    (whnf    : Expr → m Expr)
-    (isStuck : Expr → m (Option Expr))
-    (rec : RecursorVal) (recLvls : List Level) (recArgs : Array Expr) : m (Option Expr) :=
+@[specialize] def isRecStuck? {m : Type → Type} [Monad m]
+    (whnf     : Expr → m Expr)
+    (isStuck? : Expr → m (Option MVarId))
+    (rec : RecursorVal) (recLvls : List Level) (recArgs : Array Expr) : m (Option MVarId) :=
 if rec.k then
   -- TODO: improve this case
   pure none
@@ -133,7 +133,7 @@ else do
   if h : majorIdx < recArgs.size then do
     let major := recArgs.get ⟨majorIdx, h⟩;
     major ← whnf major;
-    isStuck major
+    isStuck? major
   else
     pure none
 
@@ -166,20 +166,20 @@ match rec.kind with
 | QuotKind.ind  => process 4 3
 | _             => failK ()
 
-@[specialize] def isQuotRecStuck {m : Type → Type} [Monad m]
+@[specialize] def isQuotRecStuck? {m : Type → Type} [Monad m]
     (whnf : Expr → m Expr)
-    (isStuck : Expr → m (Option Expr))
-    (rec : QuotVal) (recLvls : List Level) (recArgs : Array Expr) : m (Option Expr) :=
-let process (majorPos : Nat) : m (Option Expr) :=
+    (isStuck? : Expr → m (Option MVarId))
+    (rec : QuotVal) (recLvls : List Level) (recArgs : Array Expr) : m (Option MVarId) :=
+let process? (majorPos : Nat) : m (Option MVarId) :=
   if h : majorPos < recArgs.size then do
     let major := recArgs.get ⟨majorPos, h⟩;
     major ← whnf major;
-    isStuck major
+    isStuck? major
   else
     pure none;
 match rec.kind with
-| QuotKind.lift => process 5
-| QuotKind.ind  => process 4
+| QuotKind.lift => process? 5
+| QuotKind.ind  => process? 4
 | _             => pure none
 
 /- ===========================
@@ -187,22 +187,22 @@ match rec.kind with
    =========================== -/
 
 /-- Return `some (Expr.mvar mvarId)` if metavariable `mvarId` is blocking reduction. -/
-@[specialize] partial def getStuckMVar {m : Type → Type} [Monad m]
+@[specialize] partial def getStuckMVar? {m : Type → Type} [Monad m]
     (getConst : Name → m (Option ConstantInfo))
     (whnf     : Expr → m Expr)
-    : Expr → m (Option Expr)
-| Expr.mdata _ e _   => getStuckMVar e
-| Expr.proj _ _ e _  => do e ← whnf e; getStuckMVar e
-| e@(Expr.mvar _ _)  => pure (some e)
+    : Expr → m (Option MVarId)
+| Expr.mdata _ e _       => getStuckMVar? e
+| Expr.proj _ _ e _      => do e ← whnf e; getStuckMVar? e
+| e@(Expr.mvar mvarId _) => pure (some mvarId)
 | e@(Expr.app f _ _) =>
   let f := f.getAppFn;
   match f with
-  | Expr.mvar _ _            => pure (some f)
+  | Expr.mvar mvarId _       => pure (some mvarId)
   | Expr.const fName fLvls _ => do
     cinfo? ← getConst fName;
     match cinfo? with
-    | some $ ConstantInfo.recInfo rec  => isRecStuck whnf getStuckMVar rec fLvls e.getAppArgs
-    | some $ ConstantInfo.quotInfo rec => isQuotRecStuck whnf getStuckMVar rec fLvls e.getAppArgs
+    | some $ ConstantInfo.recInfo rec  => isRecStuck? whnf getStuckMVar? rec fLvls e.getAppArgs
+    | some $ ConstantInfo.quotInfo rec => isQuotRecStuck? whnf getStuckMVar? rec fLvls e.getAppArgs
     | _                                => pure none
   | _ => pure none
 | _ => pure none
@@ -320,14 +320,14 @@ else
     (whnf              : Expr → m Expr)
     (inferType         : Expr → m Expr)
     (isDefEq           : Expr → Expr → m Bool)
-    (synthesizePending : Expr → m Bool)
+    (synthesizePending : MVarId → m Bool)
     (getLocalDecl      : FVarId → m LocalDecl)
     (getMVarAssignment : MVarId → m (Option Expr))
     : Expr → m Expr
 | e => do
   e ← whnfCore getConst isAuxDef? whnf inferType isDefEq getLocalDecl getMVarAssignment e;
-  (some mvar) ← getStuckMVar getConst whnf e | pure e;
-  succeeded   ← synthesizePending mvar;
+  (some mvarId) ← getStuckMVar? getConst whnf e | pure e;
+  succeeded     ← synthesizePending mvarId;
   if succeeded then whnfCoreUnstuck e else pure e
 
 /-- Unfold definition using "smart unfolding" if possible. -/
@@ -337,7 +337,7 @@ else
     (whnf              : Expr → m Expr)
     (inferType         : Expr → m Expr)
     (isDefEq           : Expr → Expr → m Bool)
-    (synthesizePending : Expr → m Bool)
+    (synthesizePending : MVarId → m Bool)
     (getLocalDecl      : FVarId → m LocalDecl)
     (getMVarAssignment : MVarId → m (Option Expr))
     (e : Expr) : m (Option Expr) :=
@@ -379,7 +379,7 @@ match e with
     (isAuxDef?         : Name → m Bool)
     (inferType         : Expr → m Expr)
     (isDefEq           : Expr → Expr → m Bool)
-    (synthesizePending : Expr → m Bool)
+    (synthesizePending : MVarId → m Bool)
     (getLocalDecl      : FVarId → m LocalDecl)
     (getMVarAssignment : MVarId → m (Option Expr))
     : Expr → m Expr
diff --git a/tests/lean/run/synthPending1.lean b/tests/lean/run/synthPending1.lean
new file mode 100644
index 0000000000..d0acc0580c
--- /dev/null
+++ b/tests/lean/run/synthPending1.lean
@@ -0,0 +1,4 @@
+new_frontend
+
+theorem ex : Not (1 = 2) :=
+ofDecideEqFalse rfl