fix: take synthPendingDepth into account when caching TC results

closes #2522

src/Lean/Meta/Basic.lean

@@ -212,7 +212,17 @@ instance : Hashable InfoCacheKey :=
   ⟨fun ⟨transparency, expr, nargs⟩ => mixHash (hash transparency) <| mixHash (hash expr) (hash nargs)⟩
 end InfoCacheKey
 
-abbrev SynthInstanceCache := PersistentHashMap (LocalInstances × Expr) (Option Expr)
+structure SynthInstanceCacheKey where
+  localInsts        : LocalInstances
+  type              : Expr
+  /--
+  Value of `synthPendingDepth` when instance was synthesized or failed to be synthesized.
+  See issue #2522.
+  -/
+  synthPendingDepth : Nat
+  deriving Hashable, BEq
+
+abbrev SynthInstanceCache := PersistentHashMap SynthInstanceCacheKey (Option Expr)
 
 abbrev InferTypeCache := PersistentExprStructMap Expr
 abbrev FunInfoCache   := PersistentHashMap InfoCacheKey FunInfo

src/Lean/Meta/SynthInstance.lean

@@ -735,7 +735,8 @@ def synthInstance? (type : Expr) (maxResultSize? : Option Nat := none) : MetaM (
       unless defEq do
         trace[Meta.synthInstance] "{crossEmoji} result type{indentExpr resultType}\nis not definitionally equal to{indentExpr type}"
       return defEq
-    match s.cache.synthInstance.find? (localInsts, type) with
+    let cacheKey := { localInsts, type, synthPendingDepth := (← read).synthPendingDepth }
+    match s.cache.synthInstance.find? cacheKey with
     | some result =>
       trace[Meta.synthInstance] "result {result} (cached)"
       if let some inst := result then
@@ -782,7 +783,7 @@ def synthInstance? (type : Expr) (maxResultSize? : Option Nat := none) : MetaM (
             pure (some result)
           else
             pure none
-      modify fun s => { s with cache.synthInstance := s.cache.synthInstance.insert (localInsts, type) result? }
+      modify fun s => { s with cache.synthInstance := s.cache.synthInstance.insert cacheKey result? }
       pure result?
 
 /--