diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index dbce5f41ef4b4..b6e68cb1dc329 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6524,6 +6524,7 @@ namespace ts {
             let targetStack: Type[];
             let depth = 0;
             let inferiority = 0;
+            const visited: Map<boolean> = {};
             inferFromTypes(source, target);
 
             function isInProcess(source: Type, target: Type) {
@@ -6653,6 +6654,12 @@ namespace ts {
                             return;
                         }
 
+                        const key = source.id + "," + target.id;
+                        if (hasProperty(visited, key)) {
+                            return;
+                        }
+                        visited[key] = true;
+
                         if (depth === 0) {
                             sourceStack = [];
                             targetStack = [];
diff --git a/src/server/protocol.d.ts b/src/server/protocol.d.ts
index b8207570ea2d7..c50f211a21f56 100644
--- a/src/server/protocol.d.ts
+++ b/src/server/protocol.d.ts
@@ -469,7 +469,7 @@ declare namespace ts.server.protocol {
         placeOpenBraceOnNewLineForControlBlocks?: boolean;
 
         /** Index operator */
-        [key: string]: string | number | boolean;
+        [key: string]: string | number | boolean;
     }
 
     /**