From 709f5f2fc47eb9d28798f5933864d05711720dee Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 15 Oct 2018 10:18:54 -0700
Subject: [PATCH] Handle circular mapped type instantiations for arrays and
 tuples

---
 src/compiler/checker.ts | 12 +++++++++++-
 src/compiler/types.ts   |  1 +
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 795b893f69e..4d397bcc71c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10365,7 +10365,15 @@ namespace ts {
             if (typeVariable) {
                 const mappedTypeVariable = instantiateType(typeVariable, mapper);
                 if (typeVariable !== mappedTypeVariable) {
-                    return mapType(mappedTypeVariable, t => {
+                    // If we are already in the process of creating an instantiation of this mapped type,
+                    // return the error type. This situation only arises if we are instantiating the mapped
+                    // type for an array or tuple type, as we then need to eagerly resolve the (possibly
+                    // circular) element type(s).
+                    if (type.instantiating) {
+                        return errorType;
+                    }
+                    type.instantiating = true;
+                    const result = mapType(mappedTypeVariable, t => {
                         if (t.flags & (TypeFlags.AnyOrUnknown | TypeFlags.InstantiableNonPrimitive | TypeFlags.Object | TypeFlags.Intersection) && t !== wildcardType) {
                             const replacementMapper = createReplacementMapper(typeVariable, t, mapper);
                             return isArrayType(t) ? createArrayType(instantiateMappedTypeTemplate(type, numberType, /*isOptional*/ true, replacementMapper)) :
@@ -10375,6 +10383,8 @@ namespace ts {
                         }
                         return t;
                     });
+                    type.instantiating = false;
+                    return result;
                 }
             }
             return instantiateAnonymousType(type, mapper);
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 3bd103579f1..5763ab3eff0 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4074,6 +4074,7 @@ namespace ts {
         templateType?: Type;
         modifiersType?: Type;
         resolvedApparentType?: Type;
+        instantiating?: boolean;
     }
 
     export interface EvolvingArrayType extends ObjectType {