From ed497c414930452958961cf30a8cb1c7601dbb22 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 30 Sep 2019 08:54:02 -0700
Subject: [PATCH] Properly handle recursive type references in type inference

---
 src/compiler/checker.ts | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0731ccbf04c..61cf45c6c15 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -17021,7 +17021,7 @@ namespace ts {
         function couldContainTypeVariables(type: Type): boolean {
             const objectFlags = getObjectFlags(type);
             return !!(type.flags & TypeFlags.Instantiable ||
-                objectFlags & ObjectFlags.Reference && forEach(getTypeArguments(<TypeReference>type), couldContainTypeVariables) ||
+                objectFlags & ObjectFlags.Reference && ((<TypeReference>type).node || forEach(getTypeArguments(<TypeReference>type), couldContainTypeVariables)) ||
                 objectFlags & ObjectFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.TypeLiteral | SymbolFlags.ObjectLiteral) && type.symbol.declarations ||
                 objectFlags & ObjectFlags.Mapped ||
                 type.flags & TypeFlags.UnionOrIntersection && !(type.flags & TypeFlags.EnumLiteral) && couldUnionOrIntersectionContainTypeVariables(<UnionOrIntersectionType>type));
@@ -17318,7 +17318,8 @@ namespace ts {
                     }
                 }
                 if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (
-                    (<TypeReference>source).target === (<TypeReference>target).target || isArrayType(source) && isArrayType(target))) {
+                    (<TypeReference>source).target === (<TypeReference>target).target || isArrayType(source) && isArrayType(target)) &&
+                    !((<TypeReference>source).node && (<TypeReference>target).node)) {
                     // If source and target are references to the same generic type, infer from type arguments
                     inferFromTypeArguments(getTypeArguments(<TypeReference>source), getTypeArguments(<TypeReference>target), getVariances((<TypeReference>source).target));
                 }
@@ -17599,6 +17600,12 @@ namespace ts {
             }
 
             function inferFromObjectTypesWorker(source: Type, target: Type) {
+                if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (
+                    (<TypeReference>source).target === (<TypeReference>target).target || isArrayType(source) && isArrayType(target))) {
+                    // If source and target are references to the same generic type, infer from type arguments
+                    inferFromTypeArguments(getTypeArguments(<TypeReference>source), getTypeArguments(<TypeReference>target), getVariances((<TypeReference>source).target));
+                    return;
+                }
                 if (isGenericMappedType(source) && isGenericMappedType(target)) {
                     // The source and target types are generic types { [P in S]: X } and { [P in T]: Y }, so we infer
                     // from S to T and from X to Y.