From 7233cde0dc61156a98b8d290449a183e986dc22b Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Tue, 24 Jul 2018 14:49:22 -0700
Subject: [PATCH] Simplify logic in getBaseConstraint

---
 src/compiler/checker.ts | 41 ++++++++++++++++++-----------------------
 1 file changed, 18 insertions(+), 23 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c56d39a9ffc..c1366745c39 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4370,8 +4370,7 @@ namespace ts {
                 case TypeSystemPropertyName.ResolvedReturnType:
                     return !!(<Signature>target).resolvedReturnType;
                 case TypeSystemPropertyName.ImmediateBaseConstraint:
-                    const bc = (<Type>target).immediateBaseConstraint;
-                    return !!bc && bc !== circularConstraintType;
+                    return !!(<Type>target).immediateBaseConstraint;
             }
             return Debug.fail("Unhandled TypeSystemPropertyName " + propertyName);
         }
@@ -6987,30 +6986,26 @@ namespace ts {
          * circularly references the type variable.
          */
         function getResolvedBaseConstraint(type: InstantiableType | UnionOrIntersectionType): Type {
-            let circular: boolean | undefined;
-            if (!type.resolvedBaseConstraint) {
-                const constraint = getBaseConstraint(type);
-                type.resolvedBaseConstraint = circular ? circularConstraintType : getTypeWithThisArgument(constraint || noConstraintType, type);
+            return type.resolvedBaseConstraint ||
+                (type.resolvedBaseConstraint = getTypeWithThisArgument(getImmediateBaseConstraint(type), type));
+
+            function getImmediateBaseConstraint(t: Type): Type {
+                if (!t.immediateBaseConstraint) {
+                    if (!pushTypeResolution(t, TypeSystemPropertyName.ImmediateBaseConstraint)) {
+                        return circularConstraintType;
+                    }
+                    let result = computeBaseConstraint(getSimplifiedType(t));
+                    if (!popTypeResolution()) {
+                        result = circularConstraintType;
+                    }
+                    t.immediateBaseConstraint = result || noConstraintType;
+                }
+                return t.immediateBaseConstraint;
             }
-            return type.resolvedBaseConstraint;
 
             function getBaseConstraint(t: Type): Type | undefined {
-                if (t.immediateBaseConstraint) {
-                    return t.immediateBaseConstraint === noConstraintType ? undefined : t.immediateBaseConstraint;
-                }
-                if (!pushTypeResolution(t, TypeSystemPropertyName.ImmediateBaseConstraint)) {
-                    circular = true;
-                    t.immediateBaseConstraint = circularConstraintType;
-                    return undefined;
-                }
-                const result = computeBaseConstraint(getSimplifiedType(t));
-                if (!popTypeResolution()) {
-                    circular = true;
-                    t.immediateBaseConstraint = circularConstraintType;
-                    return undefined;
-                }
-                t.immediateBaseConstraint = !result ? noConstraintType : result;
-                return result;
+                const c = getImmediateBaseConstraint(t);
+                return c !== noConstraintType && c !== circularConstraintType ? c : undefined;
             }
 
             function computeBaseConstraint(t: Type): Type | undefined {