Simplify or get constraint in distributive conditional type, but not both

ahejlsberg · ahejlsberg · commit 7740822d02d8 · 2018-08-21T15:45:29.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -6921,7 +6921,8 @@ namespace ts {
             // over the conditional type and possibly reduced. For example, 'T extends undefined ? never : T'
             // removes 'undefined' from T.
             if (type.root.isDistributive) {
-                const constraint = getConstraintOfType(getSimplifiedType(type.checkType));
+                const simplified = getSimplifiedType(type.checkType);
+                const constraint = simplified === type.checkType ? getConstraintOfType(simplified) : simplified;
                 if (constraint) {
                     const mapper = makeUnaryTypeMapper(type.root.checkType, constraint);
                     const instantiated = getConditionalTypeInstantiation(type, combineTypeMappers(mapper, type.mapper));
