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jack-williams · jack-williams · commit c6c1e952e009 · 2019-10-28T21:51:50.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -15582,7 +15582,10 @@ namespace ts {
                             }
                         }
                     }
-                    if (unionParent && !result) {
+                    if (unionParent && !result && targetIsOptional) {
+                        result = isRelatedTo(source, undefinedType);
+                    }
+                    if (unionParent && !result && source.flags & TypeFlags.Union) {
                         // The easiest way to get the right errors here is to un-defer (which may be costly)
                         // If it turns out this is too costly too often, we can replicate the error handling logic within
                         // typeRelatedToSomeType without the discriminatable type branch (as that requires a manifest union

Original file line number	Diff line number	Diff line change
`@@ -15582,7 +15582,10 @@ namespace ts {`
`15582`	`15582`	`}`
`15583`	`15583`	`}`
`15584`	`15584`	`}`
`15585`		`- if (unionParent && !result) {`
	`15585`	`+ if (unionParent && !result && targetIsOptional) {`
	`15586`	`+ result = isRelatedTo(source, undefinedType);`
	`15587`	`+ }`
	`15588`	`+ if (unionParent && !result && source.flags & TypeFlags.Union) {`
`15586`	`15589`	`// The easiest way to get the right errors here is to un-defer (which may be costly)`
`15587`	`15590`	`// If it turns out this is too costly too often, we can replicate the error handling logic within`
`15588`	`15591`	`// typeRelatedToSomeType without the discriminatable type branch (as that requires a manifest union`