Try finding the first type with a call/construct signature when relating to unions.

DanielRosenwasser · DanielRosenwasser · commit c5c594f1e7b2 · 2018-09-04T19:25:46.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -11395,7 +11395,8 @@ namespace ts {
                     const bestMatchingType =
                         findMatchingDiscriminantType(source, target) ||
                         findMatchingTypeReferenceOrTypeAliasReference(source, target) ||
-                        findBestTypeForObjectLiteral(source, target);
+                        findBestTypeForObjectLiteral(source, target) ||
+                        findBestTypeForInvokable(source, target);
 
                     isRelatedTo(source, bestMatchingType || targetTypes[targetTypes.length - 1], /*reportErrors*/ true);
                 }
@@ -11426,6 +11427,15 @@ namespace ts {
                 }
             }
 
+            function findBestTypeForInvokable(source: Type, unionTarget: UnionOrIntersectionType) {
+                let signatureKind = SignatureKind.Call;
+                const hasSignatures = getSignaturesOfType(source, signatureKind).length > 0 ||
+                    (signatureKind = SignatureKind.Construct, getSignaturesOfType(source, signatureKind).length > 0);
+                if (hasSignatures) {
+                    return find(unionTarget.types, t => getSignaturesOfType(t, signatureKind).length > 0);
+                }
+            }
+
             // Keep this up-to-date with the same logic within `getApparentTypeOfContextualType`, since they should behave similarly
             function findMatchingDiscriminantType(source: Type, target: UnionOrIntersectionType) {
                 let match: Type | undefined;
