Initial type of variable is union of declared type and undefined

Author: ahejlsberg

src/compiler/checker.ts

@@ -7989,7 +7989,7 @@ namespace ts {
             const defaultsToDeclaredType = !strictNullChecks || type.flags & TypeFlags.Any || !declaration ||
                 getRootDeclaration(declaration).kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
                 getContainingFunctionOrModule(declaration) !== getContainingFunctionOrModule(node);
-            const flowType = getFlowTypeOfReference(node, type, defaultsToDeclaredType ? type : undefinedType);
+            const flowType = getFlowTypeOfReference(node, type, defaultsToDeclaredType ? type : addNullableKind(type, TypeFlags.Undefined));
             if (strictNullChecks && !(type.flags & TypeFlags.Any) && !(getNullableKind(type) & TypeFlags.Undefined) && getNullableKind(flowType) & TypeFlags.Undefined) {
                 error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                 // Return the declared type to reduce follow-on errors