Remove undefined when checking patterns in function declarations

Author: sandersn

src/compiler/checker.ts

@@ -3098,7 +3098,8 @@ namespace ts {
         /** Return the inferred type for a binding element */
         function getTypeForBindingElement(declaration: BindingElement): Type {
             const pattern = <BindingPattern>declaration.parent;
-            const parentType = getTypeForBindingElementParent(<VariableLikeDeclaration>pattern.parent);
+            let parentType = getTypeForBindingElementParent(<VariableLikeDeclaration>pattern.parent);
+
             // If parent has the unknown (error) type, then so does this binding element
             if (parentType === unknownType) {
                 return unknownType;
@@ -3112,6 +3113,12 @@ namespace ts {
                 }
                 return parentType;
             }
+            // In strict null checking mode, a default value of a binding pattern adds undefined,
+            // which should be removed to get the type of the elements
+            const func = getContainingFunction(declaration);
+            if (strictNullChecks && func && !func.body && getFalsyFlags(parentType) & TypeFlags.Undefined) {
+                parentType = getTypeWithFacts(parentType, TypeFacts.NEUndefined);
+            }
 
             let type: Type;
             if (pattern.kind === SyntaxKind.ObjectBindingPattern) {

src/compiler/declarationEmitter.ts

@@ -319,7 +319,7 @@ namespace ts {
             }
         }
 
-            function writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, type: TypeNode, getSymbolAccessibilityDiagnostic: GetSymbolAccessibilityDiagnostic) {
+        function writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, type: TypeNode, getSymbolAccessibilityDiagnostic: GetSymbolAccessibilityDiagnostic) {
             writer.getSymbolAccessibilityDiagnostic = getSymbolAccessibilityDiagnostic;
             write(": ");
             if (type) {