Add isOptionalUninitializedParameterProperty

Improves declaration emit and code readability

Author: sandersn

src/compiler/checker.ts

@@ -23128,6 +23128,13 @@ namespace ts {
                 !(getModifierFlags(parameter) & ModifierFlags.ParameterPropertyModifier);
         }
 
+        function isOptionalUninitializedParameterProperty(parameter: ParameterDeclaration) {
+            return strictNullChecks &&
+                isOptionalParameter(parameter) &&
+                !parameter.initializer &&
+                !!(getModifierFlags(parameter) & ModifierFlags.ParameterPropertyModifier);
+        }
+
         function getNodeCheckFlags(node: Node): NodeCheckFlags {
             return getNodeLinks(node).flags;
         }
@@ -23337,6 +23344,7 @@ namespace ts {
                 isDeclarationVisible,
                 isImplementationOfOverload,
                 isRequiredInitializedParameter,
+                isOptionalUninitializedParameterProperty,
                 writeTypeOfDeclaration,
                 writeReturnTypeOfSignatureDeclaration,
                 writeTypeOfExpression,

src/compiler/declarationEmitter.ts

@@ -340,7 +340,7 @@ namespace ts {
             // these types may need to add `undefined`.
             const shouldUseResolverType = declaration.kind === SyntaxKind.Parameter &&
                 (resolver.isRequiredInitializedParameter(declaration as ParameterDeclaration) ||
-                 (getModifierFlags(declaration) & ModifierFlags.ParameterPropertyModifier && resolver.isOptionalParameter(declaration as ParameterDeclaration)));
+                 resolver.isOptionalUninitializedParameterProperty(declaration as ParameterDeclaration));
             if (type && !shouldUseResolverType) {
                 // Write the type
                 emitType(type);

src/compiler/types.ts

@@ -2814,6 +2814,7 @@ namespace ts {
         collectLinkedAliases(node: Identifier): Node[];
         isImplementationOfOverload(node: FunctionLikeDeclaration): boolean | undefined;
         isRequiredInitializedParameter(node: ParameterDeclaration): boolean;
+        isOptionalUninitializedParameterProperty(node: ParameterDeclaration): boolean;
         writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeReturnTypeOfSignatureDeclaration(signatureDeclaration: SignatureDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeTypeOfExpression(expr: Expression, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;