Strict optional properties (#43947)

* Introduce --strictOptionalProperties compiler switch

* Accept new baselines

* Removing missingType when printing back optional properties

* Accept new baselines

* Fix linting issue

* Use getNonMissingTypeOfSymbol in getTypeOfSymbolAtLocation

* Properly elaborate errors involving optional properties

* Accept new baselines

* Properly check optional properties in tuple types

* Accept new baselines

* Add missing tuple type check

* More permissive check of strict optional properties and index signatures

* Add tests

* Fix lint issues

* Accept new baselines

Author: ahejlsberg

src/compiler/checker.ts

@@ -612,6 +612,15 @@ namespace ts {
             category: Diagnostics.Strict_Type_Checking_Options,
             description: Diagnostics.Enable_strict_checking_of_property_initialization_in_classes
         },
+        {
+            name: "strictOptionalProperties",
+            type: "boolean",
+            affectsSemanticDiagnostics: true,
+            strictFlag: true,
+            showInSimplifiedHelpView: true,
+            category: Diagnostics.Strict_Type_Checking_Options,
+            description: Diagnostics.Enable_strict_checking_of_optional_properties
+        },
         {
             name: "noImplicitThis",
             type: "boolean",

src/compiler/commandLineParser.ts

@@ -4877,6 +4877,10 @@
         "category": "Message",
         "code": 6242
     },
+    "Enable strict checking of optional properties.": {
+        "category": "Message",
+        "code": 6243
+    },
 
     "Projects to reference": {
         "category": "Message",

src/compiler/diagnosticMessages.json

@@ -3043,6 +3043,9 @@ namespace ts {
             if (options.strictPropertyInitialization && !getStrictOptionValue(options, "strictNullChecks")) {
                 createDiagnosticForOptionName(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "strictPropertyInitialization", "strictNullChecks");
             }
+            if (options.strictOptionalProperties && !getStrictOptionValue(options, "strictNullChecks")) {
+                createDiagnosticForOptionName(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "strictOptionalProperties", "strictNullChecks");
+            }
 
             if (options.isolatedModules) {
                 if (options.out) {

src/compiler/program.ts

@@ -6041,6 +6041,7 @@ namespace ts {
         strictBindCallApply?: boolean;  // Always combine with strict property
         strictNullChecks?: boolean;  // Always combine with strict property
         strictPropertyInitialization?: boolean;  // Always combine with strict property
+        strictOptionalProperties?: boolean;  // Always combine with strict property
         stripInternal?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;

src/compiler/types.ts

@@ -6086,6 +6086,7 @@ namespace ts {
         | "strictFunctionTypes"
         | "strictBindCallApply"
         | "strictPropertyInitialization"
+        | "strictOptionalProperties"
         | "alwaysStrict"
         ;
 

src/compiler/utilities.ts

@@ -2913,6 +2913,7 @@ declare namespace ts {
         strictBindCallApply?: boolean;
         strictNullChecks?: boolean;
         strictPropertyInitialization?: boolean;
+        strictOptionalProperties?: boolean;
         stripInternal?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2913,6 +2913,7 @@ declare namespace ts {
         strictBindCallApply?: boolean;
         strictNullChecks?: boolean;
         strictPropertyInitialization?: boolean;
+        strictOptionalProperties?: boolean;
         stripInternal?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;

tests/baselines/reference/api/typescript.d.ts

@@ -16,7 +16,7 @@ declare class Component<P> {
 
 class C<T> extends Component<{ x?: boolean; } & T> {}
 >C : C<T>
->Component : Component<{ x?: boolean | undefined; } & T>
+>Component : Component<{ x?: boolean; } & T>
 >x : boolean | undefined
 
 const y = new C({foobar: "example"});

tests/baselines/reference/checkJsxIntersectionElementPropsType.types

@@ -10,7 +10,7 @@ declare class Component<P> {
 >context : any
 
     readonly props: Readonly<P> & Readonly<{ children?: {} }>;
->props : Readonly<P> & Readonly<{ children?: {} | undefined; }>
+>props : Readonly<P> & Readonly<{ children?: {}; }>
 >children : {} | undefined
 }
 interface ComponentClass<P = {}> {
@@ -29,7 +29,7 @@ interface ComponentClass<P = {}> {
 }
 interface FunctionComponent<P = {}> {
     (props: P & { children?: {} }, context?: any): {} | null;
->props : P & { children?: {} | undefined; }
+>props : P & { children?: {}; }
 >children : {} | undefined
 >context : any
 >null : null

tests/baselines/reference/circularlyConstrainedMappedTypeContainingConditionalNoInfiniteInstantiationDepth.types

@@ -16,7 +16,7 @@ declare function id3<T extends (x: { foo: any }) => any>(input: T): T;
 
 declare function id4<T extends (x: { foo?: number }) => any>(input: T): T;
 >id4 : <T extends (x: {    foo?: number;}) => any>(input: T) => T
->x : { foo?: number | undefined; }
+>x : { foo?: number; }
 >foo : number | undefined
 >input : T
 
@@ -60,10 +60,10 @@ const f13 = id3(function ({ foo = 42 }) { return foo });
 >foo : any
 
 const f14 = id4(function ({ foo = 42 }) { return foo });
->f14 : ({ foo }: { foo?: number | undefined; }) => number
->id4(function ({ foo = 42 }) { return foo }) : ({ foo }: { foo?: number | undefined; }) => number
->id4 : <T extends (x: { foo?: number | undefined; }) => any>(input: T) => T
->function ({ foo = 42 }) { return foo } : ({ foo }: { foo?: number | undefined; }) => number
+>f14 : ({ foo }: { foo?: number; }) => number
+>id4(function ({ foo = 42 }) { return foo }) : ({ foo }: { foo?: number; }) => number
+>id4 : <T extends (x: { foo?: number; }) => any>(input: T) => T
+>function ({ foo = 42 }) { return foo } : ({ foo }: { foo?: number; }) => number
 >foo : number
 >42 : 42
 >foo : number