Interestnig brand type

Author: weswigham

src/compiler/core.ts

@@ -1710,7 +1710,7 @@ namespace ts {
         return moduleResolution;
     }
 
-    export function getStrictOptionValue(compilerOptions: CompilerOptions, flag: "noImplicitAny" | "noImplicitThis" | "strictNullChecks" | "strictFunctionTypes" | "alwaysStrict"): boolean {
+    export function getStrictOptionValue(compilerOptions: CompilerOptions, flag: keyof StrictOptions): boolean {
         return compilerOptions[flag] === undefined ? compilerOptions.strict : compilerOptions[flag];
     }
 

src/compiler/types.ts

@@ -3676,14 +3676,22 @@ namespace ts {
 
     export type CompilerOptionsValue = string | number | boolean | (string | number)[] | string[] | MapLike<string[]> | PluginImport[] | null | undefined;
 
-    export interface CompilerOptions {
+    export interface StrictOptions {
+        noImplicitAny?: boolean;
+        noImplicitThis?: boolean;
+        strictNullChecks?: boolean;
+        strictFunctionTypes?: boolean;
+        alwaysStrict?: boolean;
+    }
+
+    export interface CompilerOptions extends StrictOptions {
         /*@internal*/ all?: boolean;
         allowJs?: boolean;
         /*@internal*/ allowNonTsExtensions?: boolean;
         allowSyntheticDefaultImports?: boolean;
         allowUnreachableCode?: boolean;
         allowUnusedLabels?: boolean;
-        alwaysStrict?: boolean;  // Always combine with strict property
+        /*@internal*/ alwaysStrict?: never & boolean;  // Always combine with strict property
         baseUrl?: string;
         charset?: string;
         checkJs?: boolean;
@@ -3722,9 +3730,9 @@ namespace ts {
         noEmitOnError?: boolean;
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
-        noImplicitAny?: boolean;  // Always combine with strict property
+        /*@internal*/ noImplicitAny?: never & boolean;  // Always combine with strict property
         noImplicitReturns?: boolean;
-        noImplicitThis?: boolean;  // Always combine with strict property
+        /*@internal*/ noImplicitThis?: never & boolean;  // Always combine with strict property
         noStrictGenericChecks?: boolean;
         noUnusedLocals?: boolean;
         noUnusedParameters?: boolean;
@@ -3750,8 +3758,8 @@ namespace ts {
         sourceMap?: boolean;
         sourceRoot?: string;
         strict?: boolean;
-        strictFunctionTypes?: boolean;  // Always combine with strict property
-        strictNullChecks?: boolean;  // Always combine with strict property
+        /*@internal*/ strictFunctionTypes?: never & boolean;  // Always combine with strict property
+        /*@internal*/ strictNullChecks?: never & boolean;  // Always combine with strict property
         /* @internal */ stripInternal?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;