Make isTypeAssignableTo public on TypeChecker (#56448)

src/compiler/types.ts

@@ -5158,7 +5158,20 @@ export interface TypeChecker {
     /** @internal */ getPromiseLikeType(): Type;
     /** @internal */ getAsyncIterableType(): Type | undefined;
 
-    /** @internal */ isTypeAssignableTo(source: Type, target: Type): boolean;
+    /**
+     * Returns true if the "source" type is assignable to the "target" type.
+     *
+     * ```ts
+     * declare const abcLiteral: ts.Type; // Type of "abc"
+     * declare const stringType: ts.Type; // Type of string
+     *
+     * isTypeAssignableTo(abcLiteral, abcLiteral); // true; "abc" is assignable to "abc"
+     * isTypeAssignableTo(abcLiteral, stringType); // true; "abc" is assignable to string
+     * isTypeAssignableTo(stringType, abcLiteral); // false; string is not assignable to "abc"
+     * isTypeAssignableTo(stringType, stringType); // true; string is assignable to string
+     * ```
+     */
+    isTypeAssignableTo(source: Type, target: Type): boolean;
     /** @internal */ createAnonymousType(symbol: Symbol | undefined, members: SymbolTable, callSignatures: Signature[], constructSignatures: Signature[], indexInfos: IndexInfo[]): Type;
     /** @internal */ createSignature(
         declaration: SignatureDeclaration | undefined,

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

@@ -6832,6 +6832,20 @@ declare namespace ts {
          * is `never`. Instead, use `type.flags & TypeFlags.Never`.
          */
         getNeverType(): Type;
+        /**
+         * Returns true if the "source" type is assignable to the "target" type.
+         *
+         * ```ts
+         * declare const abcLiteral: ts.Type; // Type of "abc"
+         * declare const stringType: ts.Type; // Type of string
+         *
+         * isTypeAssignableTo(abcLiteral, abcLiteral); // true; "abc" is assignable to "abc"
+         * isTypeAssignableTo(abcLiteral, stringType); // true; "abc" is assignable to string
+         * isTypeAssignableTo(stringType, abcLiteral); // false; string is not assignable to "abc"
+         * isTypeAssignableTo(stringType, stringType); // true; string is assignable to string
+         * ```
+         */
+        isTypeAssignableTo(source: Type, target: Type): boolean;
         /**
          * True if this type is the `Array` or `ReadonlyArray` type from lib.d.ts.
          * This function will _not_ return true if passed a type which