Make expectError support TS2820 error introduced in TypeScript 4.5 (#139)

Author: aqrln

Diff for: source/lib/compiler.ts

@@ -37,6 +37,7 @@ const expectErrordiagnosticCodesToIgnore = new Set<DiagnosticCode>([
 	DiagnosticCode.NewExpressionTargetLackingConstructSignatureHasAnyType,
 	DiagnosticCode.MemberCannotHaveOverrideModifierBecauseItIsNotDeclaredInBaseClass,
 	DiagnosticCode.MemberMustHaveOverrideModifier,
+	DiagnosticCode.StringLiteralTypeIsNotAssignableToUnionTypeWithSuggestion,
 ]);
 
 type IgnoreDiagnosticResult = 'preserve' | 'ignore' | Location;

Diff for: source/lib/interfaces.ts

@@ -43,6 +43,7 @@ export enum DiagnosticCode {
 	PropertyMissingInType1ButRequiredInType2 = 2741,
 	NoOverloadExpectsCountOfTypeArguments = 2743,
 	NoOverloadMatches = 2769,
+	StringLiteralTypeIsNotAssignableToUnionTypeWithSuggestion = 2820,
 	MemberCannotHaveOverrideModifierBecauseItIsNotDeclaredInBaseClass = 4113,
 	MemberMustHaveOverrideModifier = 4114,
 	NewExpressionTargetLackingConstructSignatureHasAnyType = 7009,

Diff for: source/test/fixtures/expect-error/values/index.d.ts

@@ -18,3 +18,9 @@ export function atLeastOne(...expected: [unknown, ...Array<unknown>]): void;
 export interface Options<T> {}
 
 export class MyClass {}
+
+export const triggerSuggestion: {
+	// fooOrBar must be of union type to trigger TS2820, otherwise TypeScript will
+	// emit a regular TS2322 error without the "Did you mean..." suggestion.
+	fooOrBar: 'foo' | 'bar';
+};

Diff for: source/test/fixtures/expect-error/values/index.test-d.ts

@@ -1,5 +1,5 @@
 import {expectError} from '../../../..';
-import {default as one, atLeastOne, foo, getFoo, HasKey, hasProperty, MyClass, Options} from '.';
+import {default as one, atLeastOne, foo, getFoo, HasKey, hasProperty, MyClass, Options, triggerSuggestion} from '.';
 
 expectError<string>(1);
 expectError<string>('fo');
@@ -33,3 +33,7 @@ expectError(MyClass());
 
 // 'new' expression, whose target lacks a construct signature, implicitly has an 'any' type.
 expectError(new hasProperty({name: 'foo'}));
+
+expectError(() => {
+	triggerSuggestion.fooOrBar = 'fooo';
+})