Exclude symbols when checking binding types

packages/lit-analyzer/src/lib/rules/util/type/extract-binding-types.ts

@@ -50,6 +50,10 @@ export function extractBindingTypes(assignment: HtmlNodeAttrAssignment, context:
 		typeB = directiveType;
 	}
 
+	// Handle `nothing` and `noChange` symbols
+	// Since it's not possible to check the details of a symbol due to various restrictions (it's treated as a `unique symbol` or `Symbol()`), all symbols are excluded.
+	typeB = excludeSymbolsFromUnion(typeB);
+
 	// Cache the result
 	const result = { typeA, typeB };
 	cache.set(assignment, result);
@@ -82,6 +86,17 @@ export function inferTypeFromAssignment(assignment: HtmlNodeAttrAssignment, chec
 	}
 }
 
+function excludeSymbolsFromUnion(type: SimpleType): SimpleType {
+	if (type.kind !== "UNION") {
+		return type;
+	}
+
+	return {
+		...type,
+		types: type.types.filter(t => t.kind !== "ES_SYMBOL" && t.kind !== "ES_SYMBOL_UNIQUE")
+	};
+}
+
 /**
  * Relax the type so that for example "string literal" become "string" and "function" become "any"
  * This is used for javascript files to provide type checking with Typescript type inferring

packages/lit-analyzer/src/test/helpers/compile-files.ts

@@ -105,7 +105,17 @@ export function compileFiles(inputFiles: TestFile[] | TestFile = []): { program:
 
 	const program = ts.createProgram({
 		//rootNames: [...files.map(file => file.fileName!), "node_modules/typescript/lib/lib.dom.d.ts"],
-		rootNames: [...files.map(file => file.fileName!), ...(includeLib ? ["node_modules/typescript/lib/lib.dom.d.ts"] : [])],
+		rootNames: [
+			...files.map(file => file.fileName!),
+			...(includeLib
+				? [
+						"node_modules/typescript/lib/lib.dom.d.ts",
+						// We need this to enable `Symbol`.
+						// We cannot use "esnext" since it cannot be loaded by TS 4.8.4.
+						"node_modules/typescript/lib/lib.es2015.d.ts"
+				  ]
+				: [])
+		],
 		//rootNames: files.map(file => file.fileName!),
 		options: compilerOptions,
 		host: compilerHost

packages/lit-analyzer/src/test/rules/no-incompatible-type-binding.ts

@@ -293,3 +293,39 @@ tsTest("Attribute binding: the target attribute is correctly type checked when g
 
 	hasNoDiagnostics(t, diagnostics);
 });
+
+tsTest("Attribute binding: any symbols are ignored on type checking", t => {
+	const { diagnostics } = getDiagnostics(`
+declare const value: boolean | unique symbol;
+
+html\`<div aria-expanded=\${userInput}></div>\`
+	`);
+
+	hasNoDiagnostics(t, diagnostics);
+});
+
+tsTest("Attribute binding: symbols are not treated as any type", t => {
+	const { diagnostics } = getDiagnostics(`
+declare const value: "invalid" | unique symbol;
+
+html\`<div aria-expanded=\${value}></div>\`
+	`);
+
+	hasDiagnostic(t, diagnostics, "no-incompatible-type-binding");
+});
+
+tsTest("Attribute binding: lit's nothing is ignored on type checking when returned by a function", t => {
+	const { diagnostics } = getDiagnostics(`
+declare const nothing: unique symbol;
+
+function customIfDef<T>(value: T | null | undefined): T | typeof nothing {
+	return value ?? nothing;
+}
+
+declare const value: boolean | null;
+
+html\`<div aria-expanded=\${customIfDef(value)}></div>\`
+	`);
+
+	hasNoDiagnostics(t, diagnostics);
+});