The ifDefined directive should return a non-null type, not just non-undefined.

[bicknellr]

The return type of ifDefined is determined with a special rule here:

lit-analyzer/packages/lit-analyzer/src/lib/rules/util/directive/get-directive.ts

Lines 45 to 63 in 92c084d

	case "ifDefined": {
	// Example: html`<img src="${ifDefined(imageUrl)}">`;
	// Take the argument to ifDefined and remove undefined from the type union (if possible).
	// This new type becomes the actual type of the expression
	const actualType = lazy(() => {
	if (args.length >= 1) {
	const returnType = toSimpleType(checker.getTypeAtLocation(args[0]), checker);
	return removeUndefinedFromType(returnType);
	}
	return undefined;
	});
	return {
	kind: "ifDefined",
	actualType,
	args
	};
	}

But the analyzer doesn't realize that the return type can't be null in Lit 2, because it only removes undefined (matching Lit 1's behavior):

lit-analyzer/packages/lit-analyzer/src/lib/rules/util/directive/get-directive.ts

Line 52 in 92c084d

return removeUndefinedFromType(returnType);

[bicknellr]

I think it would be better if Lit 2's ifDefined didn't run through this path at all, since it doesn't have any special semantics, but TS seems to be returning the wrong type from checker.getTypeAtLocation(assignment.expression) during this stack:

lit-analyzer/packages/lit-analyzer/src/lib/rules/no-nullable-attribute-binding.ts

Line 24 in 92c084d

const { typeB } = extractBindingTypes(assignment, context);

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

Line 32 in 92c084d

const typeBInferred = inferTypeFromAssignment(assignment, checker);

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

Line 69 in 92c084d

return checker.getTypeAtLocation(assignment.expression);

in the new test:

lit-analyzer/packages/lit-analyzer/src/test/rules/no-nullable-attribute-binding.ts

Lines 29 to 35 in 942db29

	tsTest("Return type of `ifDefined` is not `null`", t => {
	const { diagnostics } = getDiagnostics(`
	const ifDefined = <T>(x: T) => x ?? "non-null";
	html\`<input some-attribute="$\{ifDefined(Math.random() < 0.5 ? 123 : null)}" />\`;
	`);
	hasNoDiagnostics(t, diagnostics);
	});

assignment.expression is the Node with text ifDefined(Math.random() < 0.5 ? 123 : null) but the result of checker.getTypeAtLocation(assignment.expression) gives the type "non-null" | 123 | null despite null not being possible. Not sure what's going on there yet.