Fix references to this in default arguments of class/object methods

game-scripts-tests/this_in_default_argument.ts

@@ -0,0 +1,35 @@
+export default ({ expect }: typeof import('vitest')) => {
+	class Foo {
+		declare foo: number
+
+		constructor() {
+			this.foo = 1
+		}
+
+		bar() {
+			return 2
+		}
+
+		baz(a = this.foo, b = this.bar()) {
+			return a + b
+		}
+	}
+
+	const obj = {
+		foo: 1,
+		bar() {
+			return 2
+		},
+		baz(a = this.foo, b = this.bar()) {
+			return a + b
+		}
+	}
+
+	const foo = (new Foo)
+
+	expect(foo.baz(3, 4)).toBe(7) // Is this the real life?
+	expect(foo.baz()).toBe(3) // Is this just fantasy?
+
+	expect(obj.baz(3, 4)).toBe(7) // Caught in a landslide
+	expect(obj.baz()).toBe(3) // No escape from reality
+}

src/processScript/transform.ts

@@ -821,6 +821,19 @@ export function transform(
 
 				let methodReferencesThis = false as boolean
 
+				for (const param of classMethod.params) {
+					traverse(param, {
+						ThisExpression(path) {
+							// We can't reference _abc_THIS_ here, because there would be
+							// nowhere to place the assignment to that!
+							// Hence, we just do the super thing directly.
+							path.replaceWith(t.callExpression(
+								t.memberExpression(t.super(), t.identifier(`valueOf`)), []
+							))
+						}
+					}, scope)
+				}
+
 				traverse(classMethod.body, {
 					ThisExpression(path) {
 						methodReferencesThis = true