checker: disallow indexing generic struct receivers

[Delta456]

Fixes #18707

🤖[deprecated] Generated by Copilot at 5ce04ba

Add a check and a test case for invalid index expressions on generic types. This fixes a compiler error or a runtime panic when trying to access a generic type by index that is not a map, array, fixed array, or gated type.

🤖[deprecated] Generated by Copilot at 5ce04ba

• Add a check for invalid index expressions on generic types that are not maps, arrays, fixed arrays, or gated types (link)
• Add a test case and source code for the error message that is expected when using an invalid index expression on a generic heap struct (link, link)

[spytheman]

Suggested change

	c.error('cannot index a generic', node.pos)
	c.error('cannot index a generic struct', node.pos)

[spytheman]

imho, adding a test like this, will also be good:

struct Number[T] {
	value T
}

fn f[T](numbers &Number[T], idx int) T {
	return unsafe { numbers[idx].value }
}

fn test_indexing_a_pointer_to_generic_instances() {
	numbers := [10]Number[int]{init: Number[int]{
		value: index * 10
	}}
	assert f(&numbers[0], 3) == 30
	assert f(&numbers[0], 9) == 90
}

It is a useful thing to have, and it compiles and runs on master, but fails with the current implementation of this PR.

[spytheman]

Closing in favor of #19949 .
Thanks @Delta456 for noticing it 🙇🏻‍♂️ . I have missed that the check there is more general.