Refine nested types

[osa1]

test(x: Result[[A, B], U32]): Result[[B], U32]
    match x:
        Result.Err(~A): Result.Ok(123u32)
        other: other

Currently this does not type check, but it should.

[osa1]

This is because we don't refine nested types when refining type of a binder:

fir/src/type_checker/pat.rs

Lines 205 to 289 in 6908582

	ast::Pat::Var(var) => {
	let (labels, extension) = match ty.normalize(tc_state.tys.tys.cons()) {
	Ty::Anonymous {
	labels,
	extension,
	kind: RecordOrVariant::Variant,
	is_row,
	} => {
	assert!(!is_row);
	collect_rows(
	tc_state.tys.tys.cons(),
	ty,
	RecordOrVariant::Variant,
	&labels,
	extension.clone(),
	)
	}
	_ => return,
	};
	let num_labels = labels.len();
	let mut unhandled_labels: Map<Id, Ty> =
	Map::with_capacity_and_hasher(num_labels, Default::default());
	'variant_label_loop: for (label, label_ty) in labels {
	let label_field_coverage = match coverage.get_variant_fields(&label) {
	Some(field_coverage) => field_coverage,
	None => {
	unhandled_labels.insert(label, label_ty);
	continue;
	}
	};
	let (label_fields, label_field_extension) =
	match &label_ty.normalize(tc_state.tys.tys.cons()) {
	ty @ Ty::Anonymous {
	labels,
	extension,
	kind,
	is_row,
	} => {
	assert!(!is_row);
	assert_eq!(*kind, RecordOrVariant::Record);
	collect_rows(
	tc_state.tys.tys.cons(),
	ty,
	RecordOrVariant::Record,
	labels,
	extension.clone(),
	)
	}
	_ => return,
	};
	assert!(label_field_extension.is_none());
	for (field_label, field_ty) in label_fields {
	let field_coverage = match label_field_coverage.get_named_field(&field_label) {
	Some(field_coverage) => field_coverage,
	None => {
	unhandled_labels.insert(label, label_ty);
	continue 'variant_label_loop;
	}
	};
	if !field_coverage.is_exhaustive(&field_ty, tc_state, &pat.loc) {
	unhandled_labels.insert(label, label_ty);
	continue 'variant_label_loop;
	}
	}
	}
	if unhandled_labels.len() != num_labels {
	let new_variant = Ty::Anonymous {
	labels: unhandled_labels,
	extension: extension.map(Box::new),
	kind: RecordOrVariant::Variant,
	is_row: false,
	};
	tc_state.env.insert(var.clone(), new_variant);
	}
	}