tests: protect generic sum type equality across method instances

vlib/v/tests/generics/generic_method_sumtype_equality_instantiations_test.v

@@ -0,0 +1,47 @@
+struct EqualityBox[T] {
+mut:
+	items []T
+}
+
+fn (box &EqualityBox[T]) has(value T) bool {
+	for item in box.items {
+		if item == value {
+			return true
+		}
+	}
+	return false
+}
+
+struct EqualityLeaf {}
+
+struct EqualityTwig {
+	n i64
+}
+
+type EqualityTree = EqualityLeaf | EqualityTwig
+
+struct EqualityParam {
+	name         string
+	default_expr &EqualityBox[EqualityTree] = unsafe { nil }
+}
+
+fn test_generic_sumtype_equality_with_an_uncalled_struct_instantiation() {
+	params := EqualityBox[EqualityParam]{}
+	_ = params
+	mut trees := EqualityBox[EqualityTree]{}
+	trees.items << EqualityTree(EqualityLeaf{})
+	assert trees.has(EqualityTree(EqualityLeaf{}))
+	assert !trees.has(EqualityTree(EqualityTwig{ n: 1 }))
+}
+
+fn test_generic_equality_keeps_both_instantiations_independent() {
+	mut trees := EqualityBox[EqualityTree]{}
+	trees.items << EqualityTree(EqualityTwig{ n: 2 })
+	mut params := EqualityBox[EqualityParam]{}
+	param := EqualityParam{ name: 'x' }
+	params.items << param
+	assert params.has(param)
+	assert !params.has(EqualityParam{ name: 'y' })
+	assert trees.has(EqualityTree(EqualityTwig{ n: 2 }))
+	assert !trees.has(EqualityTree(EqualityLeaf{}))
+}