checker: fix option mismatch checking on array initializations (fix #20409) (#20415)

vlib/v/checker/containers.v

@@ -212,6 +212,20 @@ fn (mut c Checker) array_init(mut node ast.ArrayInit) ast.Type {
 				c.check_expected(typ, elem_type) or {
 					c.error('invalid array element: ${err.msg()}', expr.pos())
 				}
+				if !elem_type.has_flag(.option)
+					&& (typ.has_flag(.option) || typ.idx() == ast.none_type_idx) {
+					typ_str, elem_type_str := c.get_string_names_of(typ, elem_type)
+					if typ.idx() == ast.none_type_idx {
+						c.error('cannot use `${typ_str}` as `${elem_type_str}`', expr.pos())
+					} else {
+						c.error('cannot use `${typ_str}` as `${elem_type_str}`, it must be unwrapped first',
+							expr.pos())
+					}
+				} else if elem_type.has_flag(.option) && !typ.has_flag(.option)
+					&& typ.idx() != ast.none_type_idx && !expr.is_pure_literal() {
+					typ_str, elem_type_str := c.get_string_names_of(typ, elem_type)
+					c.error('cannot use `${typ_str}` as `${elem_type_str}`', expr.pos())
+				}
 			}
 		}
 		if node.is_fixed {

vlib/v/checker/tests/array_init_element_option_mismatch_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/array_init_element_option_mismatch_err.vv:8:11: error: cannot use `?string` as `string`, it must be unwrapped first
+    6 | fn main() {
+    7 |     str := ?string(none)
+    8 |     _ = ['', str]
+      |              ~~~
+    9 | 
+   10 |     foo := Foo{}
+vlib/v/checker/tests/array_init_element_option_mismatch_err.vv:11:18: error: cannot use `?string` as `string`, it must be unwrapped first
+    9 | 
+   10 |     foo := Foo{}
+   11 |     _ = [foo.a, foo.b]
+      |                     ^
+   12 |     _ = [foo.b, foo.a]
+   13 | }
+vlib/v/checker/tests/array_init_element_option_mismatch_err.vv:12:18: error: cannot use `string` as `?string`
+   10 |     foo := Foo{}
+   11 |     _ = [foo.a, foo.b]
+   12 |     _ = [foo.b, foo.a]
+      |                     ^
+   13 | }

vlib/v/checker/tests/array_init_element_option_mismatch_err.vv

@@ -0,0 +1,13 @@
+struct Foo {
+	a string
+	b ?string
+}
+
+fn main() {
+	str := ?string(none)
+	_ = ['', str]
+
+	foo := Foo{}
+	_ = [foo.a, foo.b]
+	_ = [foo.b, foo.a]
+}