checker: check error for array of sumtype appendding (#13593)

vlib/v/checker/checker.v

@@ -845,6 +845,22 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 							right_pos)
 					}
 					return ast.void_type
+				} else if left_value_sym.kind == .sum_type {
+					if right_final.kind != .array {
+						if left_value_type.idx() != right_type.idx()
+							&& !c.table.sumtype_has_variant(left_value_type, right_type, false) {
+							c.error('cannot append `$right_sym.name` to `$left_sym.name`',
+								right_pos)
+						}
+					} else {
+						right_value_type := c.table.value_type(right_type)
+						if left_value_type.idx() != right_value_type.idx()
+							&& !c.table.sumtype_has_variant(left_value_type, right_value_type, false) {
+							c.error('cannot append `$right_sym.name` to `$left_sym.name`',
+								right_pos)
+						}
+					}
+					return ast.void_type
 				}
 				// []T << T or []T << []T
 				unwrapped_right_type := c.unwrap_generic(right_type)

vlib/v/checker/containers.v

@@ -110,7 +110,17 @@ pub fn (mut c Checker) array_init(mut node ast.ArrayInit) ast.Type {
 				continue
 			} else if expecting_sumtype_array {
 				if i == 0 {
-					elem_type = expected_value_type
+					if expected_value_type.idx() == typ.idx()
+						|| c.table.sumtype_has_variant(expected_value_type, typ, false) {
+						elem_type = expected_value_type
+					} else {
+						if expr.is_auto_deref_var() {
+							elem_type = ast.mktyp(typ.deref())
+						} else {
+							elem_type = ast.mktyp(typ)
+						}
+					}
+					c.expected_type = elem_type
 				}
 				continue
 			}

vlib/v/checker/tests/array_of_sumtype_append_err.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/array_of_sumtype_append_err.vv:9:9: error: cannot append `[]string` to `[]Bar`
+    7 | fn main() {
+    8 |     mut bar := []Bar{}
+    9 |     bar << ['hey']
+      |            ~~~~~~~
+   10 |     bar << 'hey'
+   11 | }
+vlib/v/checker/tests/array_of_sumtype_append_err.vv:10:9: error: cannot append `string` to `[]Bar`
+    8 |     mut bar := []Bar{}
+    9 |     bar << ['hey']
+   10 |     bar << 'hey'
+      |            ~~~~~
+   11 | }

vlib/v/checker/tests/array_of_sumtype_append_err.vv

@@ -0,0 +1,11 @@
+type Bar = BarA | BarB
+
+struct BarA {}
+
+struct BarB {}
+
+fn main() {
+	mut bar := []Bar{}
+	bar << ['hey']
+	bar << 'hey'
+}