checker: disallow array append as expression in .map and `.filter methods (#15823)

vlib/v/checker/fn.v

@@ -1944,6 +1944,12 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ ast.Type, node ast
 				c.error('type mismatch, should use e.g. `${node.name}(it > 2)`', arg_expr.pos)
 			}
 		}
+		ast.InfixExpr {
+			if arg_expr.op == .left_shift && arg_expr.is_stmt
+				&& c.table.final_sym(arg_expr.left_type).kind == .array {
+				c.error('array append cannot be used in an expression', arg_expr.pos)
+			}
+		}
 		else {}
 	}
 }

vlib/v/checker/tests/array_filter_map_array_expression_as_argument_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/array_filter_map_array_expression_as_argument_err.vv:24:19: error: array append cannot be used in an expression
+   22 |         for k, v in current.kids {
+   23 |             _ = visited[k] or {
+   24 |                 v.map(horizon << it)
+      |                               ~~
+   25 |                 true
+   26 |             }

vlib/v/checker/tests/array_filter_map_array_expression_as_argument_err.vv

@@ -0,0 +1,30 @@
+struct Graph {
+mut:
+	root   Node
+	forest []Node
+}
+
+struct Node {
+	name string
+mut:
+	kids map[string][]Node
+}
+
+fn (g Graph) pp() {
+	mut visited := map[string]bool{}
+	mut horizon := [g.root]
+
+	for horizon.len > 0 {
+		current := horizon.first()
+		horizon.delete(0)
+		println('$current.name -> $current.kids.keys()')
+		visited[current.name] = true
+		for k, v in current.kids {
+			_ = visited[k] or {
+				v.map(horizon << it)
+				true
+			}
+		}
+		println('HORIZON = $horizon')
+	}
+}