checker: add error for x as Y.field, suggesting using `(x as Y).fie…

…ld` instead for clarity (#20725)

vlib/v/checker/checker.v

@@ -1436,6 +1436,10 @@ fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 			c.expr(mut node_expr)
 			name_type = node.expr.typ
 		}
+		ast.AsCast {
+			c.add_error_detail('for example `(${node.expr.expr} as ${c.table.type_to_str(node.expr.typ)}).${node.field_name}`')
+			c.error('indeterminate `as` cast, use parenthesis to clarity', node.expr.pos)
+		}
 		else {}
 	}
 	if name_type > 0 {

vlib/v/checker/tests/as_cast_selector_expr_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/as_cast_selector_expr_err.vv:15:14: error: indeterminate `as` cast, use parenthesis to clarity
+   13 | fn main() {
+   14 |     mut bar := Foobar(Bar{y: 123})
+   15 |     println(bar as Bar.y == 123)
+      |                 ~~
+   16 | }
+Details: for example `(bar as main.Bar).y`

vlib/v/checker/tests/as_cast_selector_expr_err.vv

@@ -0,0 +1,16 @@
+struct Foo {
+mut:
+	x int
+}
+
+struct Bar {
+mut:
+	y int
+}
+
+type Foobar = Foo | Bar
+
+fn main() {
+	mut bar := Foobar(Bar{y: 123})
+	println(bar as Bar.y == 123)
+}

vlib/v/tests/as_cast_selector_test.v

@@ -14,5 +14,5 @@ fn test_main() {
 	mut bar := Foobar(Bar{
 		y: 123
 	})
-	assert bar as Bar.y == 123
+	assert (bar as Bar).y == 123
 }