checker: add checks for optional selector_expr (#8330)

vlib/v/checker/checker.v

@@ -2077,6 +2077,11 @@ pub fn (mut c Checker) selector_expr(mut selector_expr ast.SelectorExpr) table.T
 		return table.void_type
 	}
 	selector_expr.expr_type = typ
+	if selector_expr.expr_type.has_flag(.optional) && !((selector_expr.expr is ast.Ident
+		&& (selector_expr.expr as ast.Ident).kind == .constant)) {
+		c.error('cannot access fields of an optional, handle the error with `or {...}` or propagate it with `?`',
+			selector_expr.pos)
+	}
 	field_name := selector_expr.field_name
 	utyp := c.unwrap_generic(typ)
 	sym := c.table.get_type_symbol(utyp)

vlib/v/checker/tests/selector_expr_optional_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/selector_expr_optional_err.vv:4:46: error: cannot access fields of an optional, handle the error with `or {...}` or propagate it with `?`
+    2 | 
+    3 | fn main() {
+    4 |     println(http.get('https://httpbin.org/').status_code)
+      |                                              ~~~~~~~~~~~
+    5 | }

vlib/v/checker/tests/selector_expr_optional_err.vv

@@ -0,0 +1,5 @@
+import net.http
+
+fn main() {
+    println(http.get('https://httpbin.org/').status_code)
+}