checker: fix missing checker for option map indexing

vlib/v/checker/checker.v

@@ -1278,6 +1278,15 @@ fn (mut c Checker) check_expr_option_or_result_call(expr ast.Expr, ret_type ast.
 					c.check_or_expr(expr.or_expr, ret_type, ret_type.set_flag(.result),
 						expr)
 				}
+			} else if expr.left is ast.SelectorExpr && expr.left_type.has_option_or_result() {
+				with_modifier_kind := if expr.left_type.has_flag(.option) {
+					'an Option'
+				} else {
+					'a Result'
+				}
+				with_modifier := if expr.left_type.has_flag(.option) { '?' } else { '!' }
+				c.error('field `${expr.left.field_name}` is ${with_modifier_kind}, so it should have either an `or {}` block, or `${with_modifier}` at the end',
+					expr.left.pos)
 			}
 		}
 		ast.CastExpr {

vlib/v/checker/tests/option_map_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/option_map_err.vv:22:13: error: field `opmap` is an Option, so it should have either an `or {}` block, or `?` at the end
+   20 |         }
+   21 |     }
+   22 |     assert op2.opmap['1'] == 1.0
+      |                ~~~~~
+   23 | }

vlib/v/checker/tests/option_map_err.vv

@@ -0,0 +1,23 @@
+struct OptionalMap {
+	opmap ?map[string]f64
+}
+
+fn main() {
+	op := OptionalMap{
+		opmap: {
+			'1': 0.0
+		}
+	}
+	assert op.opmap or {
+		{
+			'1': 0.0
+		}
+	}['1'] == 0.0
+
+	op2 := OptionalMap{
+		opmap: {
+			'1': 1.0
+		}
+	}
+	assert op2.opmap['1'] == 1.0
+}