checker: check if unwrapped m[key] if m is Option

vlib/v/checker/checker.v

@@ -4914,6 +4914,9 @@ fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 		} else if node.left is ast.CallExpr {
 			c.error('type `?${typ_sym.name}` is an Option, it must be unwrapped with `func()?`, or use `func() or {default}`',
 				node.left.pos())
+		} else if node.left is ast.SelectorExpr && node.left.or_block.kind == .absent {
+			c.error('type `?${typ_sym.name}` is an Option, it must be unwrapped first; use `struct.var?[]` to do it',
+				node.left.pos())
 		}
 	} else if typ.has_flag(.result) {
 		c.error('type `!${typ_sym.name}` is a Result, it does not support indexing', node.left.pos())

vlib/v/checker/tests/option_map_err.out

@@ -1,3 +1,9 @@
+vlib/v/checker/tests/option_map_err.vv:22:13: error: type `?map[string]f64` is an Option, it must be unwrapped first; use `struct.var?[]` to do it
+   20 |         }
+   21 |     }
+   22 |     assert op2.opmap['1'] == 1.0
+      |                ~~~~~
+   23 | }
 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 |     }

vlib/v/checker/tests/selector_expr_opt_map_get_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/selector_expr_opt_map_get_err.vv:8:9: error: type `?map[string]string` is an Option, it must be unwrapped first; use `struct.var?[]` to do it
+    6 | fn main() {
+    7 |     x := Abc{}
+    8 |     _ := x.m['test'] or { 'test' }
+      |            ^
+    9 | }

vlib/v/checker/tests/selector_expr_opt_map_get_err.vv

@@ -0,0 +1,9 @@
+struct Abc {
+mut:
+	m ?map[string]string
+}
+
+fn main() {
+	x := Abc{}
+	_ := x.m['test'] or { 'test' }
+}