parser: check undefined variable in if guard (#16138)

vlib/v/checker/tests/undefined_ident_in_if_guard_err_a.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/undefined_ident_in_if_guard_err.vv:6:11: error: undefined variable: `re`
+vlib/v/checker/tests/undefined_ident_in_if_guard_err_a.vv:6:11: error: undefined variable: `re`
     4 |
     5 | fn main() {
     6 |     if re := re.regex_opt('a') {

vlib/v/checker/tests/undefined_ident_in_if_guard_err_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/undefined_ident_in_if_guard_err_b.vv:3:17: error: undefined variable: `id`
+    1 | fn main(){
+    2 |     items := {'a': 'A'}
+    3 |     if id := items[id] {} else {}
+      |                    ~~
+    4 | }

vlib/v/checker/tests/undefined_ident_in_if_guard_err_b.vv

@@ -0,0 +1,4 @@
+fn main(){
+	items := {'a': 'A'}
+	if id := items[id] {} else {}
+}

vlib/v/parser/assign.v

@@ -59,6 +59,10 @@ fn (mut p Parser) check_undefined_variables(exprs []ast.Expr, val ast.Expr) ? {
 				p.check_undefined_variables(exprs, arg.expr)?
 			}
 		}
+		ast.IndexExpr {
+			p.check_undefined_variables(exprs, val.left)?
+			p.check_undefined_variables(exprs, val.index)?
+		}
 		ast.InfixExpr {
 			p.check_undefined_variables(exprs, val.left)?
 			p.check_undefined_variables(exprs, val.right)?
@@ -155,6 +159,10 @@ fn (mut p Parser) check_undefined_variables_by_names(names []string, val ast.Exp
 				p.check_undefined_variables_by_names(names, arg.expr)?
 			}
 		}
+		ast.IndexExpr {
+			p.check_undefined_variables_by_names(names, val.left)?
+			p.check_undefined_variables_by_names(names, val.index)?
+		}
 		ast.InfixExpr {
 			p.check_undefined_variables_by_names(names, val.left)?
 			p.check_undefined_variables_by_names(names, val.right)?