parser: check undefined variable in if guard (#15882)

vlib/v/parser/assign.v

@@ -200,6 +200,9 @@ fn (mut p Parser) check_undefined_variables_by_names(names []string, val ast.Exp
 		ast.PrefixExpr {
 			p.check_undefined_variables_by_names(names, val.right)?
 		}
+		ast.SelectorExpr {
+			p.check_undefined_variables_by_names(names, val.expr)?
+		}
 		ast.StringInterLiteral {
 			for expr_ in val.exprs {
 				p.check_undefined_variables_by_names(names, expr_)?

vlib/v/parser/tests/if_guard_undefined_variable_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/if_guard_undefined_variable_err.vv:4:10: error: undefined variable: `f`
+    2 |
+    3 | fn main() {
+    4 |     if f := f.g.x() {
+      |             ^
+    5 |     }
+    6 | }

vlib/v/parser/tests/if_guard_undefined_variable_err.vv

@@ -0,0 +1,6 @@
+module main
+
+fn main() {
+	if f := f.g.x() {
+	}
+}