checker: fix non-bool check on use of result bool (fix #15539) (#15540)

vlib/v/checker/if.v

@@ -47,8 +47,8 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 				// check condition type is boolean
 				c.expected_type = ast.bool_type
 				cond_typ := c.unwrap_generic(c.expr(branch.cond))
-				if (cond_typ.idx() != ast.bool_type_idx || cond_typ.has_flag(.optional))
-					&& !c.pref.translated && !c.file.is_translated {
+				if (cond_typ.idx() != ast.bool_type_idx || cond_typ.has_flag(.optional)
+					|| cond_typ.has_flag(.result)) && !c.pref.translated && !c.file.is_translated {
 					c.error('non-bool type `${c.table.type_to_str(cond_typ)}` used as if condition',
 						branch.cond.pos())
 				}

vlib/v/checker/tests/if_expr_optional_err.out

@@ -1,7 +1,7 @@
-vlib/v/checker/tests/if_expr_optional_err.vv:7:5: error: non-bool type `?bool` used as if condition
+vlib/v/checker/tests/if_expr_optional_err.vv:6:5: error: non-bool type `?bool` used as if condition
+    4 |
     5 | fn main() {
-    6 | 
-    7 |     if get_bool() {
+    6 |     if get_bool() {
       |        ~~~~~~~~~~
-    8 |         println("Using plain lists")
-    9 |     }
+    7 |         println("Using plain lists")
+    8 |     }

vlib/v/checker/tests/if_expr_optional_err.vv

@@ -3,7 +3,6 @@ fn get_bool() ?bool {
 }
 
 fn main() {
-
 	if get_bool() {
 		println("Using plain lists")
 	}

vlib/v/checker/tests/if_expr_result_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/if_expr_result_err.vv:6:5: error: non-bool type `!bool` used as if condition
+    4 |
+    5 | fn main() {
+    6 |     if get_bool() {
+      |        ~~~~~~~~~~
+    7 |         println("Using plain lists")
+    8 |     }

vlib/v/checker/tests/if_expr_result_err.vv

@@ -0,0 +1,9 @@
+fn get_bool() !bool {
+	return true
+}
+
+fn main() {
+	if get_bool() {
+		println("Using plain lists")
+	}
+}