checker: check if condition optional (#10921)

vlib/v/checker/checker.v

@@ -6498,9 +6498,10 @@ 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.expr(branch.cond)
-				if cond_typ.idx() != ast.bool_type_idx && !c.pref.translated {
-					typ_sym := c.table.get_type_symbol(cond_typ)
-					c.error('non-bool type `$typ_sym.name` used as if condition', branch.cond.position())
+				if (cond_typ.idx() != ast.bool_type_idx || cond_typ.has_flag(.optional))
+					&& !c.pref.translated {
+					c.error('non-bool type `${c.table.type_to_str(cond_typ)}` used as if condition',
+						branch.cond.position())
 				}
 			}
 		}

vlib/v/checker/tests/if_expr_optional_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/if_expr_optional_err.vv:7:5: error: non-bool type `?bool` used as if condition
+    5 | fn main() {
+    6 | 
+    7 |     if get_bool() {
+      |        ~~~~~~~~~~
+    8 |         println("Using plain lists")
+    9 |     }

vlib/v/checker/tests/if_expr_optional_err.vv

@@ -0,0 +1,10 @@
+fn get_bool() ?bool {
+	return true
+}
+
+fn main() {
+
+	if get_bool() {
+		println("Using plain lists")
+	}
+}