checker,cgen: allow result if guard (#14474)

vlib/v/checker/checker.v

@@ -2150,25 +2150,25 @@ pub fn (mut c Checker) expr(node_ ast.Expr) ast.Type {
 			c.inside_if_guard = true
 			node.expr_type = c.expr(node.expr)
 			c.inside_if_guard = old_inside_if_guard
-			if !node.expr_type.has_flag(.optional) {
+			if !node.expr_type.has_flag(.optional) && !node.expr_type.has_flag(.result) {
-				mut no_opt := true
+				mut no_opt_or_res := true
 				match mut node.expr {
 					ast.IndexExpr {
-						no_opt = false
+						no_opt_or_res = false
 						node.expr_type = node.expr_type.set_flag(.optional)
 						node.expr.is_option = true
 					}
 					ast.PrefixExpr {
 						if node.expr.op == .arrow {
-							no_opt = false
+							no_opt_or_res = false
 							node.expr_type = node.expr_type.set_flag(.optional)
 							node.expr.is_option = true
 						}
 					}
 					else {}
 				}
-				if no_opt {
+				if no_opt_or_res {
-					c.error('expression should return an option', node.expr.pos())
+					c.error('expression should either return an option or a result', node.expr.pos())
 				}
 			}
 			return ast.bool_type
@@ -2676,7 +2676,7 @@ pub fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 									}
 								}
 							} else {
-								typ = obj.expr.expr_type.clear_flag(.optional)
+								typ = obj.expr.expr_type.clear_flag(.optional).clear_flag(.result)
 							}
 						} else {
 							typ = c.expr(obj.expr)

vlib/v/checker/containers.v

@@ -80,7 +80,7 @@ pub fn (mut c Checker) array_init(mut node ast.ArrayInit) ast.Type {
 		// }
 		array_info := type_sym.array_info()
 		node.elem_type = array_info.elem_type
-		// clear optional flag incase of: `fn opt_arr ?[]int { return [] }`
+		// clear optional flag incase of: `fn opt_arr() ?[]int { return [] }`
 		return if c.expected_type.has_flag(.shared_f) {
 			c.expected_type.clear_flag(.shared_f).deref()
 		} else {

vlib/v/checker/tests/expression_should_return_an_option.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/expression_should_return_an_option.vv:28:10: error: expression should return an option
+vlib/v/checker/tests/expression_should_return_an_option.vv:28:10: error: expression should either return an option or a result
    26 |     }
    27 |     // should be an checker error:
    28 |     if x := return_string() {

vlib/v/gen/c/if.v

@@ -132,11 +132,19 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 				var_name = g.new_tmp_var()
 				guard_vars[i] = var_name // for `else`
 				g.tmp_count--
+				if branch.cond.expr_type.has_flag(.optional) {
 					g.writeln('if (${var_name}.state == 0) {')
+				} else if branch.cond.expr_type.has_flag(.result) {
+					g.writeln('if (!${var_name}.is_error) {')
+				}
 			} else {
 				g.write('if ($var_name = ')
 				g.expr(branch.cond.expr)
+				if branch.cond.expr_type.has_flag(.optional) {
 					g.writeln(', ${var_name}.state == 0) {')
+				} else if branch.cond.expr_type.has_flag(.result) {
+					g.writeln(', !${var_name}.is_error) {')
+				}
 			}
 			if short_opt || branch.cond.vars[0].name != '_' {
 				base_type := g.base_type(branch.cond.expr_type)

vlib/v/tests/results_test.v

@@ -81,3 +81,16 @@ fn return_reference_type(path string) !&string {
 	str := ''
 	return &str
 }
+
+fn read() !string {
+	return ''
+}
+
+fn test_results_if_guard() {
+	if fcontent := read() {
+		assert fcontent == ''
+		assert '$fcontent' == ''
+		return
+	}
+	assert false
+}