checker: fix error for fn with multi return (#14095)

vlib/v/checker/if.v

@@ -173,6 +173,13 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 					continue
 				}
 				last_expr.typ = c.expr(last_expr.expr)
+				if c.table.type_kind(c.expected_type) == .multi_return
+					&& c.table.type_kind(last_expr.typ) == .multi_return {
+					if node.typ == ast.void_type {
+						node.is_expr = true
+						node.typ = c.expected_type
+					}
+				}
 				if !c.check_types(last_expr.typ, node.typ) {
 					if node.typ == ast.void_type {
 						// first branch of if expression
@@ -210,6 +217,15 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 					if is_noreturn_callexpr(last_expr.expr) {
 						continue
 					}
+					node_sym := c.table.sym(node.typ)
+					last_sym := c.table.sym(last_expr.typ)
+					if node_sym.kind == .multi_return && last_sym.kind == .multi_return {
+						node_types := node_sym.mr_info().types
+						last_types := last_sym.mr_info().types.map(ast.mktyp(it))
+						if node_types == last_types {
+							continue
+						}
+					}
 
 					c.error('mismatched types `${c.table.type_to_str(node.typ)}` and `${c.table.type_to_str(last_expr.typ)}`',
 						node.pos)

vlib/v/checker/match.v

@@ -72,7 +72,7 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 				expr_type := c.expr(stmt.expr)
 				if first_iteration {
 					if node.is_expr && (node.expected_type.has_flag(.optional)
-						|| c.table.type_kind(node.expected_type) == .sum_type) {
+						|| c.table.type_kind(node.expected_type) in [.sum_type, .multi_return]) {
 						ret_type = node.expected_type
 					} else {
 						ret_type = expr_type
@@ -86,6 +86,14 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 							&& (ret_type.has_flag(.generic)
 							|| c.table.is_sumtype_or_in_variant(ret_type, expr_type)))
 							&& !is_noreturn {
+							expr_sym := c.table.sym(expr_type)
+							if expr_sym.kind == .multi_return && ret_sym.kind == .multi_return {
+								ret_types := ret_sym.mr_info().types
+								expr_types := expr_sym.mr_info().types.map(ast.mktyp(it))
+								if expr_types == ret_types {
+									continue
+								}
+							}
 							c.error('return type mismatch, it should be `$ret_sym.name`',
 								stmt.expr.pos())
 						}

vlib/v/tests/fn_multi_return_test.v

@@ -0,0 +1,49 @@
+fn multret1(i int, j int) (int, int) {
+	return if i > j { i, 10 } else { 10, j }
+}
+
+fn multret2(i int, j int) (int, int) {
+	return match i > j {
+		true { i, 10 }
+		false { 10, j }
+	}
+}
+
+fn multret3(i int, j int) (int, int) {
+	if i > j {
+		return i, 10
+	} else {
+		return 10, j
+	}
+}
+
+fn multret4(i int, j int) (int, int) {
+	match i > j {
+		true { return i, 10 }
+		false { return 10, j }
+	}
+}
+
+fn test_fn_multi_return() {
+	mut a, mut b := 0, 0
+
+	println(multret1(3, 14))
+	a, b = multret1(3, 14)
+	assert a == 10
+	assert b == 14
+
+	println(multret2(3, 14))
+	a, b = multret2(3, 14)
+	assert a == 10
+	assert b == 14
+
+	println(multret3(3, 14))
+	a, b = multret3(3, 14)
+	assert a == 10
+	assert b == 14
+
+	println(multret4(3, 14))
+	a, b = multret4(3, 14)
+	assert a == 10
+	assert b == 14
+}