checker: prohibit option and `IError` in multi-return (#10689)

vlib/v/checker/checker.v

@@ -7458,6 +7458,18 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 				}
 			}
 		}
+		return_sym := c.table.get_type_symbol(node.return_type)
+		if return_sym.info is ast.MultiReturn {
+			for multi_type in return_sym.info.types {
+				if multi_type == ast.error_type {
+					c.error('type `IError` cannot be used in multi-return, return an option instead',
+						node.return_type_pos)
+				} else if multi_type.has_flag(.optional) {
+					c.error('option cannot be used in multi-return, return an option instead',
+						node.return_type_pos)
+				}
+			}
+		}
 	}
 	if node.return_type == ast.void_type {
 		for mut a in node.attrs {

vlib/v/checker/tests/ierror_in_return_tuple.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/ierror_in_return_tuple.vv:1:29: error: type `IError` cannot be used in multi-return, return an option instead
+    1 | fn return_ierror_in_tuple() (bool, IError) {
+      |                             ~~~~~~~~~~~~~~
+    2 |     return false, error('')
+    3 | }
+vlib/v/checker/tests/ierror_in_return_tuple.vv:5:29: error: option cannot be used in multi-return, return an option instead
+    3 | }
+    4 | 
+    5 | fn return_option_in_tuple() (bool, ?bool) {
+      |                             ~~~~~~~~~~~~~
+    6 |     return false, error('')
+    7 | }

vlib/v/checker/tests/ierror_in_return_tuple.vv

@@ -0,0 +1,7 @@
+fn return_ierror_in_tuple() (bool, IError) {
+	return false, error('')
+}
+
+fn return_option_in_tuple() (bool, ?bool) {
+	return false, error('')
+}