checker: add error for .map() calling a fn with multiple return values (prevent inaccessible tuple leak)

vlib/v/checker/fn.v

@@ -1707,6 +1707,10 @@ fn (mut c Checker) array_builtin_method_call(mut node ast.CallExpr, left_type as
 			else { arg_type }
 		}
 		node.return_type = c.table.find_or_register_array(c.unwrap_generic(ret_type))
+		ret_sym := c.table.sym(ret_type)
+		if ret_sym.kind == .multi_return {
+			c.error('returning multiple values is not supported in .map() calls', node.pos)
+		}
 	} else if method_name == 'filter' {
 		// check fn
 		c.check_map_and_filter(false, elem_typ, node)

vlib/v/checker/tests/map_func_return_multiple_values_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/map_func_return_multiple_values_err.vv:6:17: error: returning multiple values is not supported in .map() calls
+    4 | 
+    5 | fn main() {
+    6 |     a := [1, 2, 3].map(fun)
+      |                    ~~~~~~~~
+    7 |     println(a)
+    8 | }

vlib/v/checker/tests/map_func_return_multiple_values_err.vv

@@ -0,0 +1,8 @@
+fn fun(n int) (int, string) {
+	return n, n.str()
+}
+
+fn main() {
+	a := [1, 2, 3].map(fun)
+	println(a)
+}