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

Author: spytheman

doc/docs.md

@@ -1064,10 +1064,10 @@ the `it` built-in variable to achieve a classic `map/filter` functional paradigm
 ```v
 // using filter, map and negatives array slices
 a := ['pippo.jpg', '01.bmp', '_v.txt', 'img_02.jpg', 'img_01.JPG']
-res := a.filter(it#[-4..].to_lower() == '.jpg').map(fn (w string) (string, int) {
-	return w.to_upper(), w.len
+res := a.filter(it#[-4..].to_lower() == '.jpg').map(fn (w string) string {
+	return w.to_upper()
 })
-// [('PIPPO.JPG', 9), ('IMG_02.JPG', 10), ('IMG_01.JPG', 10)]
+// ['PIPPO.JPG', 'IMG_02.JPG', 'IMG_01.JPG']
 ```
 
 ### Fixed size arrays

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)
+}