checker: fix detection of invalid insertions, fixes #3600 (#11945)

vlib/v/checker/checker.v

@@ -1586,8 +1586,13 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 				}
 				// []T << T or []T << []T
 				unwrapped_right_type := c.unwrap_generic(right_type)
-				if c.check_types(unwrapped_right_type, left_value_type)
-					|| c.check_types(unwrapped_right_type, c.unwrap_generic(left_type)) {
+				if c.check_types(unwrapped_right_type, left_value_type) {
+					// []&T << T is wrong: we check for that, !(T.is_ptr()) && ?(&T).is_ptr()
+					if !(!unwrapped_right_type.is_ptr() && left_value_type.is_ptr()
+						&& left_value_type.share() == .mut_t) {
+						return ast.void_type
+					}
+				} else if c.check_types(unwrapped_right_type, c.unwrap_generic(left_type)) {
 					return ast.void_type
 				}
 				c.error('cannot append `$right_sym.name` to `$left_sym.name`', right_pos)

vlib/v/checker/tests/invalid_insert_references_test.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/invalid_insert_references_test.vv:8:7: error: cannot append `int literal` to `[]&int`
+    6 |     a << &c
+    7 |     c = 2
+    8 |     a << 1
+      |          ^
+    9 |     println(a)
+   10 | }

vlib/v/checker/tests/invalid_insert_references_test.vv

@@ -0,0 +1,14 @@
+// fixes https://github.com/vlang/v/issues/3600, test based on a simplified version of example by https://github.com/radare
+fn test_invalid_insert_references() {
+	b := 0
+	mut a := [&b]
+	mut c := 1
+	a << &c
+	c = 2
+	a << 1
+	println(a)
+}
+
+fn main() {
+	test_invalid_insert_references()
+}