checker: fix `&` on pointers (#6787)

vlib/v/checker/checker.v

@@ -2856,6 +2856,8 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 					}
 				}
 				return right_type.to_ptr()
+			} else if node.op == .amp && node.right !is ast.CastExpr {
+				return right_type.to_ptr()
 			}
 			if node.op == .mul {
 				if right_type.is_ptr() {

vlib/v/tests/unsafe_test.v

@@ -14,6 +14,26 @@ fn test_ptr_assign() {
 	assert v[3] == 31
 }
 
+fn test_double_ptr() {
+	i := 5
+	j := 7
+	unsafe {
+		mut x := &i
+		mut p := &x
+		(*p) = &j
+		assert x == &j
+	}
+
+	/////////
+
+	mut x := &int(0)
+	unsafe {
+		mut p := &x
+		(*p) = &int(1)
+	}
+	assert ptr_str(x) == ptr_str(&int(1))
+}
+
 fn test_ptr_infix() {
 	v := 4
 	mut q := unsafe {&v - 1}