Fix memoryview.cast over sliced memoryview

Fixes #4758

Author: tannewt

ports/unix/main.c

@@ -716,7 +716,7 @@ MP_NOINLINE int main_(int argc, char **argv) {
 }
 
 #if !MICROPY_VFS
-uint mp_import_stat(const char *path) {
+mp_import_stat_t mp_import_stat(const char *path) {
     struct stat st;
     if (stat(path, &st) == 0) {
         if (S_ISDIR(st.st_mode)) {

py/objarray.c

@@ -254,12 +254,14 @@ STATIC mp_obj_t memoryview_make_new(const mp_obj_type_t *type_in, size_t n_args,
 STATIC mp_obj_t memoryview_cast(const mp_obj_t self_in, const mp_obj_t typecode_in) {
     mp_obj_array_t *self = MP_OBJ_TO_PTR(self_in);
     const char *typecode = mp_obj_str_get_str(typecode_in);
-    size_t element_size = mp_binary_get_size('@', typecode[0], NULL);
-    size_t bytelen = self->len * mp_binary_get_size('@', self->typecode & ~MP_OBJ_ARRAY_TYPECODE_FLAG_RW, NULL);
-    if (bytelen % element_size != 0) {
+    size_t new_element_size = mp_binary_get_size('@', typecode[0], NULL);
+    size_t old_element_size = mp_binary_get_size('@', self->typecode & ~MP_OBJ_ARRAY_TYPECODE_FLAG_RW, NULL);
+    size_t bytelen = self->len * old_element_size;
+    if (bytelen % new_element_size != 0) {
         mp_raise_TypeError(MP_ERROR_TEXT("memoryview: length is not a multiple of itemsize"));
     }
-    mp_obj_array_t *result = MP_OBJ_TO_PTR(mp_obj_new_memoryview(*typecode, bytelen / element_size, self->items));
+    mp_obj_array_t *result = MP_OBJ_TO_PTR(mp_obj_new_memoryview(*typecode, bytelen / new_element_size, self->items));
+    result->memview_offset = (self->memview_offset * old_element_size) / new_element_size;
 
     // test if the object can be written to
     if (self->typecode & MP_OBJ_ARRAY_TYPECODE_FLAG_RW) {

tests/basics/memoryview_cast.py

@@ -0,0 +1,21 @@
+try:
+    memoryview(b'a').cast
+except:
+    print("SKIP")
+    raise SystemExit
+
+b = bytearray(range(16))
+
+def print_memview(mv):
+    print(", ".join(hex(v) for v in mv))
+
+mv = memoryview(b)
+print_memview(mv)
+print_memview(mv[4:])
+
+words = mv.cast("I")
+print_memview(words)
+print_memview(mv[4:].cast("I"))
+print_memview(words[1:])
+
+print_memview(words.cast("B"))