Re: [vim/vim] Make extend() accept blobs (PR #20958)

h-east left a comment (vim/vim#20958)

`VAR_ANY` there is not intentional for blobs, it is just the default.

`generate_PUSHBLOB()` goes through `generate_instr_type()`, which always passes
`&t_any` as the declared type (`vim9instr.c:88`), while `generate_NEWLIST()`
pushes a `list<any>` of its own (`vim9instr.c:1438`). `arg_same_as_prev()`
compares against `type_decl` (`evalfunc.c:1008`), so `[1]` is checked against
`list<any>` and rejects `42`, while `0z01` is checked against `any` and passes.

That default is deliberate: constants have no declared type, and
`NEWLIST`/`NEWDICT` are the exception because a container's member type can
still widen. It went unnoticed for blobs because `arg_same_as_prev()` is used
by `extend()` alone, and `extend()` did not take blobs until now.

This fixes it:

```c
    if ((isn = generate_instr_type2(cctx, ISN_PUSHBLOB, &t_blob, &t_blob))
	    == NULL)
	return FAIL;
```

It widens the declared type of every blob literal, but nothing else reads
`type_decl`: `vim9compile.c` never does, `arg_same_as_prev()` is used by
`extend()` alone, `arg_item_of_prev()` looks at it only for lists and dicts,
and blob index and slice overwrite it anyway (`vim9expr.c:227`).

Other things I noticed:

- `make test_listdict` aborts in `Test_extendnew_leak()`. `blob_extend_func()`
  puts the copy into `rettv` up front, so the `blob_unref(b1)` at `cleanup:`
  leaves `rettv->vval.v_blob` pointing at freed memory. `list_extend_func()`
  keeps its copy in a local and moves it into `rettv` at `theend:`.
  `extendnew(0z, 0z, 0z)` reaches it: the third argument is a blob, so
  `tv_get_number_chk()` fails.

- `0z0102030102030` in `Test_listdict_extend()` has an odd number of hex
  digits.

- `b->extend([])` in the same block: the second argument is not a blob.

- `EXXXX` in `errors.h` and in the `assert_fails()` needs a real number.

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