I think the issue here is the combination of FlexibleArrayBufferView and AllowShared flags, see the logic here: https://cs.chromium.org/chromium/src/third_party/blink/renderer/bindings/scripts/v8_types.py?type=cs&l=250

If FlexibleArrayBufferViews are always meant to be AllowShared, then maybe we should just check that here. Otherwise  we'll need to use the NonShared<> wrapper template with a FlexibleArrayBufferView type for these parameters.

Before https://chromium-review.googlesource.com/c/1334844 , FlexibleArrayBufferView was previously implying AllowShared. Was that intentional?

It would be OK to make that implicit; all uses of FlexibleArrayBufferView actually allow SharedArrayBuffer to be passed in.

Oops, sorry for delay in response. Sgtm either way: making it implicit or checking that we always have both. I guess it is more self-documenting for it to be explicit, though.

I'll work on CL for this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8f5133db01a76dc2ccee87ed034241e5cdacc045

commit 8f5133db01a76dc2ccee87ed034241e5cdacc045
Author: Ben Smith <binji@chromium.org>
Date: Tue Nov 27 17:44:24 2018

Require AllowShared with FlexibleArrayBufferView

All methods that have a parameter with the FlexibleArrayBufferView
extended attribute support AllowShared. In other words, it is possible
to use them with an ArrayBufferView backed by an ArrayBuffer or a
SharedArrayBuffer. This CL makes this an explicit requirement in the IDL
file.

Currently only WebGL APIs use the FlexibleArrayBufferView extended
attribute.

Bug:  chromium:905449 
Change-Id: I52f23d4ee07c169a525243f00dd8a96a58ce7075
Reviewed-on: https://chromium-review.googlesource.com/c/1345125
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611175}
[modify] https://crrev.com/8f5133db01a76dc2ccee87ed034241e5cdacc045/third_party/blink/renderer/bindings/IDLExtendedAttributes.md
[modify] https://crrev.com/8f5133db01a76dc2ccee87ed034241e5cdacc045/third_party/blink/renderer/bindings/scripts/v8_types.py
[modify] https://crrev.com/8f5133db01a76dc2ccee87ed034241e5cdacc045/third_party/blink/renderer/bindings/tests/idls/core/test_object.idl

Thanks binji@ for picking this up.