Typing of @lumino/widgets changed from 1.19 to 1.20

[fcollonval]

Description

The trouble comes from a change in the type file generated by the lumino package. In 1.19.0, the Widget.layout accessor was translated as property in the type declaration. But now in 1.20.0, the accessor is kept:

This coupled with the new constrain (Properties Overriding Accessors (and vice versa) is an Error) on Typescript v4 will break code. The error is now seen on JupyterLab yet as the lock file is pulling version 1.19.0 of @lumino/widgets.

Expected behavior

The type between minor version does not change.

Xref: jupyterlab/jupyterlab#10088

I hit this issue when upgrading the lock file on @jupyterlab/git: https://github.com/jupyterlab/jupyterlab-git/pull/926/checks?check_run_id=2359801487

[fcollonval]

Unfortunately #172 did not correct this and @lumino/widgets@1.21.0 is still defining types as in 1.20.0.

[fcollonval]

Actually the change in the produced typing file was introduced in TypeScript 3.7: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#the-usedefineforclassfields-flag-and-the-declare-property-modifier

To detect the issue around accessors, TypeScript 3.7 will now emit get/set accessors in .d.ts files so that in TypeScript can check for overridden accessors.

[fcollonval]

So it seems we are stuck with Typescript 3.6, if we want to ensure dependent packages using Typescript 4+ can build without error.