This repository contains a very minimal reproduction of the problem I'm experiencing with now and full-icu.
With .build.env.NODE_ICU_DATA=node_modules/full-icu/icudt64l.dat
Running the next project locally with $ now dev will work fine.
Deploying to Vercel with $ now will fail with
17:02:37.101 Retrieving list of deployment files...
17:02:37.436 Downloading 8 deployment files...
17:02:37.658 /node12/bin/node: could not initialize ICU (check NODE_ICU_DATA or --icu-data-dir parameters)
17:02:37.682 /node12/bin/node: could not initialize ICU (check NODE_ICU_DATA or --icu-data-dir parameters)
17:02:37.689 /node12/bin/node: could not initialize ICU (check NODE_ICU_DATA or --icu-data-dir parameters)
17:02:37.696 /node12/bin/node: could not initialize ICU (check NODE_ICU_DATA or --icu-data-dir parameters)
17:02:38.704 /node12/bin/node: could not initialize ICU (check NODE_ICU_DATA or --icu-data-dir parameters)
17:02:38.707 Done with "package.json"
With .env.NODE_ICU_DATA=node_modules/full-icu/icudt64l.dat
Running the next project locally with $ now dev will work.
Deploying to Vercel with $ now will also succeed.
The problem which occures in this case is that the SSR-rendered version
renders the date on the index-page as M/D/YYYY but it should be rendered as D.M.YYYY.
The problem is caused by the property .build.env.NODE_ICU_DATA or .env.NODE_ICU_DATA. If this configuration is removed,
the node runtime doesn't fulfill the requirements of react-intl.
This will cause this error serverside:
[React Intl Error MISSING_DATA] Missing locale data for locale: "de" in Intl.NumberFormat. Using default locale: "en" as fallback. See https://github.com/formatjs/react-intl/blob/master/docs/Getting-Started.md#runtime-requirements for more details
And this error clientside:
Warning: Text content did not match. Server: "4/29/2020" Client: "29.4.2020"
As a workaround I am currently using is using Intl.js. But this is not the way react-intl recommends.