As of mid-July 2026, the Embedded Swift documentation about existentials says:
• Embedded Swift allows and supports forming existentials of any kind
• Existentials in Embedded Swift allow the “is” and “as!” / “as?” operators to check whether an existential holds a specific concrete type (or subclass thereof)
But those features require the unreleased Swift 6.4, and won't work with WASM if you follow the Embedded Swift installation guide's instructions to run swiftly install main-snapshot, because (at least right now) Swift WASM does not post corresponding nightly builds very often.
So here are the steps to get the right versions of the Swift Toolchain, Embedded Swift, and WASM SDK:
- Go to the WASM SDK list on GitHub and scroll all the way down to find the most recent snapshot JSON file, which, as of 21 July 2026, is this July 11 snapshot JSON.
- In terminal, copy-paste the relevant URL, checksum, and "YYYY-MM-DD-a" string from the JSON's filename to run these commands:
URL="PASTE_ARTIFACTBUNDLE.ZIP_URL_HERE"
CHECKSUM=PASTE_CHECKSUM_URL_HERE
DATE="PASTE_YYYY-MM-DD-a_HERE"
swiftly run swift sdk install "$URL" --checksum $CHECKSUM
swiftly install main-snapshot-$DATE
swiftly use main-snapshot-$DATE
And now you can actually compile your Embedded Swift targets against the latest WASM using the new Swift 6.4 features.
Then you can proceed trying to make it actually run 