IR checking error for IR coming from Scala.js < 1.11 through deserialization hack

[sjrd]

The deserialization hack for the argument of a Throw node coming from Scala.js 1.11 produces invalid IR in Scala.js 1.18. It generates an UnwrapFromThrowable operation with a nullable argument (actually, literally Null()). But in 1.18 UnwrapFromThrowable is a UnaryOp that demands a non-nullable argument.

The error reads:

...(UnaryOp): java.lang.Throwable! expected but null found for tree of type org.scalajs.ir.Trees$Null
There were 1 IR checking errors.

This does not happen for throw new ... because it special-cases those away. However it happens for throw e where e is a variable or a more complicated expression.

Discovered by @plokhotnyuk on Discord.

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A reproduction with the javalib, which we can use in our BackwardsCompatTest, is the following expression:

java.util.Base64.getDecoder().decode(java.nio.ByteBuffer.wrap(Array(65, 81, 73, 61)))

because Base64.Decoder.decode(ByteBuffer) contains such a throw e.

[sjrd]

We have a similar issue with Array_length in the hacks of jlr.Array.newInstance. Although admittedly those are less likely to be hit by real users, since normally if you link with 1.18.0, you also have the scalajs-javalib v1.18.0.