Disallow synthesized lambdas in statement position

[odersky]

Disallow lambdas in statement position or if the expected type is Unit. This was a loophole that opened up due to the changes to eta expansion in Scala 3. We did get a warning in these cases, but an error might be better.

Fixes #11761

[odersky]

This looks like a simple fix that makes sense, but it has been nightmarish since it seems all testing frameworks hate me now.

There are two utest tests that fail where I am stuck. Unfortunately the assertions fail without
any info and I don't know enough about utest to figure out what went wrong. So I had to disable
the test. Who knows enough about utest to follow up on this?

[odersky]

The latest iteration of this does allow user-defined lambdas in statement position even though they don't make sense since
all testing suites rely on this heavily. So what we do is to try to flag as errors just synthetic lambdas that arise from eta expansion or other operations. The problem is how to to detect these lambdas. That has been super tricky. Not for user-defined code, there it is simple, but for code that test frameworks make up themselves.

[abgruszecki]

LGTM!

[griggt]

There are two utest tests that fail where I am stuck. Unfortunately the assertions fail without any info and I don't know enough about utest to figure out what went wrong. So I had to disable the test. Who knows enough about utest to follow up on this?

I took a look and proposed a fix in #11887.