Duplicated messages in testdata appeared because default renderer
renders diagnostic spans ambiguously. It shows only start position.
In fact, there are 3 failures, 2 of them distinguish only by the
diagnostic end offset. See youtrack for more information.
The issue about inconvenient rendering is KT-64989.
#KT-64608
This commit effectively reverts
d386712903.
The reason is that when using a release language version, you can only
"see" the subset of a pre-release library which consists of released and
supported features, so reporting an error is not helpful there. Also, it
presents a problem currently when using kotlinc 1.3 (which is
pre-release) with language version 1.2 (stable) against the bundled
stdlib of version 1.3 (pre-release)
#KT-21267 Declined
Pre-release compiler should be able to read pre-release binaries in tests
even if a stable language version is used, otherwise no stdlib API can be read.
Since we're not yet sure of the design of Experimental/UseExperimental,
we're making them "experimental" themselves in some sense, in that the
user is required to provide the magic argument
"-Xuse-experimental=kotlin.Experimental" to be allowed to use either
Experimental or UseExperimental. This is more convenient than the
previous approach of "-language-version 1.3
-Xskip-metadata-version-check" because it's simpler and does not cause
pre-release binaries to be produced
Evgeniy.Zhelenskiy