Preface: Kotlin 1.3 will be able to read metadata of .class files
produced by Kotlin 1.4 (see KT-25972). Also, to simplify implementation
and to improve diagnostic messages, we're going to advance JVM metadata
version to 1.4.0 in Kotlin 1.4, and would like to keep it in sync with
the compiler version thereafter. This presents a problem: in an unlikely
event that before releasing 1.4, we find out that the metadata-reading
implementation in 1.3 was incorrect, we'd like to be able to fix the bug
in that implementation and _forbid_ 1.3 from reading metadata of 1.4.
But prior to this commit the only way to do this was to advance the
metadata version, in this case to 1.5, and that breaks the
metadata/compiler version equivalence we'd like to keep.
The solution is to add another boolean flag to the class file, called
"strict metadata version semantics", which signifies that if this class
file has metadata version 1.X, then it can only be read by the compilers
of versions 1.X and greater. This flag effectively disables the smooth
migration scenario proposed in KT-25972 (as does increasing metadata
version by 2), and will be used only in hopeless situations as in the
case described above.
Alexander Udalov