This error actually appears to come from the Java compiler installed in Cygwin. When I ran cmake --build <build_dir> -v, I see the invocation of javac using regular Cygwin paths, but the error message using the backslash paths. Perhaps the fault is the JDK I’ve got installed in Cygwin. I did point to a v8/v1.8 JDK available on my system (JAVA_HOME=“C:\Program Files\Amazon Correto\jdk1.8.0_292”), but it seems to be ignored in favor of /cygdrive/c/jdk-15.0.2/bin/java, which I never point to.