Mixed compileOrder error with scala 3.0.0-M3 but success with 2.13.4

[giabao]

Minimized code

% mkdir -p project src/main/{scala,java}
% echo 'addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.5.1")' > project/plugins.sbt
% echo 'crossScalaVersions := Seq("2.13.4", "3.0.0-M3")' > build.sbt
% echo 'trait T { object O }' > src/main/scala/T.scala
% echo 'class J { T.O$ o; }' > src/main/java/J.java
% sbt ";clean ;++2.13.4 compile"
// success
% sbt ";clean ;++3.0.0-M3 compile"
[error] -- [E006] Not Found Error: /tmp/src/main/java/J.java:1:10 ------------
[error] 1 |class J { T.O$ o; }
[error]   |          ^
[error]   |          Not found: T

% echo 'compileOrder := CompileOrder.ScalaThenJava' >> build.sbt
% sbt ";clean ;++3.0.0-M3 compile"
// success

Output

Compile error (see above) in Mixed compileOrder with scala 3.0.0-M3 but success with 2.13.4

Expectation

Compile successfully as in scala 2.

Notes

• Minimized code in this issue is same as in jdk11: NPE at javac's ClassReader when reading .class file of object member of traits #9200 but I think this is a difference issue, maybe related to sbt-dotty?
• Same errors in java 8 & 11

% java -version
openjdk version "1.8.0_272"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_272-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.272-b10, mixed mode)
% eval "$(cs java --env --jvm 11)"
% java -version
openjdk version "11.0.9" 2020-10-20
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.9+11)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.9+11, mixed mode)

[liufengyun]

The cause of the problem is that semanticDB generates a META-INF folder, and javac gets confused somehow.

Updated: incorrect, I get confused with the info.

The reason is that the JavaParser parses T.O$ to a select node, and the typer thinks that T is a term.

The Java parser has to way to determine in a selection A.B whether A is a type or a package. The typer has to try both. For simplicity, we can think A is a type name if it's captalized.