CyclicReference exception, thrown only during incremental compilation

[marcinzh]

Compiler version

3.2.1
3.2.2-RC2

Minimized code

Important: We need 2 separete files, or else the error won't happen.

Foo.scala:

final class Foo(_that: Foo) extends FooStub(_that)

FooStub.scala:

abstract class FooStub(val that: Foo):
  val bar = 1337;

Output

Clean compile succeeds.

To trigger the error, do the following:

1. Use ~compile in sbt
2. Make a minor change to FooStub source: add or remove 1 digit from bar

[error] -- Error: /myproject/FooStub.scala:2:27 
[error] 2 |abstract class FooStub(val that: Foo):
[error]   |                           ^
[error]   |Could not read definition of class Foo in /myproject/target/scala-3.2.2-RC2/classes/Foo.class
[error]   |An exception was encountered:
[error]   |  dotty.tools.dotc.core.CyclicReference: 
[error]   |Run with -Ydebug-unpickling to see full stack trace.

Again, clean recompile succeeds. Until another minor change of source.

After adding suggested -Ydebug-unpickling and -explain options, got this:

[error] -- [E046] Cyclic Error: /myproject/FooStub.scala:3:27 
[error] 3 |abstract class FooStub(val that: Foo):
[error]   |                           ^
[error]   |                           Cyclic reference involving method <init>
[error]   |-----------------------------------------------------------------------------
[error]   | Explanation (enabled by `-explain`)
[error]   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]   | constructor FooStub is declared as part of a cycle which makes it impossible for the
[error]   | compiler to decide upon <init>'s type.
[error]   | To avoid this error, try giving <init> an explicit type.
[error]    -----------------------------------------------------------------------------
[error] Explanation
[error] ===========
[error] constructor FooStub is declared as part of a cycle which makes it impossible for the
[error] compiler to decide upon <init>'s type.
[error] To avoid this error, try giving <init> an explicit type.

Also, this thing showed up once:

OpenJDK 64-Bit Server VM warning: CodeCache is full. Compiler has been disabled.
OpenJDK 64-Bit Server VM warning: Try increasing the code cache size using -XX:ReservedCodeCacheSize=
CodeCache: size=131072Kb used=86902Kb max_used=127348Kb free=44169Kb
 bounds [0x00007f8e00000000, 0x00007f8e08000000, 0x00007f8e08000000]
 total_blobs=21123 nmethods=20197 adapters=828
 compilation: enabled

[odersky]

A way to reproduce directly:

1. Compile Foo.scala and FooStub.scala together
2. Compile just FooStub.scala

[odersky]

This is a fairly serious problem. It's because we read the complete parent constructors when unpickling. By contrast, when type-checking sources, we skip the parent constructor arguments in Namer as long as they are not needed to determine the type of the result of a constructor.

[odersky]

I think this might be a source of instability in other incremental compilation scenarios as well.

[odersky]

@marcinzh Thanks a lot for minimizing the code to illustrate the bug so concisely!