More informative error message when classes can't be found on the classpath.

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=== What steps will reproduce the problem (please be specific and use wikiformatting)? === Consider these three simple Scala source files:

File projectA/org/example/a/A.scala:

package org.example.a

class A

File projectB/org/example/b/B.scala:

package org.example.b

import org.example.a.A

class B {
  val a = new A()
}

File projectC/org/example/c/C.scala:

package org.example.c

import org.example.b.B

class C {
  val b = new B()
  b.a
}

Now, we compile the first two separately without problems:

$$ scalac -d bin/projectA/ projectA/src/org/example/a/
$$ scalac -d bin/projectB/ -cp bin/projectA/ projectB/src/

Compiling the 3rd requires both projectA and projectB on the classpath. Leaving out projectA causes an expected error, but the error message is not as helpful as it could be:

$$ scalac -d bin/projectC/ -cp bin/projectB/ projectC/src/org/example/c/C.scala
error: class file needed by B is missing.
reference value a of package org.example refers to nonexisting symbol.
one error found

If we reimplement class C in Java:

package org.example.c;

import org.example.b.B;

class C {
  public final B b;

  public C() {
    b = new B();
    b.a();
  }
}

and then compile using the Java compiler we get:

$$ javac -d bin/projectC/ -cp bin/projectB/:scala-library.jar projectC/src/org/example/c/C.java
projectC/src/org/example/c/C.java:10: cannot access org.example.a.A
class file for org.example.a.A not found
    b.a();
       ^
1 error

This error tells us exactly what is wrong.

The source and a build script is attached.

=== Additional information === Doing the same in Eclipse leads to an even better error message (actually two messages):

• The project was not built since its build path is incomplete. Cannot find the class file for org.example.a.A. Fix the build path then try building this project
• The type org.example.a.A cannot be resolved. It is indirectly referenced from required .class files

This issue was originally (incorrectly) raised as part of another ticket on the Scala IDE bug tracking system:

• http://scala-ide-portfolio.assembla.com/spaces/scala-ide/tickets/1000370

=== What versions of the following are you using? ===

• Scala: 2.9.0.r24728-b20110411140906
• Java:
  • java version "1.6.0_24"
  • Java(TM) SE Runtime Environment (build 1.6.0_24-b07-334-10M3326)
  • Java HotSpot(TM) 64-Bit Server VM (build 19.1-b02-334, mixed mode)
• javac:1.6.0_24
• Operating system: Mac OS X 10.6 (Snow Leopard)

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Imported From: https://issues.scala-lang.org/browse/SI-4492?orig=1 Reporter: Martin Gamwell Dawids (mgd) See #4876 Attachments:

• missingclass.zip (created on Apr 18, 2011 6:08:08 AM UTC, 4629 bytes)

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Martin Gamwell Dawids (mgd) said: Reproduction scenario

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@adriaanm said: Un-assigning to foster work stealing, as announced in https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/o8WG4plpNkw

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@adriaanm said: Unassigning and rescheduling to M6 as previous deadline was missed.

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@adriaanm said: Marking this as critical because the modularization may cause more of these. We should try to detect when someone needs to add scala-xml and be helpful in suggestion a dependency on scala-library-all, or a more fine-grained one.

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@adriaanm said: Tweaked error message and a position: https://github.com/scala/scala/pull/3631

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@adriaanm said: It'll be hard to reach parity with javac, but the above PR gets us close. The evaluation order during symbol loading is structured differently; thus, we complain about the package org.example.a before we get to the type org.example.a.A, making the error message a little less focussed.