This test case:

class bug
{
  Int[] indexes := Int[,]

  Int? entryForIndex(Int index) { return 1 }

  Void modify(Int start)
  {
    ip := entryForIndex(start) ?: 0
    indexes[ip] += 1
  }

  Void main() {}
}

generates this error trace:

java.lang.VerifyError: (class: fan/bug_0/bug, method: modify signature: (J)V) Expecting to find object/array on stack
  at java.lang.Class.getDeclaredFields0(Native Method)
  at java.lang.Class.privateGetDeclaredFields(Unknown Source)
  at java.lang.Class.getDeclaredFields(Unknown Source)
  at fan.sys.ClassType.finishSlots(ClassType.java:591)
  at fan.sys.ClassType.finish(ClassType.java:554)
  at fan.sys.Method$MethodFunc.isStatic(Method.java:459)
  at fan.sys.Method$MethodFunc.callList(Method.java:175)
  at fan.sys.Type.make(Type.java:242)
  at fan.sys.ClassType.make(ClassType.java:112)
  at fan.sys.Type.make(Type.java:232)
  at fanx.tools.Fan.callMain(Fan.java:138)
  at fanx.tools.Fan.executeFile(Fan.java:89)
  at fanx.tools.Fan.execute(Fan.java:35)
  at fanx.tools.Fan.run(Fan.java:241)
  at fanx.tools.Fan.main(Fan.java:279)
sys::Err: Method not mapped to java.lang.reflect correctly bug_0::bug.make
  fan.sys.Method$MethodFunc.isStatic (Method.java:466)
  fan.sys.Method$MethodFunc.callList (Method.java:175)
  fan.sys.Type.make (Type.java:242)
  fan.sys.ClassType.make (ClassType.java:112)
  fan.sys.Type.make (Type.java:232)
  fanx.tools.Fan.callMain (Fan.java:138)
  fanx.tools.Fan.executeFile (Fan.java:89)
  fanx.tools.Fan.execute (Fan.java:35)
  fanx.tools.Fan.run (Fan.java:241)
  fanx.tools.Fan.main (Fan.java:279)

Changing the line

indexes[ip] += 1

to something else makes it go away, but I can't explain this one. Maybe a problem saving a nullable inferred type into a non-nullable array? Or confusion between array.get and the int.plus?

Promoted to ticket #788 and assigned to brian

Renamed from Odd little compiler error to Coercion of indexed assignment shortcut

Ticket resolved in 1.0.47

Move the scratch variable generation from ResolveExpr step to the CheckErrors step after coercion of arguments has been performed.

changeset