Illegal arguments for bootstrap method for invokedynamic

[ddtthh]

Compiler version

3.1.3

Description

When compiling a lambda, the arguments to the generated bootstrap method in the generated class file violate the rules given in the JDK documentation. While this does not seem to cause any issues on OpenJDK, the Android Dexer d8 cannot handle this.

According to https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/invoke/LambdaMetafactory.html interfaceMethodType and dynamicMethodType have to fullfill the following constraints:

• Same arity
• Argument types and return types are either the same as in the interface, or in case of reference may be subtypes.
  There is no type conversion between primitive types and reference types

In the following example, the scala compiler generates a bootstrap method call to LambdaMetafactory.altMetafactory with
interfaceMethodType: (Ljava/lang/Object;)Ljava/lang/Object;
dynamicMethodType: (Ljava/lang/Object;)Z

This clearly violates the specifiaction of LambdaMetafactory.
The dynamicMethodeType should be (Ljava/lang/Object;)Ljava/lang/Boolean;

It is ok for the MethodHandle supplied as implementation to have a MethodType of (Ljava/lang/Object;)Z, LambdaMetafactory will handle boxing, but the dynamicMethodType has to specify the boxed types.

Minimized code

class Foo[T]():
  val f = (x: T) => true

Output

Taken from javap -private -c -v

BootstrapMethods:
  0: #29 REF_invokeStatic java/lang/invoke/LambdaMetafactory.altMetafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #14 (Ljava/lang/Object;)Ljava/lang/Object;
      #19 REF_invokeStatic Foo.$init$$$anonfun$1:(Ljava/lang/Object;)Z
      #20 (Ljava/lang/Object;)Z
      #21 5
      #22 1
      #20 (Ljava/lang/Object;)Z

Expectation

Taken from javap -private -c -v

BootstrapMethods:
  0: #29 REF_invokeStatic java/lang/invoke/LambdaMetafactory.altMetafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #14 (Ljava/lang/Object;)Ljava/lang/Object;
      #19 REF_invokeStatic Foo.$init$$$anonfun$1:(Ljava/lang/Object;)Z
      #20 (Ljava/lang/Object;)Ljava/lang/Boolean;
      #21 5
      #22 1
      #20 (Ljava/lang/Object;)Z

[lrytz]

The anonfun method in Scala 2 is also (Object)Z, but then Scala 2 emits an anonfun$adapted bridge with signature (Object)Object which is used in the indy-lambda.

Corresponding code in the compiler: https://github.com/scala/scala/blob/v2.13.16/src/compiler/scala/tools/nsc/transform/Delambdafy.scala#L162-L175

ASM of the Scala 2 compiled class:

public class Foo {
  public final static synthetic $anonfun$f$1(Ljava/lang/Object;)Z
    ...

  public final static synthetic $anonfun$f$1$adapted(Ljava/lang/Object;)Ljava/lang/Object;
    // parameter final  x
   L0
    LINENUMBER 2 L0
    ALOAD 0
    INVOKESTATIC Foo.$anonfun$f$1 (Ljava/lang/Object;)Z
    INVOKESTATIC scala/runtime/BoxesRunTime.boxToBoolean (Z)Ljava/lang/Boolean;
    ARETURN

  public <init>()V
    ...
    INVOKEDYNAMIC apply()Lscala/Function1; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.altMetafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
      // arguments:
      (Ljava/lang/Object;)Ljava/lang/Object;, 
      // handle kind 0x6 : INVOKESTATIC
      Foo.$anonfun$f$1$adapted(Ljava/lang/Object;)Ljava/lang/Object;, 
      (Ljava/lang/Object;)Ljava/lang/Object;, 
      1
    ]
}

[lrytz]

Oh, I only saw the PR now. Will take a look, probably next week though..

[lrytz]

Scala 3 also creates an $adapted bridge when it needs to unbox a parameter (https://github.com/scala/scala3/blob/3.6.3/compiler/src/dotty/tools/dotc/transform/Erasure.scala#L464-L477). For example in val f = (x: Boolean) => "".

Turns out Scala 2 is a bit more eager than needed in creating such a bridge: boxing the result value doesn't need any special casing for Scala, we can rely on the boxing adaptation implemented in LambdaMetaFactory.