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This is working as intended. When --no-desugaring is used R8 will not do any desugaring, also no invokedynamic desugaring of invokedynamic with known bootstrap methods. When this desugaring does not happen the invokedynamic will be translated directly to invoke-custom. As ART has no implementation of any of the common JVM bootstrap methods you will see the BootstrapMethodError at runtime. The code above will end up having the following two invoke-custom instructions:

invoke-custom {}, call_site_0("apply", ()Ljava/util/function/BinaryOperator;, (Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;, invoke-static@Ljava/lang/Integer;->sum(II)I, (Ljava/lang/Integer;Ljava/lang/Integer;)Ljava/lang/Integer;)@Ljava/lang/invoke/LambdaMetafactory;->metafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;

invoke-custom {p0}, call_site_1("makeConcatWithConstants", (I)Ljava/lang/String;, "c = \u0001")@Ljava/lang/invoke/StringConcatFactory;->makeConcatWithConstants(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;

The first for the Integer::sum lambda and the second for "c = " + c.

The use of --no-desugaring is to support code which has already been desugared and therefore desugaring is not needed. Currently the bazel integration use this, as there desugaring happens separately producing class file output first.

With your example it will look like this:

java -cp r8.jar com.android.tools.r8.D8 --classfile --release --output output.jar ./classes.jar
java -cp r8.jar com.android.tools.r8.R8 --no-desugaring --pg-conf rules.pro --min-api 26 --release --output ./ ./output.jar
env ANDROID_LOG_TAGS=*:f ANDROID_DATA=./ path-to-art/art --64 --no-compile -cp ./classes.dex Test

You will get the warning Type java.util.function.BinaryOperator was not found..., which can be silenced by passing an android.jar with the --lib.

Thank you for your explanation.