When applying BCV-MU via the settings plugin, if any subproject is Kotlin/Multiplatform (and probably Android too) KGP is required as a buildscript dependency.

// settings.gradle.kts

buildscript {
  dependencies {
    // BCV-MU requires the Kotlin Gradle Plugin classes are present 
    classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.8.10")
  }
}

plugins {
  id("dev.adamko.kotlin.binary-compatibility-validator") version "$bcvMuVersion"
}

Without this, BCV-MU fails because the KGP classes it uses to fetch the dependencies are missing.

@Fleshgrinder explained more here: Kotlin/binary-compatibility-validator#88 (comment)

Gradle uses classpath isolation with inheritance. The classpath of settings is isolated from the projects, and projects inherit the classpath of their settings. But, an init script for instance is entirely isolated from everything else.

Defining a dependency as compileOnly means that our classpath at runtime does not contain the classes we used for compilation. Since settings is isolated from projects we never see their classes, hence, we cannot find the classes we defined as compileOnly since nobody added them to our runtime classpath.

Us adding the classes to our runtime classpath (either by using implementation or combining compileOnly with runtimeOnly which is implementation) would force the dependency upon our consumers. Since projects inherit from settings they would inherit our version of whatever we added to our runtime classpath. This is obviously not what we want.

This is a classic problem with Gradle plugins. A good way to solve it is by shadowing the dependencies. We actually do not require a dependency on the Gradle Kotlin DSL, we only require a dependency on its API (org.jetbrains.kotlin:kotlin-gradle-plugin-api) (reducing the amount of code to shadow). By using the API we lose the KotlinMultiplatformExtension class, but we could rewrite the code to use an alternative instead that works just as well:

  private fun createKotlinMultiplatformTargets(project: Project, extension: BCVProjectExtension) {
    project.pluginManager.withPlugin("kotlin-multiplatform") {
      project.extensions.getByType<KotlinTargetsContainer>().targets.forEach {
        val platformType = it.platformType
        if (platformType == KotlinPlatformType.jvm || platformType == KotlinPlatformType.androidJvm) {
          extension.targets.register(it.targetName) {
            enabled.convention(true)
            it.compilations.configureEach {
              inputClasses.from(output.classesDirs)
            }
          }
        }
      }
    }
  }

With that it should work. Maybe there is also a way to change the current classloader, since the current thread within the project.pluginManager.withPlugin("kotlin-multiplatform") block can load the class. But, I do not know how or if this is at all possible (sounds dangerous, hopefully it is not allowed).

I have attempted to implement the proposed solution. Using KGP-api works well, but I have still encountered problems when using Shadow:

• With isEnableRelocation = true, Gradle doesn't seem to pick up the KotlinTargetsContainer extension

  An exception occurred applying plugin request [id: 'org.jetbrains.kotlin.multiplatform', version: '1.7.20']
  > Failed to apply plugin 'org.jetbrains.kotlin.multiplatform'.
     > Extension of type 'KotlinTargetsContainer' does not exist. Currently registered extension types: [ExtraPropertiesExtension, BCVProjectExtension, KotlinMultiplatformExtension, KotlinTestsRegistry, BasePluginExtension, DefaultArtifactPublicationSet, SourceSetContainer, ReportingExtension, JavaToolchainService, JavaPluginExtension, KotlinArtifactsExtensionImpl]
  

• With With isEnableRelocation = false and Kotlin/JVM subprojects there's some sort of problem

  An exception occurred applying plugin request [id: 'org.jetbrains.kotlin.jvm', version: '1.7.20']
  > Failed to apply plugin 'org.jetbrains.kotlin.jvm'.
     > Could not create an instance of type org.jetbrains.kotlin.gradle.plugin.mpp.KotlinWithJavaTarget.
        > Could not generate a decorated class for type KotlinWithJavaTarget.
           > Cannot have abstract method KotlinTarget.attributes().
  

• I've also encountered a weird 'wasm' error with Shadowing and Kotlin Multiplatform subprojects

  Output:
  
  FAILURE: Build failed with an exception.
  
  * What went wrong:
  wasm
  
  * Try:
  > Run with --info or --debug option to get more log output.
  > Run with --scan to get full insights.
  
  * Exception is:
  java.lang.NoSuchFieldError: wasm
      at org.jetbrains.kotlin.gradle.targets.js.ir.KotlinWasmTargetPreset.<init>(KotlinWasmTargetPreset.kt:20)
      at org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMultiplatformPlugin.setupDefaultPresets(KotlinMultiplatformPlugin.kt:159)
      at org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMultiplatformPlugin.apply(KotlinMultiplatformPlugin.kt:79)
      at org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMultiplatformPlugin.apply(KotlinMultiplatformPlugin.kt:47)
      at org.jetbrains.kotlin.gradle.plugin.KotlinBasePluginWrapper.apply(KotlinPluginWrapper.kt:183)
      at org.jetbrains.kotlin.gradle.plugin.AbstractKotlinMultiplatformPluginWrapper.apply(KotlinPluginWrapper.kt:297)
      at org.jetbrains.kotlin.gradle.plugin.KotlinMultiplatformPluginWrapper.apply(PluginWrappers.kt:74)
  

As there's a workaround, and the settings plugin is still experimental, this isn't critical to fix right now. I'm making this issue to collect any info in once place.

I'll commit the progress I've made so far, and the tests.