Infrastructure change: each header module now knows implementing modules list

core/descriptors/src/org/jetbrains/kotlin/descriptors/ModuleDescriptor.kt

@@ -49,6 +49,8 @@ interface ModuleDescriptor : DeclarationDescriptor {
      */
     val allDependentModules: List<ModuleDescriptor>
 
+    val allImplementingModules: Set<ModuleDescriptor>
+
     fun <T> getCapability(capability: Capability<T>): T?
 
     class Capability<T>(val name: String) {

core/descriptors/src/org/jetbrains/kotlin/descriptors/impl/ModuleDescriptorImpl.kt

@@ -52,6 +52,8 @@ class ModuleDescriptorImpl @JvmOverloads constructor(
     override val allDependentModules: List<ModuleDescriptor>
         get() = this.dependencies.sure { "Dependencies of module $id were not set" }.allDependencies.filter { it != this }
 
+    override val allImplementingModules: MutableSet<ModuleDescriptor> = mutableSetOf()
+
     override fun getPackage(fqName: FqName): PackageViewDescriptor = packages(fqName)
 
     override fun getSubPackagesOf(fqName: FqName, nameFilter: (Name) -> Boolean): Collection<FqName> {
@@ -79,6 +81,12 @@ class ModuleDescriptorImpl @JvmOverloads constructor(
     fun setDependencies(dependencies: ModuleDependencies) {
         assert(this.dependencies == null) { "Dependencies of $id were already set" }
         this.dependencies = dependencies
+        if (platformKind == PlatformKind.DEFAULT) return
+        for (dependentModule in allDependentModules) {
+            if (dependentModule.platformKind != PlatformKind.DEFAULT) continue
+            if (dependentModule.sourceKind != sourceKind) continue
+            (dependentModule as? ModuleDescriptorImpl)?.allImplementingModules?.add(this)
+        }
     }
 
     fun setDependencies(vararg descriptors: ModuleDescriptorImpl) {

core/descriptors/src/org/jetbrains/kotlin/types/ErrorUtils.java

@@ -41,6 +41,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 
+import static java.util.Collections.emptySet;
 import static kotlin.collections.CollectionsKt.emptyList;
 import static kotlin.collections.CollectionsKt.joinToString;
 
@@ -97,6 +98,12 @@ public class ErrorUtils {
                 return emptyList();
             }
 
+            @NotNull
+            @Override
+            public Set<ModuleDescriptor> getAllImplementingModules() {
+                return emptySet();
+            }
+
             @Override
             public <R, D> R accept(@NotNull DeclarationDescriptorVisitor<R, D> visitor, D data) {
                 return null;
@@ -205,13 +212,13 @@ public class ErrorUtils {
         @NotNull
         @Override
         public Set<Name> getFunctionNames() {
-            return Collections.emptySet();
+            return emptySet();
         }
 
         @NotNull
         @Override
         public Set<Name> getVariableNames() {
-            return Collections.emptySet();
+            return emptySet();
         }
 
         @NotNull