Support nested classes in KotlinCliJavaFileManagerImpl.findClass

findClass(String, GlobalSearchScope) is invoked for example when we're
resolving supertypes of classes in Java libraries. Previously, it never
found nested classes and falled back to CoreJavaFileManager's
implementation, which lacks a fix for the original issue (KT-12664,
which was fixed in JvmDependenciesIndex in 5a533a52 and 164c72e8)

 #KT-16931 Fixed

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/KotlinCliJavaFileManagerImpl.kt

@@ -53,12 +53,31 @@ class KotlinCliJavaFileManagerImpl(private val myPsiManager: PsiManager) : CoreJ
         }
     }
 
+    // this method is called from IDEA to resolve dependencies in Java code
+    // which supposedly shouldn't have errors so the dependencies exist in general
     override fun findClass(qName: String, scope: GlobalSearchScope): PsiClass? {
-        // this method is called from IDEA to resolve dependencies in Java code
-        // which supposedly shouldn't have errors so the dependencies exist in general
-        // Most classes are top level classes so we will try to find them fast
-        // but we must sometimes fallback to support finding inner/nested classes
-        return qName.toSafeTopLevelClassId()?.let { classId -> findClass(classId, scope) } ?: super.findClass(qName, scope)
+        // String cannot be reliably converted to ClassId because we don't know where the package name ends and class names begin.
+        // For example, if qName is "a.b.c.d.e", we should either look for a top level class "e" in the package "a.b.c.d",
+        // or, for example, for a nested class with the relative qualified name "c.d.e" in the package "a.b".
+        // Below, we start by looking for the top level class "e" in the package "a.b.c.d" first, then for the class "d.e" in the package
+        // "a.b.c", and so on, until we find something. Most classes are top level, so most of the times the search ends quickly
+
+        var classId = qName.toSafeTopLevelClassId() ?: return super.findClass(qName, scope)
+
+        while (true) {
+            findClass(classId, scope)?.let { return it }
+
+            val packageFqName = classId.packageFqName
+            if (packageFqName.isRoot) break
+
+            classId = ClassId(
+                    packageFqName.parent(),
+                    FqName(packageFqName.shortName().asString() + "." + classId.relativeClassName.asString()),
+                    false
+            )
+        }
+
+        return super.findClass(qName, scope)
     }
 
     override fun findClasses(qName: String, scope: GlobalSearchScope): Array<PsiClass> {