Path hash code is case-agnostic. This is to avoid changing it if e.g. path to project is reconfigured from C:\Work to c:\work

compiler/frontend.java/src/org/jetbrains/kotlin/load/kotlin/PackagePartClassUtils.java

@@ -41,10 +41,7 @@ import static org.jetbrains.kotlin.load.kotlin.PackageClassUtils.getPackageClass
 
 public class PackagePartClassUtils {
     public static int getPathHashCode(@NotNull VirtualFile file) {
-        // Conversion to system-dependent name seems to be unnecessary, but it's hard to check now:
-        // it was introduced when fixing KT-2839, which appeared again (KT-3639).
-        // If you try to remove it, run tests on Windows.
-        return FileUtil.toSystemDependentName(file.getPath()).hashCode();
+        return file.getPath().toLowerCase().hashCode();
     }
 
     @NotNull

jps-plugin/test/org/jetbrains/kotlin/jps/build/IncrementalProjectPathCaseChangedTest.kt

@@ -24,6 +24,10 @@ public class IncrementalProjectPathCaseChangedTest : AbstractIncrementalJpsTest(
         doTest("jps-plugin/testData/incremental/custom/projectPathCaseChanged/")
     }
 
+    fun testProjectPathCaseChangedMultiFile() {
+        doTest("jps-plugin/testData/incremental/custom/projectPathCaseChangedMultiFile/")
+    }
+
     override fun doTest(testDataPath: String) {
         if (SystemInfoRt.isFileSystemCaseSensitive) {
             return

jps-plugin/test/org/jetbrains/kotlin/jps/build/KotlinJpsBuildTest.java

@@ -19,6 +19,7 @@ package org.jetbrains.kotlin.jps.build;
 import com.google.common.collect.Lists;
 import com.intellij.openapi.util.Condition;
 import com.intellij.openapi.util.io.FileUtil;
+import com.intellij.openapi.util.io.FileUtilRt;
 import com.intellij.testFramework.LightVirtualFile;
 import com.intellij.util.ArrayUtil;
 import com.intellij.util.containers.ContainerUtil;
@@ -614,8 +615,8 @@ public class KotlinJpsBuildTest extends AbstractKotlinJpsBuildTestCase {
     }
 
     private String packagePartClass(String moduleName, String fileName, String packageClassFqName) {
-        File file = new File(workDir, fileName);
-        LightVirtualFile fakeVirtualFile = new LightVirtualFile(file.getPath()) {
+        String path = FileUtilRt.toSystemIndependentName(new File(workDir, fileName).getAbsolutePath());
+        LightVirtualFile fakeVirtualFile = new LightVirtualFile(path) {
             @NotNull
             @Override
             public String getPath() {

jps-plugin/testData/incremental/custom/projectPathCaseChangedMultiFile/bar.kt

@@ -0,0 +1,2 @@
+fun bar() {
+}

jps-plugin/testData/incremental/custom/projectPathCaseChangedMultiFile/build.log

@@ -0,0 +1,7 @@
+Cleaning output files:
+out/production/module/_DefaultPackage$foo$*.class
+out/production/module/_DefaultPackage.class
+End of files
+Compiling files:
+src/foo.kt
+End of files

jps-plugin/testData/incremental/custom/projectPathCaseChangedMultiFile/foo.kt

@@ -0,0 +1,2 @@
+fun foo() {
+}

jps-plugin/testData/incremental/custom/projectPathCaseChangedMultiFile/foo.kt.new

@@ -0,0 +1,2 @@
+fun foo() {
+}