Fixed import in IDEA if required environment variables are not set.

#KT-26413 Fixed

build.gradle.kts

@@ -102,12 +102,31 @@ extra["ideaUltimatePluginDir"] = project.file(ideaUltimatePluginDir)
 extra["cidrPluginDir"] = project.file(cidrPluginDir)
 extra["isSonatypeRelease"] = false
 
+// Work-around necessary to avoid setting null javaHome. Will be removed after support of lazy task configuration
+val jdkNotFoundConst = "JDK NOT FOUND"
+
 extra["JDK_16"] = jdkPath("1.6")
 extra["JDK_17"] = jdkPath("1.7")
 extra["JDK_18"] = jdkPath("1.8")
 extra["JDK_9"] = jdkPath("9")
-extra["JDK_10"] = jdkPathIfFound("10")
-extra["JDK_11"] = jdkPathIfFound("11")
+extra["JDK_10"] = jdkPath("10")
+extra["JDK_11"] = jdkPath("11")
+
+gradle.taskGraph.beforeTask() {
+    checkJDK()
+}
+
+var jdkChecked: Boolean = false
+fun checkJDK() {
+    if (jdkChecked) {
+        return
+    }
+    var unpresentJdks = JdkMajorVersion.values().filter { it.isMandatory() }.map { it -> it.name }.filter { it == null || extra[it] == jdkNotFoundConst }.toList()
+    if (!unpresentJdks.isEmpty()) {
+        throw GradleException("Please set environment variable${if (unpresentJdks.size > 1) "s" else ""}: ${unpresentJdks.joinToString()} to point to corresponding JDK installation.")
+    }
+    jdkChecked = true
+}
 
 rootProject.apply {
     from(rootProject.file("versions.gradle.kts"))
@@ -245,13 +264,18 @@ fun Task.listConfigurationContents(configName: String) {
 }
 
 val defaultJvmTarget = "1.8"
-val defaultJavaHome = jdkPath(defaultJvmTarget!!)
+val defaultJavaHome = jdkPath(defaultJvmTarget)
 val ignoreTestFailures by extra(project.findProperty("ignoreTestFailures")?.toString()?.toBoolean() ?: project.hasProperty("teamcity"))
 
 allprojects {
 
     jvmTarget = defaultJvmTarget
-    javaHome = defaultJavaHome
+    if (defaultJavaHome != null) {
+        javaHome = defaultJavaHome
+    } else {
+        logger.error("Could not find default java home. Please set environment variable JDK_${defaultJavaHome} to point to JDK ${defaultJavaHome} installation.")
+    }
+
 
     // There are problems with common build dir:
     //  - some tests (in particular js and binary-compatibility-validator depend on the fixed (default) location
@@ -314,10 +338,10 @@ allprojects {
     task("listDistJar") { listConfigurationContents("distJar") }
 
     afterEvaluate {
-        logger.info("configuring project $name to compile to the target jvm version $jvmTarget using jdk: $javaHome")
         if (javaHome != defaultJavaHome || jvmTarget != defaultJvmTarget) {
+            logger.info("configuring project $name to compile to the target jvm version $jvmTarget using jdk: $javaHome")
             configureJvmProject(javaHome!!, jvmTarget!!)
-        }
+        } // else we will actually fail during the first task execution. We could not fail before configuration is done due to impact on import in IDE
 
         fun File.toProjectRootRelativePathOrSelf() = (relativeToOrNull(rootDir)?.takeUnless { it.startsWith("..") } ?: this).path
 
@@ -644,14 +668,12 @@ configure<IdeaModel> {
     }
 }
 
-fun jdkPathIfFound(version: String): String? {
+fun jdkPath(version: String): String {
     val jdkName = "JDK_${version.replace(".", "")}"
     val jdkMajorVersion = JdkMajorVersion.valueOf(jdkName)
-    return configuredJdks.find { it.majorVersion == jdkMajorVersion }?.homeDir?.canonicalPath
+    return configuredJdks.find { it.majorVersion == jdkMajorVersion }?.homeDir?.canonicalPath?:jdkNotFoundConst
 }
 
-fun jdkPath(version: String): String = jdkPathIfFound(version)
-        ?: throw GradleException ("Please set environment variable JDK_${version.replace(".", "")} to point to JDK $version installation")
 
 fun Project.configureJvmProject(javaHome: String, javaVersion: String) {
     tasks.withType<JavaCompile> {

buildSrc/src/main/kotlin/jdksFinder.kt

@@ -8,8 +8,10 @@ import java.io.File
 import net.rubygrapefruit.platform.WindowsRegistry.Key.HKEY_LOCAL_MACHINE
 import org.gradle.internal.os.OperatingSystem
 
-enum class JdkMajorVersion {
-    JDK_16, JDK_17, JDK_18, JDK_9, JDK_10, JDK_11
+enum class JdkMajorVersion(val mandatory: Boolean = true) {
+    JDK_16, JDK_17, JDK_18, JDK_9(false), JDK_10(false), JDK_11(false);
+
+    fun isMandatory() : Boolean = mandatory
 }
 
 val jdkAlternativeVarNames = mapOf(JdkMajorVersion.JDK_9 to listOf("JDK_19"))