[Gradle] skip CheckKotlinGradlePluginConfigurationErrors if no errors

- Add an `onlyIf {}` check so CheckKotlinGradlePluginConfigurationErrors
  won't run unless there are errors,
which helps writing integration tests and preventing IJ warnings.
- remove unnecessary sequence when fetching DiagnosticsForProject
- update test assertions relating to
  CheckKotlinGradlePluginConfigurationErrors

^KT-61943 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/mpp/MppDiagnosticsIt.kt

@@ -193,6 +193,15 @@ class MppDiagnosticsIt : KGPBaseTest() {
         }
     }
 
+    @GradleTest
+    fun testErrorDiagnosticUpToDateIfNoErrors(gradleVersion: GradleVersion) {
+        project("errorDiagnosticUpToDateIfNoErrors", gradleVersion) {
+            build("assemble") {
+                assertTasksSkipped(":checkKotlinGradlePluginConfigurationErrors")
+            }
+        }
+    }
+
     private fun TestProject.expectedOutputFile(suffix: String? = null): File {
         val suffixIfAny = if (suffix != null) "-$suffix" else ""
         return projectPath.resolve("expectedOutput$suffixIfAny.txt").toFile()

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/errorDiagnosticUpToDateIfNoErrors/build.gradle.kts

@@ -0,0 +1,12 @@
+plugins {
+    kotlin("multiplatform")
+}
+
+repositories {
+    mavenLocal()
+    mavenCentral()
+}
+
+kotlin {
+    jvm()
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/errorDiagnosticUpToDateIfNoErrors/settings.gradle.kts

@@ -0,0 +1 @@
+rootProject.name = "diagnostics-rendering-smoke"

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/errorsFailOnlyRelevantProjects/expectedOutput-healthy.txt

@@ -12,4 +12,4 @@ e: [CommonMainOrTestWithDependsOnDiagnostic | ERROR] commonMain can't declare de
 
 > Configure project :healthyProject
 
-> Task :healthyProject:checkKotlinGradlePluginConfigurationErrors
+> Task :healthyProject:checkKotlinGradlePluginConfigurationErrors SKIPPED

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/errorsFailOnlyRelevantProjects/expectedOutput-root-with-continue.txt

@@ -22,4 +22,4 @@ e: [CommonMainOrTestWithDependsOnDiagnostic | ERROR] commonMain can't declare de
 #diagnostic-end
 
 
-> Task :healthyProject:checkKotlinGradlePluginConfigurationErrors
+> Task :healthyProject:checkKotlinGradlePluginConfigurationErrors SKIPPED

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/suppressGradlePluginErrors/expectedOutput.txt

@@ -8,4 +8,4 @@ Stability and future compatibility of the build is not guaranteed.
 #diagnostic-end
 
 
-> Task :checkKotlinGradlePluginConfigurationErrors
+> Task :checkKotlinGradlePluginConfigurationErrors SKIPPED

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/suppressGradlePluginWarnings/expectedOutput.txt

@@ -1,3 +1,3 @@
 > Configure project :
 
-> Task :checkKotlinGradlePluginConfigurationErrors
+> Task :checkKotlinGradlePluginConfigurationErrors SKIPPED

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/diagnostics/CheckKotlinGradlePluginConfigurationErrors.kt

@@ -15,6 +15,7 @@ import org.gradle.language.base.plugins.LifecycleBasePlugin
 import org.gradle.work.DisableCachingByDefault
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompileTool
 import org.jetbrains.kotlin.gradle.tasks.withType
+import org.jetbrains.kotlin.gradle.utils.onlyIfCompat
 
 @DisableCachingByDefault(
     because = "This task renders reported diagnostics; caching this task will hide diagnostics and obscure issues in the build"
@@ -51,14 +52,18 @@ internal fun Project.locateOrRegisterCheckKotlinGradlePluginErrorsTask(): TaskPr
         task.errorDiagnostics.set(
             provider {
                 kotlinToolingDiagnosticsCollector
-                    .getDiagnosticsForProject(this).asSequence()
+                    .getDiagnosticsForProject(this)
                     .filter { it.severity == ToolingDiagnostic.Severity.ERROR }
-                    .toList()
             }
         )
         task.renderingOptions.set(ToolingDiagnosticRenderingOptions.forProject(this))
         task.description = DESCRIPTION
         task.group = LifecycleBasePlugin.VERIFICATION_GROUP
+
+        task.onlyIfCompat("errorDiagnostics are present") {
+            require(it is CheckKotlinGradlePluginConfigurationErrors)
+            !it.errorDiagnostics.orNull.isNullOrEmpty()
+        }
     }
 
     taskProvider.addDependsOnFromTasksThatShouldFailWhenErrorsReported(tasks)