KT-41456: Incremental KAPT - check compiled sources before running incrementally

Because incremental KAPT tries to reuse .class files produced by kotlinc
and javac, it should check for their existence before starting
an incremental run. Otherwise, annotation processors that perform type
validation will fail to run.

Current check counts the number of declared types in processed .java
sources, and it makes sure the total number of .class files in compiled
sources dirs is equal or higher. Otherwise, KAPT runs non-incrementally.

Tests: KaptIncrementalWithIsolatingApt.testMissingKotlinOutputForcesNonIncrementalRun

^KT-41456 In Progress

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

@@ -403,6 +403,23 @@ class KaptIncrementalWithIsolatingApt : KaptIncrementalIT() {
             }
         }
     }
+
+    @DisplayName("KT-41456: detect missing kotlin compilation output")
+    @GradleTest
+    @DisabledOnOs(OS.WINDOWS, disabledReason = "https://youtrack.jetbrains.com/issue/KTI-405")
+    fun testMissingKotlinOutputForcesNonIncrementalRun(gradleVersion: GradleVersion) {
+        kaptProject(gradleVersion) {
+            build("clean", "assemble")
+
+            // Explicitly remove all kotlinc generated .class files
+            projectPath.resolve("build/classes/kotlin/main").toFile().listFiles()!!.forEach { it.deleteRecursively() }
+            javaSourcesDir().resolve("bar/useB.kt").modify { current -> "$current\nfun otherFunction() {}" }
+
+            build("assemble") {
+                assertOutputContains("Unable to run incrementally, processing all sources.")
+            }
+        }
+    }
 }
 
 private const val patternApt = "Processing java sources with annotation processors:"