GH-354: Add missing test case for NPE as a result of misbehaving compiler

Author: ascopes

java-compiler-testing/src/test/java/io/github/ascopes/jct/tests/unit/compilers/impl/JctCompilationFactoryImplTest.java

@@ -20,6 +20,7 @@
 import static io.github.ascopes.jct.tests.helpers.Fixtures.someBinaryName;
 import static io.github.ascopes.jct.tests.helpers.Fixtures.someFlags;
 import static io.github.ascopes.jct.tests.helpers.Fixtures.someLinesOfText;
+import static io.github.ascopes.jct.tests.helpers.Fixtures.someText;
 import static io.github.ascopes.jct.tests.helpers.Fixtures.someTraceDiagnostic;
 import static io.github.ascopes.jct.utils.IterableUtils.flatten;
 import static java.util.stream.Collectors.toList;
@@ -655,6 +656,35 @@ void exceptionsDuringCompilationGetWrappedAndRethrown() throws IOException {
         .hasCause(cause);
   }
 
+  @DisplayName("Compilers returning null outcomes will be raised as an exception")
+  @Test
+  void compilersReturningNullOutcomesWillBeRaisedAsAnException() throws IOException {
+    // Given
+    var task = mock(CompilationTask.class);
+    var name = someText();
+
+    when(jctCompiler.getName())
+        .thenReturn(name);
+
+    when(javaCompiler.getTask(any(), any(), any(), any(), any(), any()))
+        .thenReturn(task);
+    when(task.call())
+        .thenReturn(null);
+
+    // Do not inline this, it will break in Mockito's stubber backend.
+    var fileObjects = Set.of(somePathFileObject(someBinaryName()));
+    when(fileManager.list(any(), any(), any(), anyBoolean()))
+        .thenReturn(fileObjects);
+
+    // Then
+    assertThatThrownBy(() -> doCompile(null))
+        .isInstanceOf(JctCompilerException.class)
+        .hasMessage("Failed to perform compilation, an unexpected exception was raised")
+        .cause()
+        .hasMessage("Compiler %s task .call() method returned null unexpectedly!", name)
+        .isInstanceOf(NullPointerException.class);
+  }
+
   @DisplayName("The compilation result holds the failOnWarnings flag from the compiler")
   @ValueSource(booleans = {true, false})
   @ParameterizedTest(name = "for compiler.isFailOnWarnings() = {0}")