PY-24450 Suggest importing modules for unresolved decorators without arguments

Because it can be just an incomplete qualified name of the needed callable.

python/src/com/jetbrains/python/codeInsight/imports/PythonImportUtils.java

@@ -212,8 +212,11 @@ public final class PythonImportUtils {
   }
 
   private static boolean isPossibleModuleReference(PyElement node) {
-    if (node.getParent() instanceof PyCallExpression && node == ((PyCallExpression) node.getParent()).getCallee()) {
-      return false;
+    final PyCallExpression callExpression = as(node.getParent(), PyCallExpression.class);
+    if (callExpression != null && node == callExpression.getCallee()) {
+      final PyDecorator decorator = as(callExpression, PyDecorator.class);
+      // getArgumentList() still returns empty (but not null) element in this case
+      return decorator != null && !decorator.hasArgumentList();
     }
     if (node.getParent() instanceof PyArgumentList) {
       final PyArgumentList argumentList = (PyArgumentList)node.getParent();

python/testData/quickFixes/PyAddImportQuickFixTest/availableForUnqualifiedDecoratorWithoutArguments/main.py

@@ -0,0 +1,3 @@
+@<error descr="Unresolved reference 'pytest'">py<caret>test</error>
+def test_skipped():
+    pass

python/testData/quickFixes/PyAddImportQuickFixTest/availableForUnqualifiedDecoratorWithoutArguments/main_after.py

@@ -0,0 +1,6 @@
+import pytest
+
+
+@pytest
+def test_skipped():
+    pass

python/testData/quickFixes/PyAddImportQuickFixTest/unavailableForUnqualifiedDecoratorWithArguments/main.py

@@ -0,0 +1,3 @@
+@<error descr="Unresolved reference 'pytest'">py<caret>test</error>()
+def test_skipped():
+    pass

python/testSrc/com/jetbrains/python/quickFixes/PyAddImportQuickFixTest.java

@@ -119,15 +119,22 @@ public class PyAddImportQuickFixTest extends PyQuickFixTestCase {
     doMultiFileAutoImportTest("Import 'ClassB from foo.bar.baz'");
   }
 
+  // PY-24450
+  public void testAvailableForUnqualifiedDecoratorWithoutArguments() {
+    doMultiFileAutoImportTest("Import 'pytest'");
+  }
+
+  // PY-24450
+  public void testUnavailableForUnqualifiedDecoratorWithArguments() {
+    doMultiFileNegativeTest("Import 'pytest'");
+  }
+
   private void doMultiFileAutoImportTest(@NotNull String hintPrefix) {
     doMultiFileAutoImportTest(hintPrefix, null);
   }
 
   private void doMultiFileAutoImportTest(@NotNull String hintPrefix, @Nullable Processor<AutoImportQuickFix> checkQuickfix) {
-    myFixture.copyDirectoryToProject(getTestName(true), "");
-    myFixture.enableInspections(PyUnresolvedReferencesInspection.class);
-    myFixture.configureByFile("main.py");
-    myFixture.checkHighlighting(true, false, false);
+    configureMultiFileProject();
 
     final PsiElement hostUnderCaret = myFixture.getFile().findElementAt(myFixture.getCaretOffset());
     final PyReferenceExpression hostRefExpr = PsiTreeUtil.getParentOfType(hostUnderCaret, PyReferenceExpression.class);
@@ -147,4 +154,16 @@ public class PyAddImportQuickFixTest extends PyQuickFixTestCase {
       myFixture.checkResultByFile(getTestName(true) + "/main_after.py", true);
     }
   }
+
+  private void doMultiFileNegativeTest(@NotNull String hintPrefix) {
+    configureMultiFileProject();
+    assertEmpty(myFixture.filterAvailableIntentions(hintPrefix));
+  }
+
+  private void configureMultiFileProject() {
+    myFixture.copyDirectoryToProject(getTestName(true), "");
+    myFixture.enableInspections(PyUnresolvedReferencesInspection.class);
+    myFixture.configureByFile("main.py");
+    myFixture.checkHighlighting(true, false, false);
+  }
 }