Highlight init-only fields when __post_init__ is not declared (PY-27398)

python/src/com/jetbrains/python/inspections/PyDataclassInspection.kt

@@ -8,6 +8,7 @@ import com.intellij.codeInspection.ProblemHighlightType
 import com.intellij.codeInspection.ProblemsHolder
 import com.intellij.psi.PsiElementVisitor
 import com.jetbrains.python.codeInsight.stdlib.DATACLASSES_INITVAR_TYPE
+import com.jetbrains.python.codeInsight.stdlib.DUNDER_POST_INIT
 import com.jetbrains.python.codeInsight.stdlib.parseDataclassParameters
 import com.jetbrains.python.codeInsight.typing.PyTypingTypeProvider
 import com.jetbrains.python.psi.*
@@ -52,25 +53,35 @@ class PyDataclassInspection : PyInspection() {
             }
           }
 
+          val hasPostInit = node.findMethodByName(DUNDER_POST_INIT, false, myTypeEvalContext) != null
+
           node.processClassLevelDeclarations { element, _ ->
             if (element is PyTargetExpression && element.annotationValue != null) {
               val annotation = element.annotation
 
               if (annotation != null && !PyTypingTypeProvider.isClassVarAnnotation(annotation, myTypeEvalContext)) {
                 val value = element.findAssignedValue()
-                val cls = getInstancePyClass(value)
+                val valueClass = getInstancePyClass(value)
 
-                if (cls != null) {
+                if (valueClass != null) {
                   val builtinCache = PyBuiltinCache.getInstance(node)
 
-                  if (cls == builtinCache.listType?.pyClass ||
-                      cls == builtinCache.setType?.pyClass ||
-                      cls == builtinCache.tupleType?.pyClass) {
+                  if (valueClass == builtinCache.listType?.pyClass ||
+                      valueClass == builtinCache.setType?.pyClass ||
+                      valueClass == builtinCache.tupleType?.pyClass) {
                     registerProblem(value,
-                                    "mutable default '${cls.name}' is not allowed",
+                                    "mutable default '${valueClass.name}' is not allowed",
                                     ProblemHighlightType.GENERIC_ERROR)
                   }
                 }
+                else if (!hasPostInit) {
+                  val type = myTypeEvalContext.getType(element)
+                  if (type is PyClassType && type.classQName == DATACLASSES_INITVAR_TYPE) {
+                    registerProblem(element,
+                                    "attribute '${element.name}' is useless until '${DUNDER_POST_INIT}' is declared",
+                                    ProblemHighlightType.LIKE_UNUSED_SYMBOL)
+                  }
+                }
               }
             }
 

python/testData/inspections/PyDataclassInspection/uselessInitVar.py

@@ -0,0 +1,6 @@
+import dataclasses
+
+@dataclasses.dataclass
+class A:
+    a: int
+    <warning descr="attribute 'b' is useless until '__post_init__' is declared">b</warning>: dataclasses.InitVar[str]

python/testSrc/com/jetbrains/python/inspections/PyDataclassInspectionTest.java

@@ -56,6 +56,11 @@ public class PyDataclassInspectionTest extends PyInspectionTestCase {
     doTest();
   }
 
+  // PY-27398
+  public void testUselessInitVar() {
+    doTest();
+  }
+
   @Override
   protected void doTest() {
     runWithLanguageLevel(