Inspect access to dataclasses' init only fields (PY-27398)

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

@@ -7,6 +7,7 @@ import com.intellij.codeInspection.LocalInspectionToolSession
 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.parseDataclassParameters
 import com.jetbrains.python.codeInsight.typing.PyTypingTypeProvider
 import com.jetbrains.python.psi.*
@@ -126,6 +127,32 @@ class PyDataclassInspection : PyInspection() {
       }
     }
 
+    override fun visitPyReferenceExpression(node: PyReferenceExpression?) {
+      super.visitPyReferenceExpression(node)
+
+      if (node != null && node.isQualified) {
+        val cls = getInstancePyClass(node.qualifier) ?: return
+
+        if (parseDataclassParameters(cls, myTypeEvalContext) != null) {
+          cls.processClassLevelDeclarations { element, _ ->
+            if (element is PyTargetExpression && element.name == node.name) {
+              val type = myTypeEvalContext.getType(element)
+
+              if (type is PyClassType && type.classQName == DATACLASSES_INITVAR_TYPE) {
+                registerProblem(node.lastChild,
+                                "'${cls.name}' object could have no attribute '${element.name}' because it is declared as init-only",
+                                ProblemHighlightType.GENERIC_ERROR_OR_WARNING)
+
+                return@processClassLevelDeclarations false
+              }
+            }
+
+            true
+          }
+        }
+      }
+    }
+
     private fun getInstancePyClass(element: PyTypedElement?): PyClass? {
       val type = element?.let { myTypeEvalContext.getType(it) } as? PyClassType
       return if (type != null && !type.isDefinition) type.pyClass else null

python/testData/inspections/PyDataclassInspection/accessToInitVar.py

@@ -0,0 +1,14 @@
+import dataclasses
+
+@dataclasses.dataclass
+class A:
+    a: int
+    b: dataclasses.InitVar[str]
+
+    def __post_init__(self, b: str):
+        pass
+
+
+a = A(1, "a")
+print(a.a)
+print(a.<warning descr="'A' object could have no attribute 'b' because it is declared as init-only">b</warning>)

python/testData/inspections/PyDataclassInspection/dataclasses.py

@@ -1,3 +1,11 @@
+class _InitVarMeta(type):
+    def __getitem__(self, params):
+        return self
+
+class InitVar(metaclass=_InitVarMeta):
+    pass
+
+
 def dataclass(_cls=None, *, init=True, repr=True, eq=True, order=False,
               hash=None, frozen=False):
     pass

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

@@ -51,6 +51,11 @@ public class PyDataclassInspectionTest extends PyInspectionTestCase {
     doTest();
   }
 
+  // PY-27398
+  public void testAccessToInitVar() {
+    doTest();
+  }
+
   @Override
   protected void doTest() {
     runWithLanguageLevel(