From 42b43445ee515ade652f4dd2bb56c29167676299 Mon Sep 17 00:00:00 2001
From: Christian Kanesan <christian.kanesan@pix4d.com>
Date: Mon, 8 Apr 2019 09:21:26 +0200
Subject: [PATCH 1/2] Dataclasses: support mutable properties

Skip over attributes that are redefined as properties on dataclass construction
---
 mypy/plugins/dataclasses.py           |  7 +++++--
 test-data/unit/check-dataclasses.test | 28 +++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
index dc23b3f42454a..c565b65cd8932 100644
--- a/mypy/plugins/dataclasses.py
+++ b/mypy/plugins/dataclasses.py
@@ -3,7 +3,7 @@
 
 from mypy.nodes import (
     ARG_OPT, ARG_POS, MDEF, Argument, AssignmentStmt, CallExpr,
-    Context, Expression, FuncDef, JsonDict, NameExpr,
+    Context, Expression, FuncDef, JsonDict, NameExpr, OverloadedFuncDef,
     SymbolTableNode, TempNode, TypeInfo, Var, TypeVarExpr
 )
 from mypy.plugin import ClassDefContext
@@ -204,7 +204,10 @@ def collect_attributes(self) -> List[DataclassAttribute]:
                 continue
 
             node = cls.info.names[lhs.name].node
-            assert isinstance(node, Var)
+            assert isinstance(node, (Var, OverloadedFuncDef))
+
+            if isinstance(node, OverloadedFuncDef):
+                continue
 
             # x: ClassVar[int] is ignored by dataclasses.
             if node.is_classvar:
diff --git a/test-data/unit/check-dataclasses.test b/test-data/unit/check-dataclasses.test
index 0b094d64e6853..f4853acde9481 100644
--- a/test-data/unit/check-dataclasses.test
+++ b/test-data/unit/check-dataclasses.test
@@ -559,3 +559,31 @@ class Foo:
     bar: Optional[int] = field(default=None)
 [builtins fixtures/list.pyi]
 [out]
+
+[case testMutableProperty]
+# flags: --python-version 3.6
+from dataclasses import dataclass
+from typing import Optional
+
+@dataclass(init=False)
+class C:
+    a: Optional[int]
+
+    def __init__(self, a: Optional[int] = None) -> None:
+        self._a: Optional[int] = None
+        self.a = a
+
+    @property
+    def a(self) -> Optional[int]:
+        return self._a
+
+    @a.setter
+    def a(self, value: Optional[int]) -> Optional[int]:
+        self._a = value
+        return self._a
+
+c = C()
+c.a = 10
+c.a = "foo"  # E: Incompatible types in assignment (expression has type "str", variable has type "Optional[int]")
+
+[builtins fixtures/list.pyi]

From 10203fe25f1bc8a265416e97df7b07eccbebae9d Mon Sep 17 00:00:00 2001
From: Christian Kanesan <christian.kanesan@pix4d.com>
Date: Mon, 15 Apr 2019 09:08:18 +0200
Subject: [PATCH 2/2] Skip non variable assignment statements

---
 mypy/plugins/dataclasses.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
index c565b65cd8932..355a7dd2b9bb9 100644
--- a/mypy/plugins/dataclasses.py
+++ b/mypy/plugins/dataclasses.py
@@ -3,7 +3,7 @@
 
 from mypy.nodes import (
     ARG_OPT, ARG_POS, MDEF, Argument, AssignmentStmt, CallExpr,
-    Context, Expression, FuncDef, JsonDict, NameExpr, OverloadedFuncDef,
+    Context, Expression, FuncDef, JsonDict, NameExpr,
     SymbolTableNode, TempNode, TypeInfo, Var, TypeVarExpr
 )
 from mypy.plugin import ClassDefContext
@@ -204,9 +204,7 @@ def collect_attributes(self) -> List[DataclassAttribute]:
                 continue
 
             node = cls.info.names[lhs.name].node
-            assert isinstance(node, (Var, OverloadedFuncDef))
-
-            if isinstance(node, OverloadedFuncDef):
+            if not isinstance(node, Var):
                 continue
 
             # x: ClassVar[int] is ignored by dataclasses.