From bd915c5074450cc217861e2158b45f11b10ab9a7 Mon Sep 17 00:00:00 2001
From: vaggelisd <daniasevangelos@gmail.com>
Date: Tue, 6 Oct 2026 11:12:41 +0300
Subject: [PATCH] [mypyc] Fix property assignment when the setter accepts a
 wider type than the getter

---
 mypyc/codegen/emitfunc.py        |  2 +-
 mypyc/irbuild/targets.py         |  9 ++++
 mypyc/test-data/run-classes.test | 70 ++++++++++++++++++++++++++++++++
 3 files changed, 80 insertions(+), 1 deletion(-)

diff --git a/mypyc/codegen/emitfunc.py b/mypyc/codegen/emitfunc.py
index 8637a2565006d..a1864b3acd31e 100644
--- a/mypyc/codegen/emitfunc.py
+++ b/mypyc/codegen/emitfunc.py
@@ -621,7 +621,7 @@ def visit_set_attr(self, op: SetAttr) -> None:
                     rtype.setter_index(op.attr),
                     src,
                     rtype.struct_name(self.names),
-                    self.ctype(rtype.attr_type(op.attr)),
+                    self.ctype(op.propset.sig.args[1].type),
                     c_ret_type,
                     op.attr,
                 )
diff --git a/mypyc/irbuild/targets.py b/mypyc/irbuild/targets.py
index 8bc9da074f07d..67a5ed6f2ac4f 100644
--- a/mypyc/irbuild/targets.py
+++ b/mypyc/irbuild/targets.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from mypyc.common import PROPSET_PREFIX
 from mypyc.ir.ops import Register, Value
 from mypyc.ir.rtypes import RInstance, RType, object_rprimitive
 
@@ -49,6 +50,14 @@ def __init__(self, obj: Value, attr: str, can_borrow: bool = False) -> None:
             # Native attribute reference
             self.obj_type: RType = obj.type
             self.type = obj.type.attr_type(attr)
+            # A property setter may accept a wider type than its getter returns,
+            # so values assigned through it are coerced to the setter's argument type
+            for ir in obj.type.class_ir.mro:
+                setter = ir.method_decls.get(PROPSET_PREFIX + attr)
+                if setter is not None:
+                    if not setter.implicit:
+                        self.type = setter.sig.args[1].type
+                    break
         else:
             # Python attribute reference
             self.obj_type = object_rprimitive
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index 77f029b6b80b5..588c6d5c01643 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -2317,6 +2317,76 @@ import other_interpreted
 
 [out]
 
+[case testPropertySetterWiderThanGetter]
+from typing import Union
+from mypy_extensions import trait
+
+class Wrapped:
+    def __init__(self, s: str) -> None:
+        self.s = s
+
+@trait
+class T:
+    @property
+    def value(self) -> Wrapped:
+        raise NotImplementedError
+
+    @value.setter
+    def value(self, v: Union[Wrapped, str]) -> None:
+        raise NotImplementedError
+
+class C(T):
+    def __init__(self) -> None:
+        self._value = Wrapped("init")
+        self._n = 0
+
+    @property
+    def value(self) -> Wrapped:
+        return self._value
+
+    @value.setter
+    def value(self, v: Union[Wrapped, str]) -> None:
+        self._value = Wrapped(v) if isinstance(v, str) else v
+
+    @property
+    def n(self) -> int:
+        return self._n
+
+    @n.setter
+    def n(self, v: Union[int, str]) -> None:
+        self._n = int(v)
+
+def set_via_class(c: C, v: Union[Wrapped, str]) -> str:
+    c.value = v
+    return c.value.s
+
+def set_via_trait(t: T, v: Union[Wrapped, str]) -> str:
+    t.value = v
+    return t.value.s
+
+def set_unboxed(c: C, v: Union[int, str]) -> int:
+    c.n = v
+    return c.n
+
+def test_setter_wider_than_getter() -> None:
+    c = C()
+    assert set_via_class(c, "a") == "a"
+    assert set_via_class(c, Wrapped("b")) == "b"
+    assert set_via_trait(c, "c") == "c"
+    assert set_via_trait(c, Wrapped("d")) == "d"
+    assert set_unboxed(c, "5") == 5
+    assert set_unboxed(c, 6) == 6
+
+[file driver.py]
+from native import C, test_setter_wider_than_getter
+test_setter_wider_than_getter()
+
+c = C()
+c.value = "py"
+assert c.value.s == "py"
+c.n = "7"
+assert c.n == 7
+
 [case testAttributeOverridesProperty]
 from typing import Any
 from mypy_extensions import trait