Make NoReturn compatible with all other types

docs/changelog.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Make `NoReturn` compatible with all other types (#442)
 - Fix treatment of walrus operator in `and`, `or`, and `if/else`
   expressions (#441)
 - Refactor `isinstance()` support (#440)

pyanalyze/test_name_check_visitor.py

@@ -658,31 +658,6 @@ def capybara():
             assert_is_value(x, KnownValue(3))
 
 
-class TestNoReturn(TestNameCheckVisitorBase):
-    @assert_passes()
-    def test_no_return(self):
-        from typing_extensions import NoReturn
-        from typing import Optional
-
-        def f() -> NoReturn:
-            raise Exception
-
-        def capybara(x: Optional[int]) -> None:
-            if x is None:
-                f()
-            assert_is_value(x, TypedValue(int))
-
-    @assert_fails(ErrorCode.incompatible_argument)
-    def test_no_return_parameter(self):
-        from typing_extensions import NoReturn
-
-        def assert_unreachable(x: NoReturn) -> None:
-            pass
-
-        def capybara():
-            assert_unreachable(1)
-
-
 class TestSubclassValue(TestNameCheckVisitorBase):
     # In 3.7 the behavior of Type[] changed.
     @only_before((3, 7))

pyanalyze/test_never.py

@@ -0,0 +1,37 @@
+# static analysis: ignore
+from .value import TypedValue
+from .implementation import assert_is_value
+from .test_node_visitor import assert_passes
+from .test_name_check_visitor import TestNameCheckVisitorBase
+
+
+class TestNoReturn(TestNameCheckVisitorBase):
+    @assert_passes()
+    def test_no_return(self):
+        from typing_extensions import NoReturn
+        from typing import Optional
+
+        def f() -> NoReturn:
+            raise Exception
+
+        def capybara(x: Optional[int]) -> None:
+            if x is None:
+                f()
+            assert_is_value(x, TypedValue(int))
+
+    @assert_passes()
+    def test_no_return_parameter(self):
+        from typing_extensions import NoReturn
+
+        def assert_unreachable(x: NoReturn) -> None:
+            pass
+
+        def capybara():
+            assert_unreachable(1)  # E: incompatible_argument
+
+    @assert_passes()
+    def test_assignability(self):
+        from typing_extensions import NoReturn
+
+        def takes_never(x: NoReturn):
+            print(x)

pyanalyze/value.py

@@ -85,6 +85,9 @@ def can_assign(self, other: "Value", ctx: "CanAssignContext") -> "CanAssign":
             ctx.record_any_used()
             return {}
         elif isinstance(other, MultiValuedValue):
+            # The bottom type is assignable to every other type.
+            if other is NO_RETURN_VALUE:
+                return {}
             tv_maps = []
             for val in other.vals:
                 tv_map = self.can_assign(val, ctx)