Do not emit unreachable warnings for lines that return NotImplemented.

Author: tyralla

mypy/checker.py

@@ -3147,6 +3147,8 @@ def is_noop_for_reachability(self, s: Statement) -> bool:
         """
         if isinstance(s, AssertStmt) and is_false_literal(s.expr):
             return True
+        elif isinstance(s, ReturnStmt) and is_literal_not_implemented(s.expr):
+            return True
         elif isinstance(s, (RaiseStmt, PassStmt)):
             return True
         elif isinstance(s, ExpressionStmt):

test-data/unit/check-unreachable-code.test

@@ -1620,6 +1620,26 @@ reveal_type(bar().attr)  # N: Revealed type is "Never"
 reveal_type(foo().attr)  # N: Revealed type is "Never"
 1  # E: Statement is unreachable
 
+[case testIgnoreReturningNotImplemented]
+# flags: --warn-unreachable
+
+class C:
+    def __add__(self, o: C) -> C:
+        if not isinstance(o, C):
+            return NotImplemented
+        return C()
+    def __sub__(self, o: C) -> C:
+        if isinstance(o, C):
+            return C()
+        return NotImplemented
+    def __mul__(self, o: C) -> C:
+        if isinstance(o, C):
+            return C()
+        else:
+            return NotImplemented
+
+[builtins fixtures/isinstance.pyi]
+
 [case testUnreachableStatementPrettyHighlighting]
 # flags: --warn-unreachable --pretty
 def x() -> None: