Add no-overload-impl error code (python#11944)

Author: Brian Phillips

docs/source/error_code_list.rst

@@ -657,6 +657,28 @@ consistently when using the call-based syntax. Example:
     # Error: First argument to namedtuple() should be "Point2D", not "Point"
     Point2D = NamedTuple("Point", [("x", int), ("y", int)])
 
+Check that overloaded functions have an implementation [no-overload-impl]
+-------------------------------------------------------------------------
+
+Overloaded functions outside of stub files must be followed by a non overloaded
+implementation.
+
+.. code-block:: python
+
+   from typing import overload
+
+   @overload
+   def func(value: int) -> int:
+       ...
+
+   @overload
+   def func(value: str) -> str:
+       ...
+
+   # presence of required function below is checked
+   def func(value):
+       pass  # actual implementation
+
 Report syntax errors [syntax]
 -----------------------------
 

mypy/errorcodes.py

@@ -133,6 +133,11 @@ def __str__(self) -> str:
 NAME_MATCH: Final = ErrorCode(
     "name-match", "Check that type definition has consistent naming", "General"
 )
+NO_OVERLOAD_IMPL: Final = ErrorCode(
+    "no-overload-impl",
+    "Check that overloaded functions outside stub files have an implementation",
+    "General",
+)
 
 
 # Syntax errors are often blocking.

mypy/semanal.py

@@ -880,7 +880,7 @@ def handle_missing_overload_implementation(self, defn: OverloadedFuncDef) -> Non
             else:
                 self.fail(
                     "An overloaded function outside a stub file must have an implementation",
-                    defn)
+                    defn, code=codes.NO_OVERLOAD_IMPL)
 
     def process_final_in_overload(self, defn: OverloadedFuncDef) -> None:
         """Detect the @final status of an overloaded function (and perform checks)."""

test-data/unit/check-errorcodes.test

@@ -898,6 +898,17 @@ if lst:
     pass
 [builtins fixtures/list.pyi]
 
+[case testNoOverloadImplementation]
+from typing import overload
+
+@overload  # E: An overloaded function outside a stub file must have an implementation  [no-overload-impl]
+def f(arg: int) -> int:
+    ...
+
+@overload
+def f(arg: str) -> str:
+    ...
+
 [case testSliceInDict39]
 # flags: --python-version 3.9 --show-column-numbers
 from typing import Dict