override http.HTTPMessage methods to only use str for the header …

…type (#11114)

stdlib/http/client.pyi

@@ -99,6 +99,24 @@ responses: dict[int, str]
 
 class HTTPMessage(email.message.Message):
     def getallmatchingheaders(self, name: str) -> list[str]: ...  # undocumented
+    # override below all of Message's methods that use `_HeaderType` / `_HeaderTypeParam` with `str`
+    # `HTTPMessage` breaks the Liskov substitution principle by only intending for `str` headers
+    # This is easier than making `Message` generic
+    def __getitem__(self, name: str) -> str | None: ...
+    def __setitem__(self, name: str, val: str) -> None: ...  # type: ignore[override]
+    def values(self) -> list[str]: ...
+    def items(self) -> list[tuple[str, str]]: ...
+    @overload
+    def get(self, name: str, failobj: None = None) -> str | None: ...
+    @overload
+    def get(self, name: str, failobj: _T) -> str | _T: ...
+    @overload
+    def get_all(self, name: str, failobj: None = None) -> list[str] | None: ...
+    @overload
+    def get_all(self, name: str, failobj: _T) -> list[str] | _T: ...
+    def replace_header(self, _name: str, _value: str) -> None: ...  # type: ignore[override]
+    def set_raw(self, name: str, value: str) -> None: ...  # type: ignore[override]
+    def raw_items(self) -> Iterator[tuple[str, str]]: ...
 
 def parse_headers(fp: io.BufferedIOBase, _class: Callable[[], email.message.Message] = ...) -> HTTPMessage: ...