Consistently use literal-required error code for TypedDicts (#14621)

Ref #7178

This code is used for some TypedDict errors, but `misc` was used for
others. I make it more consistent. Also this code looks undocumented, so
I add some basic docs.

docs/source/error_code_list.rst

@@ -804,6 +804,35 @@ 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 literal is used where expected [literal-required]
+------------------------------------------------------------
+
+There are some places where only a (string) literal value is expected for
+the purposes of static type checking, for example a ``TypedDict`` key, or
+a ``__match_args__`` item. Providing a ``str``-valued variable in such contexts
+will result in an error. Note however, in many cases you can use ``Final``,
+or ``Literal`` variables, for example:
+
+.. code-block:: python
+
+   from typing import Final, Literal, TypedDict
+
+   class Point(TypedDict):
+       x: int
+       y: int
+
+   def test(p: Point) -> None:
+       X: Final = "x"
+       p[X]  # OK
+
+       Y: Literal["y"] = "y"
+       p[Y]  # OK
+
+       key = "x"  # Inferred type of key is `str`
+       # Error: TypedDict key must be a string literal;
+       #   expected one of ("x", "y")  [literal-required]
+       p[key]
+
 Check that overloaded functions have an implementation [no-overload-impl]
 -------------------------------------------------------------------------
 

mypy/checkexpr.py

@@ -724,7 +724,11 @@ def validate_typeddict_kwargs(self, kwargs: DictExpr) -> dict[str, Expression] |
                     literal_value = values[0]
             if literal_value is None:
                 key_context = item_name_expr or item_arg
-                self.chk.fail(message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL, key_context)
+                self.chk.fail(
+                    message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL,
+                    key_context,
+                    code=codes.LITERAL_REQ,
+                )
                 return None
             else:
                 item_names.append(literal_value)

mypy/plugins/default.py

@@ -3,6 +3,7 @@
 from functools import partial
 from typing import Callable
 
+import mypy.errorcodes as codes
 from mypy import message_registry
 from mypy.nodes import DictExpr, IntExpr, StrExpr, UnaryExpr
 from mypy.plugin import (
@@ -264,7 +265,11 @@ def typed_dict_pop_callback(ctx: MethodContext) -> Type:
     ):
         keys = try_getting_str_literals(ctx.args[0][0], ctx.arg_types[0][0])
         if keys is None:
-            ctx.api.fail(message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL, ctx.context)
+            ctx.api.fail(
+                message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL,
+                ctx.context,
+                code=codes.LITERAL_REQ,
+            )
             return AnyType(TypeOfAny.from_error)
 
         value_types = []
@@ -319,7 +324,11 @@ def typed_dict_setdefault_callback(ctx: MethodContext) -> Type:
     ):
         keys = try_getting_str_literals(ctx.args[0][0], ctx.arg_types[0][0])
         if keys is None:
-            ctx.api.fail(message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL, ctx.context)
+            ctx.api.fail(
+                message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL,
+                ctx.context,
+                code=codes.LITERAL_REQ,
+            )
             return AnyType(TypeOfAny.from_error)
 
         default_type = ctx.arg_types[1][0]
@@ -357,7 +366,11 @@ def typed_dict_delitem_callback(ctx: MethodContext) -> Type:
     ):
         keys = try_getting_str_literals(ctx.args[0][0], ctx.arg_types[0][0])
         if keys is None:
-            ctx.api.fail(message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL, ctx.context)
+            ctx.api.fail(
+                message_registry.TYPEDDICT_KEY_MUST_BE_STRING_LITERAL,
+                ctx.context,
+                code=codes.LITERAL_REQ,
+            )
             return AnyType(TypeOfAny.from_error)
 
         for key in keys: