Allow using expand_type during semantic analyzis (#15201)

This is another PR to prepare for variadic type aliases.

mypy/expandtype.py

@@ -21,6 +21,7 @@
     ParamSpecType,
     PartialType,
     ProperType,
+    TrivialSyntheticTypeTranslator,
     TupleType,
     Type,
     TypeAliasType,
@@ -30,7 +31,6 @@
     TypeVarLikeType,
     TypeVarTupleType,
     TypeVarType,
-    TypeVisitor,
     UnboundType,
     UninhabitedType,
     UnionType,
@@ -46,6 +46,10 @@
     split_with_prefix_and_suffix,
 )
 
+# WARNING: these functions should never (directly or indirectly) depend on
+# is_subtype(), meet_types(), join_types() etc.
+# TODO: add a static dependency test for this.
+
 
 @overload
 def expand_type(typ: CallableType, env: Mapping[TypeVarId, Type]) -> CallableType:
@@ -182,7 +186,7 @@ def visit_type_alias_type(self, t: TypeAliasType) -> Type:
         return t.copy_modified(args=[arg.accept(self) for arg in t.args])
 
 
-class ExpandTypeVisitor(TypeVisitor[Type]):
+class ExpandTypeVisitor(TrivialSyntheticTypeTranslator):
     """Visitor that substitutes type variables with values."""
 
     variables: Mapping[TypeVarId, Type]  # TypeVar id -> TypeVar value

mypy/semanal_typeddict.py

@@ -6,6 +6,7 @@
 
 from mypy import errorcodes as codes, message_registry
 from mypy.errorcodes import ErrorCode
+from mypy.expandtype import expand_type
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.messages import MessageBuilder
 from mypy.nodes import (
@@ -45,7 +46,6 @@
     TypedDictType,
     TypeOfAny,
     TypeVarLikeType,
-    replace_alias_tvars,
 )
 
 TPDICT_CLASS_ERROR: Final = (
@@ -243,21 +243,18 @@ def map_items_to_base(
     ) -> dict[str, Type]:
         """Map item types to how they would look in their base with type arguments applied.
 
-        We would normally use expand_type() for such task, but we can't use it during
-        semantic analysis, because it can (indirectly) call is_subtype() etc., and it
-        will crash on placeholder types. So we hijack replace_alias_tvars() that was initially
-        intended to deal with eager expansion of generic type aliases during semantic analysis.
+        Note it is safe to use expand_type() during semantic analysis, because it should never
+        (indirectly) call is_subtype().
         """
         mapped_items = {}
         for key in valid_items:
             type_in_base = valid_items[key]
             if not tvars:
                 mapped_items[key] = type_in_base
                 continue
-            mapped_type = replace_alias_tvars(
-                type_in_base, tvars, base_args, type_in_base.line, type_in_base.column
+            mapped_items[key] = expand_type(
+                type_in_base, {t.id: a for (t, a) in zip(tvars, base_args)}
             )
-            mapped_items[key] = mapped_type
         return mapped_items
 
     def analyze_typeddict_classdef_fields(