[PEP 747] Recognize TypeForm[T] type and values (#9773)

User must opt-in to use TypeForm with --enable-incomplete-feature=TypeForm

In particular:
* Recognize TypeForm[T] as a kind of type that can be used in a type expression
* Recognize a type expression literal as a TypeForm value in:
    - assignments
    - function calls
    - return statements
* Define the following relationships between TypeForm values:
    - is_subtype
    - join_types
    - meet_types
* Recognize the TypeForm(...) expression
* Alter isinstance(typx, type) to narrow TypeForm[T] to Type[T]

Author: davidfstr

mypy/checker.py

@@ -7459,7 +7459,10 @@ def add_any_attribute_to_type(self, typ: Type, name: str) -> Type:
             fallback = typ.fallback.copy_with_extra_attr(name, any_type)
             return typ.copy_modified(fallback=fallback)
         if isinstance(typ, TypeType) and isinstance(typ.item, Instance):
-            return TypeType.make_normalized(self.add_any_attribute_to_type(typ.item, name))
+            return TypeType.make_normalized(
+                self.add_any_attribute_to_type(typ.item, name),
+                is_type_form=typ.is_type_form,
+            )
         if isinstance(typ, TypeVarType):
             return typ.copy_modified(
                 upper_bound=self.add_any_attribute_to_type(typ.upper_bound, name),

mypy/checkexpr.py

@@ -90,6 +90,7 @@
     TypeAliasExpr,
     TypeApplication,
     TypedDictExpr,
+    TypeFormExpr,
     TypeInfo,
     TypeVarExpr,
     TypeVarTupleExpr,
@@ -4601,6 +4602,10 @@ def visit_cast_expr(self, expr: CastExpr) -> Type:
         )
         return target_type
 
+    def visit_type_form_expr(self, expr: TypeFormExpr) -> Type:
+        typ = expr.type
+        return TypeType.make_normalized(typ, line=typ.line, column=typ.column, is_type_form=True)
+
     def visit_assert_type_expr(self, expr: AssertTypeExpr) -> Type:
         source_type = self.accept(
             expr.expr,
@@ -5831,6 +5836,7 @@ def accept(
         old_is_callee = self.is_callee
         self.is_callee = is_callee
         try:
+            p_type_context = get_proper_type(type_context)
             if allow_none_return and isinstance(node, CallExpr):
                 typ = self.visit_call_expr(node, allow_none_return=True)
             elif allow_none_return and isinstance(node, YieldFromExpr):
@@ -5839,6 +5845,17 @@ def accept(
                 typ = self.visit_conditional_expr(node, allow_none_return=True)
             elif allow_none_return and isinstance(node, AwaitExpr):
                 typ = self.visit_await_expr(node, allow_none_return=True)
+            elif (
+                isinstance(p_type_context, TypeType) and
+                p_type_context.is_type_form and
+                node.as_type is not None
+            ):
+                typ = TypeType.make_normalized(
+                    node.as_type,
+                    line=node.as_type.line,
+                    column=node.as_type.column,
+                    is_type_form=True,
+                )
             else:
                 typ = node.accept(self)
         except Exception as err:

mypy/copytype.py

@@ -120,7 +120,7 @@ def visit_overloaded(self, t: Overloaded) -> ProperType:
 
     def visit_type_type(self, t: TypeType) -> ProperType:
         # Use cast since the type annotations in TypeType are imprecise.
-        return self.copy_common(t, TypeType(cast(Any, t.item)))
+        return self.copy_common(t, TypeType(cast(Any, t.item), is_type_form=t.is_type_form))
 
     def visit_type_alias_type(self, t: TypeAliasType) -> ProperType:
         assert False, "only ProperTypes supported"

mypy/erasetype.py

@@ -133,7 +133,7 @@ def visit_union_type(self, t: UnionType) -> ProperType:
         return make_simplified_union(erased_items)
 
     def visit_type_type(self, t: TypeType) -> ProperType:
-        return TypeType.make_normalized(t.item.accept(self), line=t.line)
+        return TypeType.make_normalized(t.item.accept(self), line=t.line, is_type_form=t.is_type_form)
 
     def visit_type_alias_type(self, t: TypeAliasType) -> ProperType:
         raise RuntimeError("Type aliases should be expanded before accepting this visitor")

mypy/evalexpr.py

@@ -75,6 +75,9 @@ def visit_comparison_expr(self, o: mypy.nodes.ComparisonExpr) -> object:
     def visit_cast_expr(self, o: mypy.nodes.CastExpr) -> object:
         return o.expr.accept(self)
 
+    def visit_type_form_expr(self, o: mypy.nodes.TypeFormExpr) -> object:
+        return UNKNOWN
+
     def visit_assert_type_expr(self, o: mypy.nodes.AssertTypeExpr) -> object:
         return o.expr.accept(self)
 

mypy/expandtype.py

@@ -500,7 +500,7 @@ def visit_type_type(self, t: TypeType) -> Type:
         # union of instances or Any).  Sadly we can't report errors
         # here yet.
         item = t.item.accept(self)
-        return TypeType.make_normalized(item)
+        return TypeType.make_normalized(item, is_type_form=t.is_type_form)
 
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:
         # Target of the type alias cannot contain type variables (not bound by the type

mypy/join.py

@@ -657,7 +657,11 @@ def visit_partial_type(self, t: PartialType) -> ProperType:
 
     def visit_type_type(self, t: TypeType) -> ProperType:
         if isinstance(self.s, TypeType):
-            return TypeType.make_normalized(join_types(t.item, self.s.item), line=t.line)
+            return TypeType.make_normalized(
+                join_types(t.item, self.s.item),
+                line=t.line,
+                is_type_form=self.s.is_type_form or t.is_type_form,
+            )
         elif isinstance(self.s, Instance) and self.s.type.fullname == "builtins.type":
             return self.s
         else:

mypy/literals.py

@@ -47,6 +47,7 @@
     TypeAliasExpr,
     TypeApplication,
     TypedDictExpr,
+    TypeFormExpr,
     TypeVarExpr,
     TypeVarTupleExpr,
     UnaryExpr,
@@ -230,6 +231,9 @@ def visit_slice_expr(self, e: SliceExpr) -> None:
     def visit_cast_expr(self, e: CastExpr) -> None:
         return None
 
+    def visit_type_form_expr(self, e: TypeFormExpr) -> None:
+        return None
+
     def visit_assert_type_expr(self, e: AssertTypeExpr) -> None:
         return None
 

mypy/meet.py

@@ -157,12 +157,27 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
     elif isinstance(narrowed, TypeVarType) and is_subtype(narrowed.upper_bound, declared):
         return narrowed
     elif isinstance(declared, TypeType) and isinstance(narrowed, TypeType):
-        return TypeType.make_normalized(narrow_declared_type(declared.item, narrowed.item))
+        return TypeType.make_normalized(
+            narrow_declared_type(declared.item, narrowed.item),
+            is_type_form=declared.is_type_form and narrowed.is_type_form,
+        )
     elif (
         isinstance(declared, TypeType)
         and isinstance(narrowed, Instance)
         and narrowed.type.is_metaclass()
     ):
+        if declared.is_type_form:
+            # The declared TypeForm[T] after narrowing must be a kind of
+            # type object at least as narrow as Type[T]
+            return narrow_declared_type(
+                TypeType.make_normalized(
+                    declared.item,
+                    line=declared.line,
+                    column=declared.column,
+                    is_type_form=False,
+                ),
+                original_narrowed,
+            )
         # We'd need intersection types, so give up.
         return original_declared
     elif isinstance(declared, Instance):
@@ -1039,7 +1054,11 @@ def visit_type_type(self, t: TypeType) -> ProperType:
         if isinstance(self.s, TypeType):
             typ = self.meet(t.item, self.s.item)
             if not isinstance(typ, NoneType):
-                typ = TypeType.make_normalized(typ, line=t.line)
+                typ = TypeType.make_normalized(
+                    typ,
+                    line=t.line,
+                    is_type_form=self.s.is_type_form and t.is_type_form,
+                )
             return typ
         elif isinstance(self.s, Instance) and self.s.type.fullname == "builtins.type":
             return t

mypy/messages.py

@@ -2675,7 +2675,10 @@ def format_literal_value(typ: LiteralType) -> str:
     elif isinstance(typ, UninhabitedType):
         return "Never"
     elif isinstance(typ, TypeType):
-        type_name = "type" if options.use_lowercase_names() else "Type"
+        if typ.is_type_form:
+            type_name = "TypeForm"
+        else:
+            type_name = "type" if options.use_lowercase_names() else "Type"
         return f"{type_name}[{format(typ.item)}]"
     elif isinstance(typ, FunctionLike):
         func = typ