Type variable defaults aren't checked properly against TypeAliasType(...) aliases

Bug Report, To Reproduce, & Actual Behaviour

Using K = TypeAliasType("K", int) fails here, whereas K: TypeAlias = int does not.

from typing_extensions import TypeAliasType, TypeVar

K = TypeAliasType("K", int)
KeyT = TypeVar("KeyT", str, K, default=K)  # E: TypeVar default must be one of the constraint types  [misc]

Expected Behavior

No issues

Additional information

no_args is True for K: TypeAlias = int, while it is False for K = TypeAliasType("K", int). A quick test indicates that commenting out and not pep_695 here prevents the error from occurring.

mypy/mypy/semanal.py

Lines 4048 to 4058 in 4fb187f

    
           # Note: with the new (lazy) type alias representation we only need to set no_args to True 
        
           # if the expected number of arguments is non-zero, so that aliases like `A = List` work 
        
           # but not aliases like `A = TypeAliasType("A", List)` as these need explicit type params. 
        
           # However, eagerly expanding aliases like Text = str is a nice performance optimization. 
        
           no_args = ( 
        
               isinstance(res, ProperType) 
        
               and isinstance(res, Instance) 
        
               and not res.args 
        
               and not empty_tuple_index 
        
               and not pep_695 
        
           )

The effect of no_args = False causes the constraints of KeyT to be evaluated as a TypeAliasType instead of being evaluated to an Instance,

mypy/mypy/typeanal.py

Lines 2199 to 2205 in 4fb187f

    
           if max_tv_count == 0 and act_len == 0: 
        
               if no_args: 
        
                   assert isinstance(node.target, Instance)  # type: ignore[misc] 
        
                   # Note: this is the only case where we use an eager expansion. See more info about 
        
                   # no_args aliases like L = List in the docstring for TypeAlias class. 
        
                   return Instance(node.target.type, [], line=ctx.line, column=ctx.column) 
        
               return TypeAliasType(node, [], line=ctx.line, column=ctx.column)

which then later causes the equality check p_default == value to fail:

mypy/mypy/checkexpr.py

Lines 6203 to 6204 in 4fb187f

    
           if e.values and not any(p_default == value for value in e.values): 
        
               self.chk.fail("TypeVar default must be one of the constraint types", e)

Your Environment

Mypy version used: 1.15, master
Mypy command-line flags: None
Mypy configuration options from mypy.ini (and other config files): None
Python version used: 3.9, 3.13

Merge remote-tracking branch 'upstream/master' into bugfix/pythongh-1…

Merge branch 'master' into bugfix/pythongh-18862-typevar-default-alias

Support type aliases, `NamedTuple` and `TypedDict` in constrained Typ…

Navigation Menu

Repository navigation

Type variable defaults aren't checked properly against `TypeAliasType(...)` aliases #18862

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

	# Note: with the new (lazy) type alias representation we only need to set no_args to True
	# if the expected number of arguments is non-zero, so that aliases like `A = List` work
	# but not aliases like `A = TypeAliasType("A", List)` as these need explicit type params.
	# However, eagerly expanding aliases like Text = str is a nice performance optimization.
	no_args = (
	isinstance(res, ProperType)
	and isinstance(res, Instance)
	and not res.args
	and not empty_tuple_index
	and not pep_695
	)

	if max_tv_count == 0 and act_len == 0:
	if no_args:
	assert isinstance(node.target, Instance) # type: ignore[misc]
	# Note: this is the only case where we use an eager expansion. See more info about
	# no_args aliases like L = List in the docstring for TypeAlias class.
	return Instance(node.target.type, [], line=ctx.line, column=ctx.column)
	return TypeAliasType(node, [], line=ctx.line, column=ctx.column)

	if e.values and not any(p_default == value for value in e.values):
	self.chk.fail("TypeVar default must be one of the constraint types", e)

Type variable defaults aren't checked properly against TypeAliasType(...) aliases #18862

Description

Activity

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

Type variable defaults aren't checked properly against `TypeAliasType(...)` aliases #18862