Type variable defaults, solved for partially-parameterised generic types, do not reset

[bzoracler]

Bug Report, To Reproduce, & Actual Behaviour

See mypy Playground:

from typing_extensions import Generic, TypeVar

T1 = TypeVar("T1")
T2 = TypeVar("T2", default=T1)

class MyClass(Generic[T1, T2]): ...

reveal_type(MyClass[int]) # N: Revealed type is "def [T2 = builtins.int] () -> __main__.MyClass[builtins.int, T2`2 = builtins.int]"

# `T2` now defaults to `int` for `MyClass` from now on
reveal_type(MyClass[str]) # N: Revealed type is "def [T2 = builtins.int] () -> __main__.MyClass[builtins.str, T2`2 = builtins.int]"
reveal_type(MyClass[bytes]) # N: Revealed type is "def [T2 = builtins.int] () -> __main__.MyClass[builtins.bytes, T2`2 = builtins.int]"

Expected Behavior

reveal_type(MyClass[str]) # N: Revealed type is "def [T2 = builtins.str] () -> __main__.MyClass[builtins.str, T2`2 = builtins.str]"
reveal_type(MyClass[bytes]) # N: Revealed type is "def [T2 = builtins.bytes] () -> __main__.MyClass[builtins.bytes, T2`2 = builtins.bytes]"

Your Environment

• Mypy version used: 1.15, 1.16
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9, 3.12

[sterliakov]

This is likely the same issue as #19186: type vars defaulting to other type vars share their default across all use sites. We need deep copies of default whenever the variable is allocated to another class/function, but AFAIC we don't have a type transform visitor that can create deep copies.