Disallow TypeVar constraints parameterized by type variables

[brianschubert]

Emit an error for this test case from the conformance tests:

class Test(Generic[T]):
    BadConstraint2 = TypeVar("BadConstraint2", str, list[T])  # E

[brianschubert]

Conformance results diff for this PR (generics_basic now passes):

diff --git a/conformance/results/mypy/generics_basic.toml b/conformance/results/mypy/generics_basic.toml
index 00c8244..b15fe50 100644
--- a/conformance/results/mypy/generics_basic.toml
+++ b/conformance/results/mypy/generics_basic.toml
@@ -6,6 +6,7 @@ output = """
 generics_basic.py:40: error: Value of type variable "AnyStr" of "concat" cannot be "Sequence[object]"  [type-var]
 generics_basic.py:41: error: Value of type variable "AnyStr" of "concat" cannot be "Sequence[object]"  [type-var]
 generics_basic.py:49: error: Type variable must have at least two constrained types  [misc]
+generics_basic.py:55: error: TypeVar constraint type cannot be parametrized by type variables  [misc]
 generics_basic.py:69: error: Value of type variable "AnyStr" of "concat" cannot be "Sequence[object]"  [type-var]
 generics_basic.py:121: error: Duplicate type variables in Generic[...] or Protocol[...]  [misc]
 generics_basic.py:157: error: Invalid index type "int" for "MyMap1[str, int]"; expected type "str"  [index]
@@ -15,7 +16,6 @@ generics_basic.py:191: error: Type variable "generics_basic.T" is unbound  [vali
 generics_basic.py:191: note: (Hint: Use "Generic[T]" or "Protocol[T]" base class to bind "T" inside a class)
 generics_basic.py:191: note: (Hint: Use "T" in function signature to bind "T" inside a function)
 """
-conformance_automated = "Fail"
+conformance_automated = "Pass"
 errors_diff = """
-Line 55: Expected 1 errors
 """
diff --git a/conformance/results/mypy/version.toml b/conformance/results/mypy/version.toml
index 51d97b4..59e0dd2 100644
--- a/conformance/results/mypy/version.toml
+++ b/conformance/results/mypy/version.toml
@@ -1,2 +1,2 @@
-version = "mypy 1.14.0+dev.499adaed8adbded1a180e30d071438fef81779ec"
-test_duration = 2.4
+version = "mypy 1.14.0+dev.841096633fe6255acb7840e4a7b0fce975b8ca24"
+test_duration = 4.4

[brianschubert]

Two approaches, not sure which is better:

1. Original approach (8410966), uses modified FindTypeVarVisitor, more complex, robust to placeholder types
2. Second approach (a8171c6), uses has_no_typevars, simpler, but may have false negatives if a constraint type includes both a placeholder type and a type variable.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅