Fix type comments crash inside generic definitions (python#16849)

Closes python#16649

It's the first time I am contributing to mypy so I am not very familiar
with how it works entirely behind the scene. The issue that I had is
that a crash happens when using tuple type comments inside
functions/classes that depend on a *constrained* type variable.

After investigation, the reason is that the type checker generates all
possible definitions (since constraints are known) and expands the
functions definitions and bodies accordingly. However, by doing so, a
tuple type comment ('# type: (int, float)') would have a FakeInfo, so
`ExpandTypeVisitor` would fail since it queries `t.type.fullname`.

By the way, feel free to change where my test should lie.

mypy/expandtype.py

@@ -2,7 +2,7 @@
 
 from typing import Final, Iterable, Mapping, Sequence, TypeVar, cast, overload
 
-from mypy.nodes import ARG_STAR, Var
+from mypy.nodes import ARG_STAR, FakeInfo, Var
 from mypy.state import state
 from mypy.types import (
     ANY_STRATEGY,
@@ -208,6 +208,16 @@ def visit_erased_type(self, t: ErasedType) -> Type:
 
     def visit_instance(self, t: Instance) -> Type:
         args = self.expand_types_with_unpack(list(t.args))
+
+        if isinstance(t.type, FakeInfo):
+            # The type checker expands function definitions and bodies
+            # if they depend on constrained type variables but the body
+            # might contain a tuple type comment (e.g., # type: (int, float)),
+            # in which case 't.type' is not yet available.
+            #
+            # See: https://github.com/python/mypy/issues/16649
+            return t.copy_modified(args=args)
+
         if t.type.fullname == "builtins.tuple":
             # Normalize Tuple[*Tuple[X, ...], ...] -> Tuple[X, ...]
             arg = args[0]

test-data/unit/check-typevar-values.test

@@ -706,3 +706,29 @@ Func = Callable[[], T]
 
 class A: ...
 class B: ...
+
+[case testTypeCommentInGenericTypeWithConstrainedTypeVar]
+from typing import Generic, TypeVar
+
+NT = TypeVar("NT", int, float)
+
+class Foo1(Generic[NT]):
+    p = 1  # type: int
+
+class Foo2(Generic[NT]):
+    p, q = 1, 2.0  # type: (int, float)
+
+class Foo3(Generic[NT]):
+    def bar(self) -> None:
+        p = 1  # type: int
+
+class Foo4(Generic[NT]):
+    def bar(self) -> None:
+        p, q = 1, 2.0  # type: (int, float)
+
+def foo3(x: NT) -> None:
+    p = 1  # type: int
+
+def foo4(x: NT) -> None:
+    p, q = 1, 2.0  # type: (int, float)
+[builtins fixtures/tuple.pyi]