Raise errors on unbound TypeVars with values (#15732)

Completes a `TODO` item :)

Refs #15724

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: sobolevn

mypy/checker.py

@@ -1069,6 +1069,7 @@ def check_func_def(
         """Type check a function definition."""
         # Expand type variables with value restrictions to ordinary types.
         expanded = self.expand_typevars(defn, typ)
+        original_typ = typ
         for item, typ in expanded:
             old_binder = self.binder
             self.binder = ConditionalTypeBinder()
@@ -1126,6 +1127,12 @@ def check_func_def(
                             message_registry.RETURN_TYPE_CANNOT_BE_CONTRAVARIANT, typ.ret_type
                         )
                     self.check_unbound_return_typevar(typ)
+                elif (
+                    isinstance(original_typ.ret_type, TypeVarType) and original_typ.ret_type.values
+                ):
+                    # Since type vars with values are expanded, the return type is changed
+                    # to a raw value. This is a hack to get it back.
+                    self.check_unbound_return_typevar(original_typ)
 
                 # Check that Generator functions have the appropriate return type.
                 if defn.is_generator:

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

@@ -15,8 +15,7 @@ def g() -> U: # E: A function returning TypeVar should receive at least one argu
 
 V = TypeVar('V', int, str)
 
-# TODO: this should also give an error
-def h() -> V:
+def h() -> V:  # E: A function returning TypeVar should receive at least one argument containing the same TypeVar
     ...
 
 [case testInnerFunctionTypeVar]

test-data/unit/deps-generics.test

@@ -159,7 +159,7 @@ class D: pass
 T = TypeVar('T', A, B)
 S = TypeVar('S', C, D)
 
-def f(x: T) -> S:
+def f(x: T, y: S) -> S:
     pass
 [out]
 <m.A> -> <m.T>, <m.f>, m, m.A, m.f