Add unbound typevar check (#13166)

### Description

This should fix Issue #13061 .

Mypy should now raise an error when it encounters unbound plain TypeVar.
It should not get triggered when the return type contains a TypeVar (e.g. List[T]).
It should not get triggered when a same TypeVar is present in the scope (e.g. inner function returns T, and T is an argument of outer function).

## Test Plan

5 tests were added:

- a plain unbound typevar triggering the new error
- a typevar inner function not triggering the error
- a function returning an iterable of typevars, not triggering the error
- a plain bound typevar, not triggering the error
- nested bound typevars, not triggering the error

We also changed the other functions triggering our error for the tests to pass.

This is our 1st contribution to Mypy. Please, guide us if there is anything we can improve in this PR.

This PR was made with @anilbey 


* Add simple unbound typevar check and rudimentary test

* Add test for unbound func returning iterables of TypeVars

* Add appropriate error message and make the new test pass

* add CollectArgTypes to get set of argument types

* lint fix

* fix type error

* extract check_unbound_return_typevar as method

* check if return type is instantiated in typ.variables

* Fix some tests that are affected by new implementation

* add testInnerFunctionTypeVar test

* fix testErrorCodeNeedTypeAnnotation test

* add TYPE_VAR error code to unbound error failure

* Fix the tests

* move new tests in new test file

* add 'check-typevar-unbound.test' to testcheck

* add more nested tests for unbound type

* microoptimise check_unbound_return_typevar check ret_type first

* add missing builtins fixture to failing test

Co-authored-by: Jaquier Aurélien Tristan <ajaquier@bb-fvfcv295mnhx.epfl.ch>
Co-authored-by: Anil Tuncel <anil.tuncel@epfl.ch>
Co-authored-by: Anil Tuncel <tuncel.manil@gmail.com>
Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

mypy/checker.py

@@ -12,6 +12,7 @@
 from typing_extensions import Final, TypeAlias as _TypeAlias
 
 from mypy.backports import nullcontext
+from mypy.errorcodes import TYPE_VAR
 from mypy.errors import Errors, report_internal_error, ErrorWatcher
 from mypy.nodes import (
     SymbolTable, Statement, MypyFile, Var, Expression, Lvalue, Node,
@@ -40,6 +41,7 @@
     get_proper_types, is_literal_type, TypeAliasType, TypeGuardedType, ParamSpecType,
     OVERLOAD_NAMES, UnboundType
 )
+from mypy.typetraverser import TypeTraverserVisitor
 from mypy.sametypes import is_same_type
 from mypy.messages import (
     MessageBuilder, make_inferred_type_note, append_invariance_notes, pretty_seq,
@@ -918,6 +920,7 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
                     if typ.ret_type.variance == CONTRAVARIANT:
                         self.fail(message_registry.RETURN_TYPE_CANNOT_BE_CONTRAVARIANT,
                                   typ.ret_type)
+                    self.check_unbound_return_typevar(typ)
 
                 # Check that Generator functions have the appropriate return type.
                 if defn.is_generator:
@@ -1062,6 +1065,16 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
 
             self.binder = old_binder
 
+    def check_unbound_return_typevar(self, typ: CallableType) -> None:
+        """Fails when the return typevar is not defined in arguments."""
+        if (typ.ret_type in typ.variables):
+            arg_type_visitor = CollectArgTypes()
+            for argtype in typ.arg_types:
+                argtype.accept(arg_type_visitor)
+
+            if typ.ret_type not in arg_type_visitor.arg_types:
+                self.fail(message_registry.UNBOUND_TYPEVAR, typ.ret_type, code=TYPE_VAR)
+
     def check_default_args(self, item: FuncItem, body_is_trivial: bool) -> None:
         for arg in item.arguments:
             if arg.initializer is None:
@@ -5862,6 +5875,15 @@ class Foo(Enum):
                 and member_type.fallback.type == parent_type.type_object())
 
 
+class CollectArgTypes(TypeTraverserVisitor):
+    """Collects the non-nested argument types in a set."""
+    def __init__(self) -> None:
+        self.arg_types: Set[TypeVarType] = set()
+
+    def visit_type_var(self, t: TypeVarType) -> None:
+        self.arg_types.add(t)
+
+
 @overload
 def conditional_types(current_type: Type,
                       proposed_type_ranges: Optional[List[TypeRange]],

mypy/message_registry.py

@@ -167,6 +167,9 @@ def format(self, *args: object, **kwargs: object) -> "ErrorMessage":
 TYPEVAR_VARIANCE_DEF: Final = 'TypeVar "{}" may only be a literal bool'
 TYPEVAR_BOUND_MUST_BE_TYPE: Final = 'TypeVar "bound" must be a type'
 TYPEVAR_UNEXPECTED_ARGUMENT: Final = 'Unexpected argument to "TypeVar()"'
+UNBOUND_TYPEVAR: Final = (
+    'A function returning TypeVar should receive at least '
+    'one argument containing the same Typevar')
 
 # Super
 TOO_MANY_ARGS_FOR_SUPER: Final = ErrorMessage('Too many arguments for "super"')

test-data/unit/check-classes.test

@@ -3269,10 +3269,11 @@ def new_pro(pro_c: Type[P]) -> P:
     return new_user(pro_c)
 wiz = new_pro(WizUser)
 reveal_type(wiz)
-def error(u_c: Type[U]) -> P:
+def error(u_c: Type[U]) -> P: # Error here, see below
     return new_pro(u_c)  # Error here, see below
 [out]
 main:11: note: Revealed type is "__main__.WizUser"
+main:12: error: A function returning TypeVar should receive at least one argument containing the same Typevar
 main:13: error: Value of type variable "P" of "new_pro" cannot be "U"
 main:13: error: Incompatible return value type (got "U", expected "P")
 

test-data/unit/check-errorcodes.test

@@ -335,7 +335,7 @@ z: y  # E: Variable "__main__.y" is not valid as a type  [valid-type] \
 from typing import TypeVar
 
 T = TypeVar('T')
-def f() -> T: pass
+def f() -> T: pass # E: A function returning TypeVar should receive at least one argument containing the same Typevar  [type-var]
 x = f()  # E: Need type annotation for "x"  [var-annotated]
 y = []  # E: Need type annotation for "y" (hint: "y: List[<type>] = ...")  [var-annotated]
 [builtins fixtures/list.pyi]

test-data/unit/check-generics.test

@@ -1533,9 +1533,9 @@ A = TypeVar('A')
 B = TypeVar('B')
 
 def f1(x: A) -> A: ...
-def f2(x: A) -> B: ...
+def f2(x: A) -> B: ... # E: A function returning TypeVar should receive at least one argument containing the same Typevar
 def f3(x: B) -> B: ...
-def f4(x: int) -> A: ...
+def f4(x: int) -> A: ... # E: A function returning TypeVar should receive at least one argument containing the same Typevar
 
 y1 = f1
 if int():
@@ -1584,8 +1584,8 @@ B = TypeVar('B')
 T = TypeVar('T')
 def outer(t: T) -> None:
     def f1(x: A) -> A: ...
-    def f2(x: A) -> B: ...
-    def f3(x: T) -> A: ...
+    def f2(x: A) -> B: ... # E: A function returning TypeVar should receive at least one argument containing the same Typevar
+    def f3(x: T) -> A: ... # E: A function returning TypeVar should receive at least one argument containing the same Typevar
     def f4(x: A) -> T: ...
     def f5(x: T) -> T: ...
 
@@ -1754,7 +1754,7 @@ from typing import TypeVar
 A = TypeVar('A')
 B = TypeVar('B')
 def f1(x: int, y: A) -> A: ...
-def f2(x: int, y: A) -> B: ...
+def f2(x: int, y: A) -> B: ... # E: A function returning TypeVar should receive at least one argument containing the same Typevar
 def f3(x: A, y: B) -> B: ...
 g = f1
 g = f2

test-data/unit/check-inference.test

@@ -448,7 +448,7 @@ g(None) # Ok
 f()     # Ok because not used to infer local variable type
 g(a)
 
-def f() -> T: pass
+def f() -> T: pass # E: A function returning TypeVar should receive at least one argument containing the same Typevar
 def g(a: T) -> None: pass
 [out]
 
@@ -2341,7 +2341,7 @@ def main() -> None:
 [case testDontMarkUnreachableAfterInferenceUninhabited]
 from typing import TypeVar
 T = TypeVar('T')
-def f() -> T: pass
+def f() -> T: pass # E: A function returning TypeVar should receive at least one argument containing the same Typevar
 
 class C:
     x = f() # E: Need type annotation for "x"

test-data/unit/check-literal.test

@@ -972,15 +972,15 @@ b: bt                           # E: Variable "__main__.bt" is not valid as a ty
 [out]
 
 [case testLiteralDisallowTypeVar]
-from typing import TypeVar
+from typing import TypeVar, Tuple
 from typing_extensions import Literal
 
 T = TypeVar('T')
 
 at = Literal[T]  # E: Parameter 1 of Literal[...] is invalid
 a: at
 
-def foo(b: Literal[T]) -> T: pass   # E: Parameter 1 of Literal[...] is invalid
+def foo(b: Literal[T]) -> Tuple[T]: pass   # E: Parameter 1 of Literal[...] is invalid
 [builtins fixtures/tuple.pyi]
 [out]
 

test-data/unit/check-parameter-specification.test

@@ -1062,7 +1062,7 @@ def callback(func: Callable[[Any], Any]) -> None: ...
 class Job(Generic[P]): ...
 
 @callback
-def run_job(job: Job[...]) -> T: ...
+def run_job(job: Job[...]) -> T: ... # E: A function returning TypeVar should receive at least one argument containing the same Typevar
 [builtins fixtures/tuple.pyi]
 
 [case testTupleAndDictOperationsOnParamSpecArgsAndKwargs]

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

@@ -93,4 +93,3 @@ args: Tuple[bool, int, str, int, str, object]
 reveal_type(g(args))  # N: Revealed type is "Tuple[builtins.str, builtins.str, builtins.int]"
 reveal_type(h(args))  # N: Revealed type is "Tuple[builtins.str, builtins.str, builtins.int]"
 [builtins fixtures/tuple.pyi]
-

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

@@ -0,0 +1,60 @@
+
+[case testUnboundTypeVar]
+from typing import TypeVar
+
+T = TypeVar('T')
+
+def f() -> T: # E: A function returning TypeVar should receive at least one argument containing the same Typevar
+    ...
+
+f()
+
+
+[case testInnerFunctionTypeVar]
+
+from typing import TypeVar
+
+T = TypeVar('T')
+
+def g(a: T) -> T:
+    def f() -> T:
+        ...
+    return f()
+
+
+[case testUnboundIterableOfTypeVars]
+from typing import Iterable, TypeVar
+
+T = TypeVar('T')
+
+def f() -> Iterable[T]:
+    ...
+
+f()
+
+[case testBoundTypeVar]
+from typing import TypeVar
+
+T = TypeVar('T')
+
+def f(a: T, b: T, c: int) -> T:
+    ...
+
+
+[case testNestedBoundTypeVar]
+from typing import Callable, List, Union, Tuple, TypeVar
+
+T = TypeVar('T')
+
+def f(a: Union[int, T], b: str) -> T:
+    ...
+
+def g(a: Callable[..., T], b: str) -> T:
+    ...
+
+def h(a: List[Union[Callable[..., T]]]) -> T:
+    ...
+
+def j(a: List[Union[Callable[..., Tuple[T, T]], int]]) -> T:
+    ...
+[builtins fixtures/tuple.pyi]