Fix wrong type argument value inference based on context

Ignore type inference context if the return value of a function is a
type variable type with values.

Fix #360.

Author: JukkaL

mypy/checkexpr.py

@@ -275,7 +275,14 @@ def infer_function_type_arguments_using_context(
         # special ErasedType type. On the other hand, class type variables are
         # valid results.
         erased_ctx = replace_func_type_vars(ctx, ErasedType())
-        args = infer_type_arguments(callable.type_var_ids(), callable.ret_type,
+        ret_type = callable.ret_type
+        if isinstance(ret_type, TypeVar):
+            if ret_type.values:
+                # The return type is a type variable with values, but we can't easily restrict
+                # type inference to conform to the valid values. Give up and just use function
+                # arguments for type inference.
+                ret_type = NoneTyp()
+        args = infer_type_arguments(callable.type_var_ids(), ret_type,
                                     erased_ctx, self.chk.basic_types())
         # Only substite non-None and non-erased types.
         new_args = []  # type: List[Type]

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

@@ -411,6 +411,31 @@ main, line 10: Incompatible types in assignment (expression has type "str", vari
 main, line 11: "int" has no attribute "foo"
 main, line 11: "str" has no attribute "foo"
 
+[case testTypeVarWithValueInferredFromObjectReturnTypeContext]
+from typing import typevar
+T = typevar('T', values=(int, str))
+def c1(x: object) -> None: pass
+def c2(x: int) -> None: pass
+def c3(x: str) -> None: pass
+def g(x: T) -> T: pass
+c1(g(''))
+c2(g(1))
+c3(g(''))
+c2(g(''))  # E: Argument 1 to "c2" has incompatible type "str"; expected "int"
+c3(g(1))   # E: Argument 1 to "c3" has incompatible type "int"; expected "str"
+
+[case testTypeVarWithValueInferredFromObjectReturnTypeContext2]
+from typing import typevar
+T = typevar('T', values=(int, str))
+class ss(str): pass
+def c(x: ss) -> None: pass
+def g(x: T) -> T: pass
+c(g(''))
+c(g(1))
+[out]
+main, line 6: Argument 1 to "c" has incompatible type "str"; expected "ss"
+main, line 7: Argument 1 to "c" has incompatible type "int"; expected "ss"
+
 
 -- Special cases
 -- -------------