Work in progress for fixing python#462

mypy/checkexpr.py

@@ -281,10 +281,14 @@ def infer_function_type_arguments_using_context(
         erased_ctx = replace_func_type_vars(ctx, ErasedType())
         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.
+            if ret_type.values or (not isinstance(ret_type, Instance) or
+                                   not cast(Instance, ret_type).args):
+                # The return type is a type variable. If it has values, we can't easily restrict
+                # type inference to conform to the valid values. If it's unrestricted, we could
+                # infer a too general type for the type variable if we use context. Give up and
+                # just use function arguments for type inference.
+                #
+                # See also github issues #462 and #360.
                 ret_type = NoneTyp()
         args = infer_type_arguments(callable.type_var_ids(), ret_type, erased_ctx)
         # Only substite non-None and non-erased types.

mypy/test/data/check-inference-context.test

@@ -719,3 +719,12 @@ f([1])
 f('')
 f(1) # E: Argument 1 to "f" has incompatible type "int"; expected "Union[List[None], str]"
 [builtins fixtures/isinstancelist.py]
+
+[case testIgnoringInferenceContext]
+from typing import typevar, List
+T = typevar('T')
+def f(x: List[T]) -> T: pass
+def g(y: object) -> None: pass
+a = [1]
+g(f(a))
+[builtins fixtures/list.py]

mypy/test/data/check-overloading.test

@@ -123,7 +123,7 @@ t = typevar('t')
 ab, ac, b, c = Undefined, Undefined, Undefined, Undefined # type: (A[B], A[C], B, C)
 b = f(ab)
 c = f(ac)
-b = f(ac) # E: Argument 1 to "f" has incompatible type A[C]; expected A[B]
+b = f(ac) # E: Incompatible types in assignment (expression has type "C", variable has type "B")
 b = f(b)
 c = f(b)  # E: Incompatible types in assignment (expression has type "B", variable has type "C")
 @overload

mypy/test/data/check-varargs.test

@@ -114,9 +114,9 @@ b = Undefined # type: B
 c = Undefined # type: C
 o = Undefined # type: object
 
-a = f(o) # E: Argument 1 to "f" has incompatible type "object"; expected "A"
-b = f(b, a) # E: Argument 2 to "f" has incompatible type "A"; expected "B"
-b = f(a, b) # E: Argument 1 to "f" has incompatible type "A"; expected "B"
+a = f(o) # E: Incompatible types in assignment (expression has type "object", variable has type "A")
+b = f(b, a) # E: Incompatible types in assignment (expression has type "A", variable has type "B")
+b = f(a, b) # E: Incompatible types in assignment (expression has type "A", variable has type "B")
 
 o = f()
 a = f(a)
@@ -139,10 +139,10 @@ T = typevar('T')
 a = Undefined # type: A
 o = Undefined # type: object
 
-a = f(o) # E: Argument 1 to "f" has incompatible type "object"; expected "A"
-a = f(a, o) # E: Argument 2 to "f" has incompatible type "object"; expected "A"
-a = f(a, a, o) # E: Argument 3 to "f" has incompatible type "object"; expected "A"
-a = f(a, a, a, o) # E: Argument 4 to "f" has incompatible type "object"; expected "A"
+a = f(o) # E: Incompatible types in assignment (expression has type "object", variable has type "A")
+a = f(a, o) # E: Incompatible types in assignment (expression has type "object", variable has type "A")
+a = f(a, a, o) # E: Incompatible types in assignment (expression has type "object", variable has type "A")
+a = f(a, a, a, o) # E: Incompatible types in assignment (expression has type "object", variable has type "A")
 
 a = f(a)
 a = f(a, a)