Infer constraints against tuple fallbacks (#4087)

Fixes #1485.

Tuple fallback was overlooked in constraint inference. As usual, I place nominal 
before structural.

Author: ilevkivskyi

mypy/constraints.py

@@ -373,12 +373,8 @@ def visit_instance(self, template: Instance) -> List[Constraint]:
                 cb = infer_constraints(template.args[0], item, SUPERTYPE_OF)
                 res.extend(cb)
             return res
-        elif (isinstance(actual, TupleType) and template.type.is_protocol and
-              self.direction == SUPERTYPE_OF):
-            if mypy.subtypes.is_subtype(actual.fallback, erase_typevars(template)):
-                res.extend(infer_constraints(template, actual.fallback, self.direction))
-                return res
-            return []
+        elif isinstance(actual, TupleType) and self.direction == SUPERTYPE_OF:
+            return infer_constraints(template, actual.fallback, self.direction)
         else:
             return []
 

test-data/unit/check-tuples.test

@@ -941,6 +941,21 @@ reveal_type(t[x:])  # E: Revealed type is 'Union[builtins.int, builtins.str]'
 t[y:]  # E: Slice index must be an integer or None
 [builtins fixtures/tuple.pyi]
 
+[case testInferTupleTypeFallbackAgainstInstance]
+from typing import TypeVar, Generic, Tuple
+T = TypeVar('T')
+
+class Base(Generic[T]): pass
+def f(x: Base[T]) -> T: pass
+
+class DT(Tuple[str, str], Base[int]):
+    pass
+
+reveal_type(f(DT())) # E: Revealed type is 'builtins.int*'
+
+[builtins fixtures/tuple.pyi]
+[out]
+
 [case testTypeTupleClassmethod]
 from typing import Tuple, Type
 

test-data/unit/pythoneval.test

@@ -1418,6 +1418,17 @@ async def main() -> None:
 _testAsyncioGatherPreciseType.py:9: error: Revealed type is 'builtins.str'
 _testAsyncioGatherPreciseType.py:10: error: Revealed type is 'builtins.str'
 
+[case testMultipleInheritanceWorksWithTupleTypeGeneric]
+from typing import SupportsAbs, NamedTuple
+
+class Point(NamedTuple('Point', [('x', int), ('y', int)]), SupportsAbs[int]):
+    def __abs__(p) -> int:
+        return abs(p.x) + abs(p.y)
+
+def test(a: Point) -> bool:
+    return abs(a) == 2
+[out]
+
 [case testNoCrashOnGenericUnionUnpacking]
 from typing import Union, Dict