Fix overload resolution on Tuples

Fixes python#1578, since list's __setitem__ is overloaded.

Author: rwbarton

mypy/checkexpr.py

@@ -1682,6 +1682,8 @@ def overload_arg_similarity(actual: Type, formal: Type) -> int:
     if isinstance(formal, UnionType):
         return max(overload_arg_similarity(actual, item)
                    for item in formal.items)
+    if isinstance(formal, TupleType):
+        formal = formal.fallback
     if isinstance(formal, Instance):
         if isinstance(actual, CallableType):
             actual = actual.fallback

mypy/test/data/check-overloading.test

@@ -678,3 +678,20 @@ def f(x: int) -> None: pass
 def f(x: B) -> str: pass # E: Overloaded function signatures 2 and 3 overlap with incompatible return types
 @overload
 def f(x: A) -> int: pass
+
+[case testOverloadTuple]
+from typing import overload, Tuple
+@overload
+def f(x: int, y: Tuple[str, ...]) -> None: pass
+@overload
+def f(x: int, y: str) -> None: pass
+f(1, ('2', '3'))
+f(1, (2, '3')) # E: Argument 2 to "f" has incompatible type "Tuple[int, str]"; expected Tuple[str, ...]
+f(1, ('2',))
+f(1, '2')
+f(1, (2, 3)) # E: Argument 2 to "f" has incompatible type "Tuple[int, int]"; expected Tuple[str, ...]
+x = ('2', '3')  # type: Tuple[str, ...]
+f(1, x)
+y = (2, 3)  # type: Tuple[int, ...]
+f(1, y) # E: Argument 2 to "f" has incompatible type Tuple[int, ...]; expected Tuple[str, ...]
+[builtins fixtures/tuple.py]

mypy/test/data/pythoneval.test

@@ -1211,3 +1211,12 @@ re.subn(spat, lambda m: '', '')[0] + ''
 [out]
 _program.py:7: error: Type argument 1 of "search" has incompatible value "object"
 _program.py:9: error: Cannot infer type argument 1 of "search"
+
+[case testListSetitemTuple]
+from typing import List, Tuple
+a = []  # type: List[Tuple[str, int]]
+a[0] = 'x', 1
+a[1] = 2, 'y'
+a[:] = [('z', 3)]
+[out]
+_program.py:4: error: Incompatible types in assignment