Fine-grained: Add tests for unpacking expressions (and fix crash)

Also fix crash on incompatible dictionary unpack. Fixes #4959.

Work towards #4951.

Author: JukkaL

mypy/messages.py

@@ -639,7 +639,7 @@ def incompatible_argument(self, n: int, m: int, callee: CallableType, arg_type:
             # For function calls with keyword arguments, display the argument name rather than the
             # number.
             arg_label = str(n)
-            if isinstance(context, CallExpr):
+            if isinstance(context, CallExpr) and len(context.arg_names) >= n:
                 arg_name = context.arg_names[n - 1]
                 if arg_name is not None:
                     arg_label = '"{}"'.format(arg_name)

test-data/unit/fine-grained.test

@@ -5800,3 +5800,87 @@ class M(type):
 [out]
 ==
 a.py:2: error: Argument 1 to "f" of "M" has incompatible type "int"; expected "str"
+
+[case testExtendedUnpacking-skip-cache]
+from typing import List
+from a import g
+def f() -> List[int]:
+    a, *b = g()
+    return b
+
+[file a.py]
+from typing import Tuple
+def g() -> Tuple[str, int, int]: pass
+
+[file a.py.2]
+from typing import Tuple
+def g() -> Tuple[str, str]: pass
+
+[builtins fixtures/tuple.pyi]
+[out]
+==
+main:5: error: Incompatible return value type (got "List[str]", expected "List[int]")
+
+[case testUnpackInExpression1-skip-cache]
+from typing import Tuple, List
+from a import t
+
+def f() -> Tuple[int, int]:
+    return (1, *t())
+
+def g() -> List[int]:
+    return [1, *t()]
+
+[file a.py]
+from typing import Tuple
+def t() -> Tuple[int]: ...
+
+[file a.py.2]
+from typing import Tuple
+def t() -> Tuple[str]: ...
+
+[builtins fixtures/list.pyi]
+[out]
+==
+main:5: error: Incompatible return value type (got "Tuple[int, str]", expected "Tuple[int, int]")
+main:8: error: List item 1 has incompatible type "Tuple[str]"; expected "int"
+
+[case testUnpackInExpression2-skip-cache]
+from typing import Set
+from a import t
+
+def f() -> Set[int]:
+    return {1, *t()}
+
+[file a.py]
+from typing import Tuple
+def t() -> Tuple[int]: pass
+
+[file a.py.2]
+from typing import Tuple
+def t() -> Tuple[str]: pass
+
+[builtins fixtures/set.pyi]
+[out]
+==
+main:5: error: Argument 2 to <set> has incompatible type "*Tuple[str]"; expected "int"
+
+[case testUnpackInExpression3-skip-cache]
+from typing import Dict
+from a import d
+
+def f() -> Dict[int, str]:
+    return {1: '', **d()}
+
+[file a.py]
+from typing import Dict
+def d() -> Dict[int, str]: pass
+
+[file a.py.2]
+from typing import Dict
+def d() -> Dict[int, int]: pass
+
+[builtins fixtures/dict.pyi]
+[out]
+==
+main:5: error: Argument 1 to "update" of "dict" has incompatible type "Dict[int, int]"; expected "Mapping[int, str]"

test-data/unit/pythoneval.test

@@ -1259,3 +1259,14 @@ class B:
 [out]
 _testInvalidSlots.py:2: error: Incompatible types in assignment (expression has type "int", base class "object" defined the type as "Union[str, Iterable[str], None]")
 _testInvalidSlots.py:4: error: Incompatible types in assignment (expression has type "Tuple[int, int]", base class "object" defined the type as "Union[str, Iterable[str], None]")
+
+[case testDictWithStarStarSpecialCase]
+from typing import Dict
+
+def f() -> Dict[int, str]:
+    return {1: '', **d()}
+
+def d() -> Dict[int, int]:
+    return {}
+[out]
+_testDictWithStarStarSpecialCase.py:4: error: Argument 1 to "update" of "dict" has incompatible type "Dict[int, int]"; expected "Mapping[int, str]"