Make dict expression inference more consistent (#15174)

Fixes #12977

IMO current dict expression inference logic is quite arbitrary: we only
take the non-star items to infer resulting type, then enforce it on the
remaining (star) items. In this PR I simplify the logic to simply put
all expressions as arguments into the same call. This has following
benefits:
* Makes everything more consistent/predictable.
* Fixes one of top upvoted bugs
* Makes dict item indexes correct (previously we reshuffled them causing
wrong indexes for non-star items after star items)
* No more weird wordings like `List item <n>` or `Argument <n> to
"update" of "dict"`
* I also fix the end position of generated expressions to show correct
spans in errors

The only downside is that we will see `Cannot infer type argument` error
instead of `Incompatible type` more often. This is because
`SupportsKeysAndGetItem` (used for star items) is invariant in key type.
I think this is fine however, since:
* This only affects key types, that are mixed much less often than value
types (they are usually just strings), and for latter we use joins.
* I added a dedicated note for this case

Author: ilevkivskyi

mypy/checkexpr.py

@@ -4319,12 +4319,19 @@ def visit_dict_expr(self, e: DictExpr) -> Type:
         if dt:
             return dt
 
+        # Define type variables (used in constructors below).
+        kt = TypeVarType("KT", "KT", -1, [], self.object_type())
+        vt = TypeVarType("VT", "VT", -2, [], self.object_type())
+
         # Collect function arguments, watching out for **expr.
-        args: list[Expression] = []  # Regular "key: value"
-        stargs: list[Expression] = []  # For "**expr"
+        args: list[Expression] = []
+        expected_types: list[Type] = []
         for key, value in e.items:
             if key is None:
-                stargs.append(value)
+                args.append(value)
+                expected_types.append(
+                    self.chk.named_generic_type("_typeshed.SupportsKeysAndGetItem", [kt, vt])
+                )
             else:
                 tup = TupleExpr([key, value])
                 if key.line >= 0:
@@ -4333,52 +4340,23 @@ def visit_dict_expr(self, e: DictExpr) -> Type:
                 else:
                     tup.line = value.line
                     tup.column = value.column
+                tup.end_line = value.end_line
+                tup.end_column = value.end_column
                 args.append(tup)
-        # Define type variables (used in constructors below).
-        kt = TypeVarType("KT", "KT", -1, [], self.object_type())
-        vt = TypeVarType("VT", "VT", -2, [], self.object_type())
-        rv = None
-        # Call dict(*args), unless it's empty and stargs is not.
-        if args or not stargs:
-            # The callable type represents a function like this:
-            #
-            #   def <unnamed>(*v: Tuple[kt, vt]) -> Dict[kt, vt]: ...
-            constructor = CallableType(
-                [TupleType([kt, vt], self.named_type("builtins.tuple"))],
-                [nodes.ARG_STAR],
-                [None],
-                self.chk.named_generic_type("builtins.dict", [kt, vt]),
-                self.named_type("builtins.function"),
-                name="<dict>",
-                variables=[kt, vt],
-            )
-            rv = self.check_call(constructor, args, [nodes.ARG_POS] * len(args), e)[0]
-        else:
-            # dict(...) will be called below.
-            pass
-        # Call rv.update(arg) for each arg in **stargs,
-        # except if rv isn't set yet, then set rv = dict(arg).
-        if stargs:
-            for arg in stargs:
-                if rv is None:
-                    constructor = CallableType(
-                        [
-                            self.chk.named_generic_type(
-                                "_typeshed.SupportsKeysAndGetItem", [kt, vt]
-                            )
-                        ],
-                        [nodes.ARG_POS],
-                        [None],
-                        self.chk.named_generic_type("builtins.dict", [kt, vt]),
-                        self.named_type("builtins.function"),
-                        name="<list>",
-                        variables=[kt, vt],
-                    )
-                    rv = self.check_call(constructor, [arg], [nodes.ARG_POS], arg)[0]
-                else:
-                    self.check_method_call_by_name("update", rv, [arg], [nodes.ARG_POS], arg)
-        assert rv is not None
-        return rv
+                expected_types.append(TupleType([kt, vt], self.named_type("builtins.tuple")))
+
+        # The callable type represents a function like this (except we adjust for **expr):
+        #   def <dict>(*v: Tuple[kt, vt]) -> Dict[kt, vt]: ...
+        constructor = CallableType(
+            expected_types,
+            [nodes.ARG_POS] * len(expected_types),
+            [None] * len(expected_types),
+            self.chk.named_generic_type("builtins.dict", [kt, vt]),
+            self.named_type("builtins.function"),
+            name="<dict>",
+            variables=[kt, vt],
+        )
+        return self.check_call(constructor, args, [nodes.ARG_POS] * len(args), e)[0]
 
     def find_typeddict_context(
         self, context: Type | None, dict_expr: DictExpr

mypy/messages.py

@@ -679,11 +679,13 @@ def incompatible_argument(
                 name.title(), n, actual_type_str, expected_type_str
             )
             code = codes.LIST_ITEM
-        elif callee_name == "<dict>":
+        elif callee_name == "<dict>" and isinstance(
+            get_proper_type(callee.arg_types[n - 1]), TupleType
+        ):
             name = callee_name[1:-1]
             n -= 1
             key_type, value_type = cast(TupleType, arg_type).items
-            expected_key_type, expected_value_type = cast(TupleType, callee.arg_types[0]).items
+            expected_key_type, expected_value_type = cast(TupleType, callee.arg_types[n]).items
 
             # don't increase verbosity unless there is need to do so
             if is_subtype(key_type, expected_key_type):
@@ -710,6 +712,14 @@ def incompatible_argument(
                 expected_value_type_str,
             )
             code = codes.DICT_ITEM
+        elif callee_name == "<dict>":
+            value_type_str, expected_value_type_str = format_type_distinctly(
+                arg_type, callee.arg_types[n - 1], options=self.options
+            )
+            msg = "Unpacked dict entry {} has incompatible type {}; expected {}".format(
+                n - 1, value_type_str, expected_value_type_str
+            )
+            code = codes.DICT_ITEM
         elif callee_name == "<list-comprehension>":
             actual_type_str, expected_type_str = map(
                 strip_quotes,
@@ -1301,6 +1311,12 @@ def could_not_infer_type_arguments(
         callee_name = callable_name(callee_type)
         if callee_name is not None and n > 0:
             self.fail(f"Cannot infer type argument {n} of {callee_name}", context)
+            if callee_name == "<dict>":
+                # Invariance in key type causes more of these errors than we would want.
+                self.note(
+                    "Try assigning the literal to a variable annotated as dict[<key>, <val>]",
+                    context,
+                )
         else:
             self.fail("Cannot infer function type argument", context)
 

test-data/unit/check-expressions.test

@@ -1800,11 +1800,12 @@ a = {'a': 1}
 b = {'z': 26, **a}
 c = {**b}
 d = {**a, **b, 'c': 3}
-e = {1: 'a', **a}  # E: Argument 1 to "update" of "dict" has incompatible type "Dict[str, int]"; expected "SupportsKeysAndGetItem[int, str]"
-f = {**b}  # type: Dict[int, int]  # E: List item 0 has incompatible type "Dict[str, int]"; expected "SupportsKeysAndGetItem[int, int]"
+e = {1: 'a', **a}  # E: Cannot infer type argument 1 of <dict> \
+                   # N: Try assigning the literal to a variable annotated as dict[<key>, <val>]
+f = {**b}  # type: Dict[int, int]  # E: Unpacked dict entry 0 has incompatible type "Dict[str, int]"; expected "SupportsKeysAndGetItem[int, int]"
 g = {**Thing()}
 h = {**a, **Thing()}
-i = {**Thing()}  # type: Dict[int, int]  # E: List item 0 has incompatible type "Thing"; expected "SupportsKeysAndGetItem[int, int]" \
+i = {**Thing()}  # type: Dict[int, int]  # E: Unpacked dict entry 0 has incompatible type "Thing"; expected "SupportsKeysAndGetItem[int, int]" \
                  # N: Following member(s) of "Thing" have conflicts: \
                  # N:     Expected: \
                  # N:         def __getitem__(self, int, /) -> int \
@@ -1814,16 +1815,8 @@ i = {**Thing()}  # type: Dict[int, int]  # E: List item 0 has incompatible type
                  # N:         def keys(self) -> Iterable[int] \
                  # N:     Got: \
                  # N:         def keys(self) -> Iterable[str]
-j = {1: 'a', **Thing()}  # E: Argument 1 to "update" of "dict" has incompatible type "Thing"; expected "SupportsKeysAndGetItem[int, str]" \
-                         # N: Following member(s) of "Thing" have conflicts: \
-                         # N:     Expected: \
-                         # N:         def __getitem__(self, int, /) -> str \
-                         # N:     Got: \
-                         # N:         def __getitem__(self, str, /) -> int \
-                         # N:     Expected: \
-                         # N:         def keys(self) -> Iterable[int] \
-                         # N:     Got: \
-                         # N:         def keys(self) -> Iterable[str]
+j = {1: 'a', **Thing()}  # E: Cannot infer type argument 1 of <dict> \
+                         # N: Try assigning the literal to a variable annotated as dict[<key>, <val>]
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-medium.pyi]
 

test-data/unit/check-generics.test

@@ -2711,3 +2711,25 @@ class G(Generic[T]):
     def g(self, x: S) -> Union[S, T]: ...
 
 f(lambda x: x.g(0))  # E: Cannot infer type argument 1 of "f"
+
+[case testDictStarInference]
+class B: ...
+class C1(B): ...
+class C2(B): ...
+
+dict1 = {"a": C1()}
+dict2 = {"a": C2(), **dict1}
+reveal_type(dict2)  # N: Revealed type is "builtins.dict[builtins.str, __main__.B]"
+[builtins fixtures/dict.pyi]
+
+[case testDictStarAnyKeyJoinValue]
+from typing import Any
+
+class B: ...
+class C1(B): ...
+class C2(B): ...
+
+dict1: Any
+dict2 = {"a": C1(), **{x: C2() for x in dict1}}
+reveal_type(dict2)  # N: Revealed type is "builtins.dict[Any, __main__.B]"
+[builtins fixtures/dict.pyi]

test-data/unit/check-python38.test

@@ -812,3 +812,18 @@ if sys.version_info < (3, 6):
 else:
     42  # type: ignore  # E: Unused "type: ignore" comment
 [builtins fixtures/ops.pyi]
+
+[case testDictExpressionErrorLocations]
+# flags: --pretty
+from typing import Dict
+
+other: Dict[str, str]
+dct: Dict[str, int] = {"a": "b", **other}
+[builtins fixtures/dict.pyi]
+[out]
+main:5: error: Dict entry 0 has incompatible type "str": "str"; expected "str": "int"
+    dct: Dict[str, int] = {"a": "b", **other}
+                           ^~~~~~~~
+main:5: error: Unpacked dict entry 1 has incompatible type "Dict[str, str]"; expected "SupportsKeysAndGetItem[str, int]"
+    dct: Dict[str, int] = {"a": "b", **other}
+                                       ^~~~~

test-data/unit/fine-grained.test

@@ -7546,7 +7546,7 @@ 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 "SupportsKeysAndGetItem[int, str]"
+main:5: error: Unpacked dict entry 1 has incompatible type "Dict[int, int]"; expected "SupportsKeysAndGetItem[int, str]"
 
 [case testAwaitAndAsyncDef-only_when_nocache]
 from a import g

test-data/unit/pythoneval.test

@@ -1350,7 +1350,7 @@ def f() -> Dict[int, str]:
 def d() -> Dict[int, int]:
     return {}
 [out]
-_testDictWithStarStarSpecialCase.py:4: error: Argument 1 to "update" of "MutableMapping" has incompatible type "Dict[int, int]"; expected "SupportsKeysAndGetItem[int, str]"
+_testDictWithStarStarSpecialCase.py:4: error: Unpacked dict entry 1 has incompatible type "Dict[int, int]"; expected "SupportsKeysAndGetItem[int, str]"
 
 [case testLoadsOfOverloads]
 from typing import overload, Any, TypeVar, Iterable, List, Dict, Callable, Union