Some cleanup in partial plugin (#17423)

Fixes #17405

Apart from fixing the crash I fix two obvious bugs I noticed while
making this PR.

mypy/checkexpr.py

@@ -1228,6 +1228,8 @@ def apply_function_plugin(
                 formal_arg_exprs[formal].append(args[actual])
                 if arg_names:
                     formal_arg_names[formal].append(arg_names[actual])
+                else:
+                    formal_arg_names[formal].append(None)
                 formal_arg_kinds[formal].append(arg_kinds[actual])
 
         if object_type is None:

mypy/plugins/functools.py

@@ -17,7 +17,6 @@
     Type,
     TypeOfAny,
     UnboundType,
-    UninhabitedType,
     get_proper_type,
 )
 
@@ -132,6 +131,9 @@ def partial_new_callback(ctx: mypy.plugin.FunctionContext) -> Type:
     if fn_type is None:
         return ctx.default_return_type
 
+    # We must normalize from the start to have coherent view together with TypeChecker.
+    fn_type = fn_type.with_unpacked_kwargs().with_normalized_var_args()
+
     defaulted = fn_type.copy_modified(
         arg_kinds=[
             (
@@ -146,10 +148,25 @@ def partial_new_callback(ctx: mypy.plugin.FunctionContext) -> Type:
         # Make up a line number if we don't have one
         defaulted.set_line(ctx.default_return_type)
 
-    actual_args = [a for param in ctx.args[1:] for a in param]
-    actual_arg_kinds = [a for param in ctx.arg_kinds[1:] for a in param]
-    actual_arg_names = [a for param in ctx.arg_names[1:] for a in param]
-    actual_types = [a for param in ctx.arg_types[1:] for a in param]
+    # Flatten actual to formal mapping, since this is what check_call() expects.
+    actual_args = []
+    actual_arg_kinds = []
+    actual_arg_names = []
+    actual_types = []
+    seen_args = set()
+    for i, param in enumerate(ctx.args[1:], start=1):
+        for j, a in enumerate(param):
+            if a in seen_args:
+                # Same actual arg can map to multiple formals, but we need to include
+                # each one only once.
+                continue
+            # Here we rely on the fact that expressions are essentially immutable, so
+            # they can be compared by identity.
+            seen_args.add(a)
+            actual_args.append(a)
+            actual_arg_kinds.append(ctx.arg_kinds[i][j])
+            actual_arg_names.append(ctx.arg_names[i][j])
+            actual_types.append(ctx.arg_types[i][j])
 
     # Create a valid context for various ad-hoc inspections in check_call().
     call_expr = CallExpr(
@@ -188,7 +205,7 @@ def partial_new_callback(ctx: mypy.plugin.FunctionContext) -> Type:
     for i, actuals in enumerate(formal_to_actual):
         if len(bound.arg_types) == len(fn_type.arg_types):
             arg_type = bound.arg_types[i]
-            if isinstance(get_proper_type(arg_type), UninhabitedType):
+            if not mypy.checker.is_valid_inferred_type(arg_type):
                 arg_type = fn_type.arg_types[i]  # bit of a hack
         else:
             # TODO: I assume that bound and fn_type have the same arguments. It appears this isn't
@@ -210,7 +227,7 @@ def partial_new_callback(ctx: mypy.plugin.FunctionContext) -> Type:
             partial_names.append(fn_type.arg_names[i])
 
     ret_type = bound.ret_type
-    if isinstance(get_proper_type(ret_type), UninhabitedType):
+    if not mypy.checker.is_valid_inferred_type(ret_type):
         ret_type = fn_type.ret_type  # same kind of hack as above
 
     partially_applied = fn_type.copy_modified(

test-data/unit/check-functools.test

@@ -372,3 +372,55 @@ def foo(cls3: Type[B[T]]):
     reveal_type(functools.partial(cls3, 2)())  # N: Revealed type is "__main__.B[T`-1]" \
                                                # E: Argument 1 to "B" has incompatible type "int"; expected "T"
 [builtins fixtures/tuple.pyi]
+
+[case testFunctoolsPartialTypedDictUnpack]
+from typing_extensions import TypedDict, Unpack
+from functools import partial
+
+class Data(TypedDict, total=False):
+    x: int
+
+def f(**kwargs: Unpack[Data]) -> None: ...
+def g(**kwargs: Unpack[Data]) -> None:
+    partial(f, **kwargs)()
+
+class MoreData(TypedDict, total=False):
+    x: int
+    y: int
+
+def f_more(**kwargs: Unpack[MoreData]) -> None: ...
+def g_more(**kwargs: Unpack[MoreData]) -> None:
+    partial(f_more, **kwargs)()
+
+class Good(TypedDict, total=False):
+    y: int
+class Bad(TypedDict, total=False):
+    y: str
+
+def h(**kwargs: Unpack[Data]) -> None:
+    bad: Bad
+    partial(f_more, **kwargs)(**bad)  # E: Argument "y" to "f_more" has incompatible type "str"; expected "int"
+    good: Good
+    partial(f_more, **kwargs)(**good)
+[builtins fixtures/dict.pyi]
+
+[case testFunctoolsPartialNestedGeneric]
+from functools import partial
+from typing import Generic, TypeVar, List
+
+T = TypeVar("T")
+def get(n: int, args: List[T]) -> T: ...
+first = partial(get, 0)
+
+x: List[str]
+reveal_type(first(x))  # N: Revealed type is "builtins.str"
+reveal_type(first([1]))  # N: Revealed type is "builtins.int"
+
+first_kw = partial(get, n=0)
+reveal_type(first_kw(args=[1]))  # N: Revealed type is "builtins.int"
+
+# TODO: this is indeed invalid, but the error is incomprehensible.
+first_kw([1])  # E: Too many positional arguments for "get" \
+               # E: Too few arguments for "get" \
+               # E: Argument 1 to "get" has incompatible type "List[int]"; expected "int"
+[builtins fixtures/list.pyi]