Handle functools.Placeholder in partial

[MestreY0d4-Uninter]

Fixes #21313.

Summary

This updates the functools.partial plugin to treat functools.Placeholder as an unbound positional slot instead of a normal bound argument.

The generated partial signature now keeps placeholder-backed parameters available for later calls, while still using non-placeholder arguments for type inference.

Test plan

• PYTHONPATH=. uv run --no-project --isolated --with-requirements test-requirements.txt python -m pytest -q mypy/test/testcheck.py::TypeCheckSuite::check-functools.test
• PYTHONPATH=. uv run --no-project --isolated --with-requirements test-requirements.txt python -m mypy --config-file mypy_self_check.ini mypy/plugins/functools.py

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

trio (https://github.com/python-trio/trio)
+ src/trio/_tests/test_threads.py:881: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
+ https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
+ Please report a bug at https://github.com/python/mypy/issues
+ version: 2.1.0+dev.32e79425cb79ef91dfcdfdc2f765454bd386695b
+ src/trio/_tests/test_threads.py:881: note: use --pdb to drop into pdb
+ Traceback (most recent call last):
+   File "", line 6, in <module>
+     sys.exit(console_entry())
+   File "/__main__.py", line 16, in console_entry
+     main()
+   File "/main.py", line 154, in main
+     res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
+   File "/main.py", line 244, in run_build
+     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
+   File "/build.py", line 422, in build
+     result = build_inner(
+   File "/build.py", line 537, in build_inner
+     graph = dispatch(sources, manager, stdout, connect_threads)
+   File "/build.py", line 4150, in dispatch
+     process_graph(graph, manager)
+   File "/build.py", line 4618, in process_graph
+     done, still_working, results = manager.wait_for_done(graph)
+   File "/build.py", line 1492, in wait_for_done
+     process_stale_scc(graph, next_scc, self)
+   File "/build.py", line 4792, in process_stale_scc
+     graph[id].type_check_first_pass()
+   File "/build.py", line 3414, in type_check_first_pass
+     self.type_checker().check_first_pass(recurse_into_functions=recurse_into_functions)
+   File "/checker.py", line 618, in check_first_pass
+     self.accept(d)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1159, in accept
+     return visitor.visit_func_def(self)
+   File "/checker.py", line 1332, in visit_func_def
+     self.visit_func_def_impl(defn)
+   File "/checker.py", line 1336, in visit_func_def_impl
+     self.check_func_item(defn, name=defn.name)
+   File "/checker.py", line 1369, in check_func_item
+     self.check_func_def(defn, typ, name, allow_empty)
+   File "/checker.py", line 1551, in check_func_def
+     self.accept(item.body)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1816, in accept
+     return visitor.visit_block(self)
+   File "/checker.py", line 3275, in visit_block
+     self.accept(s)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1159, in accept
+     return visitor.visit_func_def(self)
+   File "/checker.py", line 1332, in visit_func_def
+     self.visit_func_def_impl(defn)
+   File "/checker.py", line 1336, in visit_func_def_impl
+     self.check_func_item(defn, name=defn.name)
+   File "/checker.py", line 1369, in check_func_item
+     self.check_func_def(defn, typ, name, allow_empty)
+   File "/checker.py", line 1551, in check_func_def
+     self.accept(item.body)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1816, in accept
+     return visitor.visit_block(self)
+   File "/checker.py", line 3275, in visit_block
+     self.accept(s)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 2146, in accept
+     return visitor.visit_try_stmt(self)
+   File "/checker.py", line 5433, in visit_try_stmt
+     self.accept(s.finally_body)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1816, in accept
+     return visitor.visit_block(self)
+   File "/checker.py", line 3275, in visit_block
+     self.accept(s)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 2179, in accept
+     return visitor.visit_with_stmt(self)
+   File "/checker.py", line 5873, in visit_with_stmt
+     self.accept(s.body)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1816, in accept
+     return visitor.visit_block(self)
+   File "/checker.py", line 3275, in visit_block
+     self.accept(s)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 2146, in accept
+     return visitor.visit_try_stmt(self)
+   File "/checker.py", line 5428, in visit_try_stmt
+     self.visit_try_without_finally(s, try_frame=bool(s.finally_body))
+   File "/checker.py", line 5469, in visit_try_without_finally
+     self.accept(s.body)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1816, in accept
+     return visitor.visit_block(self)
+   File "/checker.py", line 3275, in visit_block
+     self.accept(s)
+   File "/checker.py", line 765, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1836, in accept
+     return visitor.visit_expression_stmt(self)
+   File "/checker.py", line 5139, in visit_expression_stmt
+     expr_type = self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
+   File "/checkexpr.py", line 6132, in accept
+     typ = self.visit_await_expr(node, allow_none_return=True)
+   File "/checkexpr.py", line 6305, in visit_await_expr
+     actual_type = get_proper_type(self.accept(e.expr, expected_type))
+   File "/checkexpr.py", line 6183, in accept
+     typ = self.accept_maybe_cache(node, type_context=type_context)
+   File "/checkexpr.py", line 6218, in accept_maybe_cache
+     typ = node.accept(self)
+   File "/nodes.py", line 2539, in accept
+     return visitor.visit_call_expr(self)
+   File "/checkexpr.py", line 502, in visit_call_expr
+     return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
+   File "/checkexpr.py", line 633, in visit_call_expr_inner
+     ret_type = self.check_call_expr_with_callee_type(
+   File "/checkexpr.py", line 1483, in check_call_expr_with_callee_type
+     ret_type, callee_type = self.check_call(
+   File "/checkexpr.py", line 1576, in check_call
+     return self.check_callable_call(
+   File "/checkexpr.py", line 1755, in check_callable_call
+     callee = self.infer_function_type_arguments(
+   File "/checkexpr.py", line 2120, in infer_function_type_arguments
+     arg_types = self.infer_arg_types_in_context(
+   File "/checkexpr.py", line 2006, in infer_arg_types_in_context
+     res.append(self.accept(arg, ctx))
+   File "/checkexpr.py", line 6183, in accept
+     typ = self.accept_maybe_cache(node, type_context=type_context)
+   File "/checkexpr.py", line 6218, in accept_maybe_cache
+     typ = node.accept(self)
+   File "/nodes.py", line 2539, in accept
+     return visitor.visit_call_expr(self)
+            ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
+   File "/checkexpr.py", line 502, in visit_call_expr
+     return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
+            ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/checkexpr.py", line 633, in visit_call_expr_inner
+     ret_type = self.check_call_expr_with_callee_type(
+         callee_type, e, fullname, object_type, member
+     )
+   File "/checkexpr.py", line 1483, in check_call_expr_with_callee_type
+     ret_type, callee_type = self.check_call(
+                             ~~~~~~~~~~~~~~~^
+         callee_type,
+         ^^^^^^^^^^^^
+     ...<6 lines>...
+         object_type=object_type,
+         ^^^^^^^^^^^^^^^^^^^^^^^^
+     )
+     ^
+   File "/checkexpr.py", line 1576, in check_call
+     return self.check_callable_call(
+            ~~~~~~~~~~~~~~~~~~~~~~~~^
+         callee,
+         ^^^^^^^
+     ...<6 lines>...
+         object_type,
+         ^^^^^^^^^^^^
+     )
+     ^
+   File "/checkexpr.py", line 1859, in check_callable_call
+     new_ret_type = self.apply_function_plugin(
+         callee,
+     ...<7 lines>...
+         context,
+     )
+   File "/checkexpr.py", line 1257, in apply_function_plugin
+     return callback(
+         FunctionContext(
+     ...<8 lines>...
+         )
+     )
+   File "/plugins/functools.py", line 159, in partial_new_callback
+     return handle_partial_with_callee(ctx, callee=ctx.arg_types[0][0])
+   File "/plugins/functools.py", line 307, in handle_partial_with_callee
+     inferred_args = infer_type_arguments(
+         fn_type.variables, fn_type.arg_types[i], bound.arg_types[i]
+     )
+   File "/infer.py", line 76, in infer_type_arguments
+     constraints = infer_constraints(template, actual, SUPERTYPE_OF if is_supertype else SUBTYPE_OF)
+   File "/constraints.py", line 318, in infer_constraints
+     return _infer_constraints(template, actual, direction, skip_neg_op)
+   File "/constraints.py", line 427, in _infer_constraints
+     return template.accept(ConstraintBuilderVisitor(actual, direction, skip_neg_op))
+            ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/types.py", line 1237, in accept
+     return visitor.visit_unpack_type(self)
+            ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
+   File "/constraints.py", line 716, in visit_unpack_type
+     raise RuntimeError("Mypy bug: unpack should be handled at a higher level.")
+ RuntimeError: Mypy bug: unpack should be handled at a higher level.
+