Fix signature generation for historical positional-only syntax

Author: brianschubert

mypyc/ir/func_ir.py

@@ -412,11 +412,20 @@ def get_text_signature(fn: FuncIR) -> str | None:
     """
     parameters = []
     mark_self = fn.class_name is not None and fn.decl.kind != FUNC_STATICMETHOD
-    for arg in fn.decl.sig.args:
+    # Pre-scan for end of positional-only parameters.
+    # This is needed to handle signatures like 'def foo(self, __x)', where mypy
+    # currently sees 'self' as being positional-or-keyword and '__x' as positional-only.
+    pos_only_idx = -1
+    for idx, arg in enumerate(fn.decl.sig.args):
+        if arg.pos_only and arg.kind in (ArgKind.ARG_POS, ArgKind.ARG_OPT):
+            pos_only_idx = idx
+    for idx, arg in enumerate(fn.decl.sig.args):
         if arg.name.startswith("__bitmap") or arg.name == "__mypyc_self__":
             continue
         kind = (
-            inspect.Parameter.POSITIONAL_ONLY if arg.pos_only else _ARG_KIND_TO_INSPECT[arg.kind]
+            inspect.Parameter.POSITIONAL_ONLY
+            if idx <= pos_only_idx
+            else _ARG_KIND_TO_INSPECT[arg.kind]
         )
         default: object = inspect.Parameter.empty
         if arg.optional:
@@ -428,7 +437,7 @@ def get_text_signature(fn: FuncIR) -> str | None:
         curr_param = inspect.Parameter(arg.name, kind, default=default)
         parameters.append(curr_param)
         if mark_self:
-            # Parameter.__init__ does not accept $
+            # Parameter.__init__/Parameter.replace do not accept $
             curr_param._name = f"${arg.name}"  # type: ignore[attr-defined]
             mark_self = False
     sig = inspect.Signature(parameters)

mypyc/test-data/run-signatures.test

@@ -123,3 +123,25 @@ def test_methods() -> None:
 
     assert getattr(Foo().f3, "__text_signature__") == "(x)"
     assert str(inspect.signature(Foo().f3)) == "(x)"
+
+[case testSignaturesHistoricalPositionalOnly]
+import inspect
+
+def f1(__x): pass
+def f2(__x, y): pass
+def f3(*, __y): pass
+def f4(x, *, __y): pass
+def f5(__x, *, __y): pass
+
+class A:
+    def func(self, __x): pass
+
+def test_historical_positional_only() -> None:
+    assert str(inspect.signature(f1)) == "(__x, /)"
+    assert str(inspect.signature(f2)) == "(__x, /, y)"
+    assert str(inspect.signature(f3)) == "(*, __y)"
+    assert str(inspect.signature(f4)) == "(x, *, __y)"
+    assert str(inspect.signature(f5)) == "(__x, /, *, __y)"
+
+    assert str(inspect.signature(A.func)) == "(self, __x, /)"
+    assert str(inspect.signature(A().func)) == "(__x, /)"