Fix RaisesGroup calling check() on contained exceptions instead of ExceptionGroup

Author: kelliott1

src/_pytest/raises.py

@@ -1207,14 +1207,35 @@ def matches(
             reason = (
                 cast(str, self._fail_reason) + f" on the {type(exception).__name__}"
             )
+
+            suggest_subexception_check = False
             if (
-                len(actual_exceptions) == len(self.expected_exceptions) == 1
+                self.check is not None
+                and len(actual_exceptions) == len(self.expected_exceptions) == 1
                 and isinstance(expected := self.expected_exceptions[0], type)
-                # we explicitly break typing here :)
-                and self._check_check(actual_exceptions[0])  # type: ignore[arg-type]
+                and isinstance(actual_exceptions[0], expected)
             ):
+                annotations = getattr(self.check, "__annotations__", {})
+                param_names = [name for name in annotations if name != "return"]
+                if param_names:
+                    param_annotation = annotations[param_names[0]]
+
+                    if isinstance(param_annotation, str):
+                        suggest_subexception_check = (
+                            "ExceptionGroup" not in param_annotation
+                            and "BaseExceptionGroup" not in param_annotation
+                        )
+                    else:
+                        origin = get_origin(param_annotation) or param_annotation
+                        if isinstance(origin, type):
+                            suggest_subexception_check = not issubclass(
+                                origin, BaseExceptionGroup
+                            )
+
+            if suggest_subexception_check:
                 self._fail_reason = reason + (
-                    f", but did return True for the expected {self._repr_expected(expected)}."
+                    f", but the single contained exception matches the expected "
+                    f"{self._repr_expected(expected)}."
                     f" You might want RaisesGroup(RaisesExc({expected.__name__}, check=<...>))"
                 )
             else:

testing/python/raises_group.py

@@ -412,9 +412,12 @@ def is_exc(e: ExceptionGroup[ValueError]) -> bool:
         return e is exc
 
     is_exc_repr = repr_callable(is_exc)
+
+    # this should pass (same object)
     with RaisesGroup(ValueError, check=is_exc):
         raise exc
 
+    # this should fail WITHOUT suggestion
     with (
         fails_raises_group(
             f"check {is_exc_repr} did not return True on the ExceptionGroup"
@@ -426,16 +429,28 @@ def is_exc(e: ExceptionGroup[ValueError]) -> bool:
     def is_value_error(e: BaseException) -> bool:
         return isinstance(e, ValueError)
 
-    # helpful suggestion if the user thinks the check is for the sub-exception
+    # this should fail WITH suggestion (because check looks like it's for inner exception)
     with (
         fails_raises_group(
-            f"check {is_value_error} did not return True on the ExceptionGroup, but did return True for the expected ValueError. You might want RaisesGroup(RaisesExc(ValueError, check=<...>))"
+            f"check {is_value_error} did not return True on the ExceptionGroup, but the single contained exception matches the expected ValueError. You might want RaisesGroup(RaisesExc(ValueError, check=<...>))"
         ),
         RaisesGroup(ValueError, check=is_value_error),
     ):
         raise ExceptionGroup("", (ValueError(),))
 
 
+def test_check_called_only_with_group() -> None:
+    seen = []
+
+    def check(exc_group: ExceptionGroup[ValueError]) -> bool:
+        seen.append(type(exc_group))
+        return len(exc_group.exceptions) == 1
+
+    with RaisesGroup(ValueError, match="Main message", check=check):
+        raise ExceptionGroup("Main message", [ValueError("foo")])
+
+    assert seen == [ExceptionGroup]
+
 def test_unwrapped_match_check() -> None:
     def my_check(e: object) -> bool:  # pragma: no cover
         return True