Fix ExceptionInfo.for_later() not populating _striptext

changelog/12175.bugfix.rst

@@ -0,0 +1 @@
+Fixed :meth:`ExceptionInfo.exconly(tryshort=True) <ExceptionInfo.exconly>` not stripping the ``AssertionError`` prefix when the :class:`ExceptionInfo` was created via :meth:`ExceptionInfo.for_later` (e.g. when using :func:`pytest.raises`).

src/_pytest/_code/code.py

@@ -591,6 +591,12 @@ def fill_unfilled(self, exc_info: tuple[type[E], E, TracebackType]) -> None:
         """Fill an unfilled ExceptionInfo created with ``for_later()``."""
         assert self._excinfo is None, "ExceptionInfo was already filled"
         self._excinfo = exc_info
+        if isinstance(exc_info[1], AssertionError):
+            exprinfo = getattr(exc_info[1], "msg", None)
+            if exprinfo is None:
+                exprinfo = saferepr(exc_info[1])
+            if exprinfo and exprinfo.startswith(self._assert_start_repr):
+                self._striptext = "AssertionError: "
 
     @property
     def type(self) -> type[E]:

testing/code/test_excinfo.py

@@ -365,6 +365,27 @@ def test_excinfo_for_later() -> None:
     assert "for raises" in str(e)
 
 
+def test_excinfo_for_later_strips_assertion(pytester: Pytester) -> None:
+    """ExceptionInfo created via for_later() should strip AssertionError prefix
+    with exconly(tryshort=True) for rewritten assertions (#12175)."""
+    pytester.makepyfile(
+        """
+        import pytest
+
+        def test_tryshort():
+            with pytest.raises(AssertionError) as exc_info:
+                assert 1 == 2
+            # tryshort should strip 'AssertionError: ' from rewritten assertions
+            result = exc_info.exconly(tryshort=True)
+            assert not result.startswith("AssertionError"), (
+                f"Expected stripped prefix, got: {result!r}"
+            )
+        """
+    )
+    result = pytester.runpytest()
+    result.assert_outcomes(passed=1)
+
+
 def test_excinfo_errisinstance():
     excinfo = pytest.raises(ValueError, h)
     assert excinfo.errisinstance(ValueError)