diff --git a/AUTHORS b/AUTHORS
index 229e4315078..2f3db95cb54 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -276,6 +276,7 @@ Kojo Idrissa
 Kostis Anagnostopoulos
 Kristoffer Nordström
 Kyle Altendorf
+Kyue
 Lawrence Mitchell
 Lee Kamentsky
 Leonardus Chen
diff --git a/changelog/14775.bugfix.rst b/changelog/14775.bugfix.rst
new file mode 100644
index 00000000000..3aba430f9d9
--- /dev/null
+++ b/changelog/14775.bugfix.rst
@@ -0,0 +1 @@
+Fixed an internal ``AssertionError`` that could occur when a fixture's setup fails before its underlying function is even called (e.g. when the class-scoped-fixture-as-instance-method deprecation warning is turned into an error via ``-W error``). The failure is now cached like any other setup error instead of leaking finalizer state into the next test.
diff --git a/src/_pytest/fixtures.py b/src/_pytest/fixtures.py
index b516c0a75d1..d7a43e2741f 100644
--- a/src/_pytest/fixtures.py
+++ b/src/_pytest/fixtures.py
@@ -1320,21 +1320,22 @@ def pytest_fixture_setup(
     for argname in fixturedef.argnames:
         kwargs[argname] = request.getfixturevalue(argname)
 
-    fixturefunc = resolve_fixture_function(fixturedef, request)
     my_cache_key = fixturedef.cache_key(request)
 
-    if inspect.isasyncgenfunction(fixturefunc) or inspect.iscoroutinefunction(
-        fixturefunc
-    ):
-        auto_str = " with autouse=True" if fixturedef._autouse else ""
-        fail(
-            f"{request.node.name!r} requested an async fixture {request.fixturename!r}{auto_str}, "
-            "with no plugin or hook that handled it. This is an error, as pytest does not natively support it.\n"
-            "See: https://docs.pytest.org/en/stable/deprecations.html#sync-test-depending-on-async-fixture",
-            pytrace=False,
-        )
-
     try:
+        fixturefunc = resolve_fixture_function(fixturedef, request)
+
+        if inspect.isasyncgenfunction(fixturefunc) or inspect.iscoroutinefunction(
+            fixturefunc
+        ):
+            auto_str = " with autouse=True" if fixturedef._autouse else ""
+            fail(
+                f"{request.node.name!r} requested an async fixture {request.fixturename!r}{auto_str}, "
+                "with no plugin or hook that handled it. This is an error, as pytest does not natively support it.\n"
+                "See: https://docs.pytest.org/en/stable/deprecations.html#sync-test-depending-on-async-fixture",
+                pytrace=False,
+            )
+
         result = call_fixture_func(fixturefunc, request, kwargs)
     except TEST_OUTCOME as e:
         if isinstance(e, skip.Exception):
diff --git a/testing/python/fixtures.py b/testing/python/fixtures.py
index 69044226b6d..fdd79b1ade9 100644
--- a/testing/python/fixtures.py
+++ b/testing/python/fixtures.py
@@ -3801,6 +3801,42 @@ def test_3(bad): pass
         lines2 = failures[2].longrepr.reprtraceback.reprentries[0].lines
         assert len(lines1) == len(lines2)
 
+    def test_exception_before_fixture_func_called_is_cached(
+        self, pytester: Pytester
+    ) -> None:
+        """A failure that happens while resolving the fixture function itself
+        (e.g. a deprecation warning promoted to an error by ``-W error``, before
+        the fixture body ever runs) must be cached on the FixtureDef like any
+        other setup failure. Otherwise leftover finalizer state leaks into the
+        next test using the same class-scoped fixture and crashes pytest
+        internals instead of reporting the (repeated) original error (#14775).
+        """
+        pytester.makepyfile(
+            """
+            import pytest
+
+            class TestFixt:
+                @pytest.fixture(scope="class")
+                def fixt(self):
+                    yield
+
+                def test_1(self, fixt):
+                    pass
+
+                def test_2(self, fixt):
+                    pass
+            """
+        )
+        result = pytester.runpytest("-Werror")
+        result.assert_outcomes(errors=2)
+        assert "AssertionError" not in result.stdout.str()
+        result.stdout.fnmatch_lines(
+            [
+                "*PytestRemovedIn10Warning: Class-scoped fixtures defined as "
+                "instance methods are deprecated.*",
+            ]
+        )
+
 
 class TestShowFixtures:
     def test_funcarg_compat(self, pytester: Pytester) -> None: