Repository: PrefectHQ/prefect
Issue category: Bug Report

Issue:
flow.validate_parameters regression
### Bug summary

Certain flow deployments that worked fine before cannot be executed anymore after updating from Prefect 2.20.9 to any newer Prefect version.
This is due to a change in the parameter validation.

MRE:
```python
from prefect import flow

@flow
def my_flow(param: dict[str, str]):
    pass

flow_run_params = {
    "param": {'foo': 'bar'}
}

my_flow.validate_parameters(flow_run_params)
```
runs fine for Prefect <= 2.20.9.

But raises
```python
Traceback (most recent call last):
  File "mre.py", line 13, in <module>
    my_flow.validate_parameters(flow_run_params)
  File "mre.py", line 532, in validate_parameters
    has_v1_models = any(
  File "mre.py", line 533, in <genexpr>
    issubclass(param.annotation, V1BaseModel)
  File "/usr/lib/python3.10/abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class
```
for Prefect >= 2.20.10


### Version info

```Text
Version:             2.20.14
API version:         0.8.4
Python version:      3.10.12
Git commit:          fb919c67
Built:               Mon, Nov 18, 2024 4:41 PM
OS/Arch:             linux/x86_64
Server type:         cloud
```


### Additional context

related to https://github.com/PrefectHQ/prefect/pull/15608


Hints:
hi @j-tr - thanks for the issue!

can you share what version of `pydantic` you are using? I cannot seem to directly reproduce this
```python
In [1]: from prefect import flow
   ...:
   ...: @flow
   ...: def my_flow(param: dict[str, str]):
   ...:     pass
   ...:
   ...: flow_run_params = {
   ...:     "param": {'foo': 'bar'}
   ...: }
   ...:
   ...: my_flow.validate_parameters(flow_run_params)

Out[1]: {'param': {'foo': 'bar'}}

In [2]:

In [2]: !uv pip list | rg 'prefect|pydantic'
prefect                             2.20.14                           /Users/nate/github.com/prefecthq/prefect
pydantic                            2.10.2
pydantic-core                       2.27.1
```

---

**EDIT**: interesting, ok I reproduce it on python 3.10...

Patch:
diff --git a/src/prefect/flows.py b/src/prefect/flows.py
index 27dc6e7c4a8a..cc4f781b6499 100644
--- a/src/prefect/flows.py
+++ b/src/prefect/flows.py
@@ -10,6 +10,7 @@
 import importlib.util
 import inspect
 import os
+import sys
 import tempfile
 import warnings
 from functools import partial, update_wrapper
@@ -137,6 +138,16 @@
 if TYPE_CHECKING:
     from prefect.deployments.runner import FlexibleScheduleList, RunnerDeployment
 
+# Handle Python 3.8 compatibility for GenericAlias
+if sys.version_info >= (3, 9):
+    from types import GenericAlias  # novermin
+
+    GENERIC_ALIAS = (GenericAlias,)
+else:
+    from typing import _GenericAlias
+
+    GENERIC_ALIAS = (_GenericAlias,)
+
 
 @PrefectObjectRegistry.register_instances
 class Flow(Generic[P, R]):
@@ -530,18 +541,22 @@ def validate_parameters(self, parameters: Dict[str, Any]) -> Dict[str, Any]:
                 is_v1_type(param.annotation) for param in sig.parameters.values()
             )
             has_v1_models = any(
-                issubclass(param.annotation, V1BaseModel)
-                if isinstance(param.annotation, type)
-                else False
+                (
+                    isinstance(param.annotation, type)
+                    and not isinstance(param.annotation, GENERIC_ALIAS)
+                    and issubclass(param.annotation, V1BaseModel)
+                )
                 for param in sig.parameters.values()
             )
             has_v2_types = any(
                 is_v2_type(param.annotation) for param in sig.parameters.values()
             )
             has_v2_models = any(
-                issubclass(param.annotation, V2BaseModel)
-                if isinstance(param.annotation, type)
-                else False
+                (
+                    isinstance(param.annotation, type)
+                    and not isinstance(param.annotation, GENERIC_ALIAS)
+                    and issubclass(param.annotation, V2BaseModel)
+                )
                 for param in sig.parameters.values()
             )
 
@@ -1601,7 +1616,9 @@ def flow(
 
 
 def select_flow(
-    flows: Iterable[Flow], flow_name: str = None, from_message: str = None
+    flows: Iterable[Flow],
+    flow_name: Optional[str] = None,
+    from_message: Optional[str] = None,
 ) -> Flow:
     """
     Select the only flow in an iterable or a flow specified by name.


Test patch:
diff --git a/tests/test_flows.py b/tests/test_flows.py
index 47bfea76ec4a..097018f97563 100644
--- a/tests/test_flows.py
+++ b/tests/test_flows.py
@@ -1517,6 +1517,23 @@ def my_flow(secret: SecretStr):
             "secret": SecretStr("my secret")
         }
 
+    @pytest.mark.skipif(
+        sys.version_info < (3, 9), reason="Python 3.9+ required for GenericAlias"
+    )
+    def test_flow_signature_can_contain_generic_type_hints(self):
+        """Test that generic type hints like dict[str, str] work correctly
+
+        this is a regression test for https://github.com/PrefectHQ/prefect/issues/16105
+        """
+
+        @flow
+        def my_flow(param: dict[str, str]):  # novermin
+            return param
+
+        test_data = {"foo": "bar"}
+        assert my_flow(test_data) == test_data
+        assert my_flow.validate_parameters({"param": test_data}) == {"param": test_data}
+
 
 class TestSubflowTaskInputs:
     async def test_subflow_with_one_upstream_task_future(self, prefect_client):