Copy alias to validation_alias

fastapi/params.py

@@ -560,6 +560,12 @@ class Body(FieldInfo):
             )
         current_json_schema_extra = json_schema_extra or extra
         if PYDANTIC_V2:
+            if serialization_alias in (_Unset, None) and isinstance(alias, str):
+                serialization_alias = alias
+
+            if validation_alias in (_Unset, None):
+                validation_alias = alias
+
             kwargs.update(
                 {
                     "annotation": annotation,

tests/test_openapi_form_alias.py

@@ -0,0 +1,28 @@
+from fastapi import FastAPI, Form
+from fastapi.testclient import TestClient
+
+app = FastAPI()
+
+
+@app.post("/")
+def route_with_form(form_param: str = Form(alias="aliased-field")):
+    return {}
+
+
+client = TestClient(app)
+
+
+def test_get_route():
+    response = client.post("/", data={"aliased-field": "Hello, World!"})
+    assert response.status_code == 200, response.text
+    assert response.json() == {}
+
+
+def test_openapi():
+    response = client.get("/openapi.json")
+    assert response.status_code == 200, response.text
+    form_properties = (
+        response.json()
+        ["components"]["schemas"]["Body_route_with_form__post"]["properties"]
+    )
+    assert "aliased-field" in form_properties