Fix --trace for parametrized tests

I'm not sure if this is the correct / best way to fix so please check
critically when reviewing

Without this, the second time it tries to stop in a parametrized
function it raises instead:

`ValueError: --trace can't be used with a fixture named func!`

changelog/6099-bugfix.rst

@@ -0,0 +1 @@
+-Fix ``--trace`` when used with parametrized functions

src/_pytest/debugging.py

@@ -275,12 +275,14 @@ def _test_pytest_function(pyfuncitem):
     _pdb = pytestPDB._init_pdb("runcall")
     testfunction = pyfuncitem.obj
     pyfuncitem.obj = _pdb.runcall
-    if "func" in pyfuncitem._fixtureinfo.argnames:  # pragma: no branch
+    if "func" in pyfuncitem.funcargs:  # pragma: no branch
         raise ValueError("--trace can't be used with a fixture named func!")
     pyfuncitem.funcargs["func"] = testfunction
-    new_list = list(pyfuncitem._fixtureinfo.argnames)
-    new_list.append("func")
-    pyfuncitem._fixtureinfo.argnames = tuple(new_list)
+    if "func" not in pyfuncitem._fixtureinfo.argnames:
+        # if using e.g. parametrize the _fixtureinfo is shared
+        new_list = list(pyfuncitem._fixtureinfo.argnames)
+        new_list.append("func")
+        pyfuncitem._fixtureinfo.argnames = tuple(new_list)
 
 
 def _enter_pdb(node, excinfo, rep):

testing/test_pdb.py

@@ -1025,6 +1025,30 @@ def test_3():
         assert "Exit: Quitting debugger" not in child.before.decode("utf8")
         TestPDB.flush(child)
 
+    def test_trace_with_parametrize(self, testdir):
+        p1 = testdir.makepyfile(
+            """
+            import pytest
+
+            @pytest.mark.parametrize('x', [1,2])
+            def test_1(x):
+                assert True
+            """
+        )
+        child = testdir.spawn_pytest("--trace " + str(p1))
+        child.expect("test_1")
+        child.expect("Pdb")
+        child.sendline("c")
+        child.expect("test_1")
+        child.expect("Pdb")
+        child.sendline("c")
+        rest = child.read().decode("utf8")
+        assert "2 passed in" in rest
+        assert "reading from stdin while output" not in rest
+        # Only printed once - not on stderr.
+        assert "Exit: Quitting debugger" not in child.before.decode("utf8")
+        TestPDB.flush(child)
+
 
 def test_trace_after_runpytest(testdir):
     """Test that debugging's pytest_configure is re-entrant."""