Fix --trace for parametrized tests

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!`

Implementation idea, test (and changelog tweaks) thanks to blueyed

Co-Authored-By: Ronny Pfannschmidt <opensource@ronnypfannschmidt.de>
Co-Authored-By: Daniel Hahler <git@thequod.de>

changelog/6099.bugfix.rst

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

src/_pytest/debugging.py

@@ -1,5 +1,6 @@
 """ interactive debugging with PDB, the Python Debugger. """
 import argparse
+import functools
 import pdb
 import sys
 from doctest import UnexpectedException
@@ -274,13 +275,16 @@ def pytest_pyfunc_call(self, pyfuncitem):
 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
-        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)
+
+    # we can't just return `partial(pdb.runcall, testfunction)` because (on
+    # python < 3.7.4) runcall's first param is `func`, which means we'd get
+    # an exception if one of the kwargs to testfunction was called `func`
+    @functools.wraps(testfunction)
+    def wrapper(*args, **kwargs):
+        func = functools.partial(testfunction, *args, **kwargs)
+        _pdb.runcall(func)
+
+    pyfuncitem.obj = wrapper
 
 
 def _enter_pdb(node, excinfo, rep):

testing/test_pdb.py

@@ -1025,6 +1025,51 @@ def test_3():
         assert "Exit: Quitting debugger" not in child.before.decode("utf8")
         TestPDB.flush(child)
 
+    def test_trace_with_parametrize_handles_shared_fixtureinfo(self, testdir):
+        p1 = testdir.makepyfile(
+            """
+            import pytest
+            @pytest.mark.parametrize('myparam', [1,2])
+            def test_1(myparam, request):
+                assert myparam in (1, 2)
+                assert request.function.__name__ == "test_1"
+            @pytest.mark.parametrize('func', [1,2])
+            def test_func(func, request):
+                assert func in (1, 2)
+                assert request.function.__name__ == "test_func"
+            @pytest.mark.parametrize('myparam', [1,2])
+            def test_func_kw(myparam, request, func="func_kw"):
+                assert myparam in (1, 2)
+                assert func == "func_kw"
+                assert request.function.__name__ == "test_func_kw"
+            """
+        )
+        child = testdir.spawn_pytest("--trace " + str(p1))
+        for func, argname in [
+            ("test_1", "myparam"),
+            ("test_func", "func"),
+            ("test_func_kw", "myparam"),
+        ]:
+            child.expect_exact("> PDB runcall (IO-capturing turned off) >")
+            child.expect_exact(func)
+            child.expect_exact("Pdb")
+            child.sendline("args")
+            child.expect_exact("{} = 1\r\n".format(argname))
+            child.expect_exact("Pdb")
+            child.sendline("c")
+            child.expect_exact("Pdb")
+            child.sendline("args")
+            child.expect_exact("{} = 2\r\n".format(argname))
+            child.expect_exact("Pdb")
+            child.sendline("c")
+            child.expect_exact("> PDB continue (IO-capturing resumed) >")
+        rest = child.read().decode("utf8")
+        assert "6 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."""
@@ -1150,7 +1195,6 @@ def set_trace(self, *args):
 
                 def runcall(self, *args, **kwds):
                     print("runcall_called", args, kwds)
-                    assert "func" in kwds
         """,
     )
     result = testdir.runpytest(