Check that param sets match number of args during _for_parametrize

It makes sense to validate them during creation of the parameter set

src/_pytest/mark/structures.py

@@ -111,7 +111,19 @@ class ParameterSet(namedtuple("ParameterSet", "values, marks, id")):
         ]
         del argvalues
 
-        if not parameters:
+        if parameters:
+            # check all parameter sets have the correct number of values
+            for param in parameters:
+                if len(param.values) != len(argnames):
+                    raise ValueError(
+                        'In "parametrize" the number of values ({}) must be '
+                        "equal to the number of names ({})".format(
+                            param.values, argnames
+                        )
+                    )
+        else:
+            # empty parameter set (likely computed at runtime): create a single
+            # parameter set with NOSET values, with the "empty parameter set" mark applied to it
             mark = get_empty_parameterset_mark(config, argnames, func)
             parameters.append(
                 ParameterSet(values=(NOTSET,) * len(argnames), marks=[mark], id=None)

src/_pytest/python.py

@@ -8,7 +8,6 @@ import os
 import collections
 import warnings
 from textwrap import dedent
-from itertools import count
 
 
 import py
@@ -887,22 +886,14 @@ class Metafunc(fixtures.FuncargnamesCompatAttr):
         # of all calls
         newcalls = []
         for callspec in self._calls or [CallSpec2(self)]:
-            elements = zip(ids, parameters, count())
-            for a_id, param, param_index in elements:
-                if len(param.values) != len(argnames):
-                    raise ValueError(
-                        'In "parametrize" the number of values ({}) must be '
-                        "equal to the number of names ({})".format(
-                            param.values, argnames
-                        )
-                    )
+            for param_index, (param_id, param_set) in enumerate(zip(ids, parameters)):
                 newcallspec = callspec.copy()
                 newcallspec.setmulti2(
                     arg_values_types,
                     argnames,
-                    param.values,
-                    a_id,
-                    param.marks,
+                    param_set.values,
+                    param_id,
+                    param_set.marks,
                     scopenum,
                     param_index,
                 )