[mypyc] Fix fastcall positional fast path when parser is uninitialized

In `CPyArg_ParseStackAndKeywordsSimple`, we don't check if the parser is
initialized, so on the first call, `min` and `max` will be zero and so
zero arguments being passed will always pass the test. Add an initialization
check.

Fixes #872.

mypyc/lib-rt/getargsfast.c

@@ -21,6 +21,8 @@
 /* None of this is supported on Python 3.6 or earlier */
 #if PY_VERSION_HEX >= 0x03070000
 
+#define PARSER_INITED(parser) ((parser)->kwtuple != NULL)
+
 /* Forward */
 static int
 vgetargskeywordsfast_impl(PyObject *const *args, Py_ssize_t nargs,
@@ -93,7 +95,8 @@ CPyArg_ParseStackAndKeywordsSimple(PyObject *const *args, Py_ssize_t nargs, PyOb
     va_list va;
 
     va_start(va, parser);
-    if (kwnames == NULL && nargs >= parser->min && nargs <= parser->max) {
+    if (kwnames == NULL && PARSER_INITED(parser) &&
+            nargs >= parser->min && nargs <= parser->max) {
         // Fast path: correct number of positional arguments only
         PyObject **p;
         Py_ssize_t i;
@@ -124,7 +127,7 @@ parser_init(CPyArg_Parser *parser)
     PyObject *kwtuple;
 
     assert(parser->keywords != NULL);
-    if (parser->kwtuple != NULL) {
+    if (PARSER_INITED(parser)) {
         return 1;
     }
 

mypyc/test-data/run-functions.test

@@ -968,6 +968,10 @@ assert onearg(*t) == 2
 d = {'x': 5}
 assert onearg(**d) == 6
 
+# Test a bogus call to twoargs before any correct calls are made
+with assertRaises(TypeError, "twoargs() missing required argument 'x' (pos 1)"):
+    twoargs()
+
 assert twoargs(5, 'foo') == 8
 assert twoargs(4, y='foo') == 7
 assert twoargs(y='foo', x=7) == 10
@@ -1002,6 +1006,8 @@ with assertRaises(TypeError, "onearg() missing required argument 'x' (pos 1)"):
 with assertRaises(TypeError, "onearg() takes at most 1 argument (2 given)"):
     onearg(1, y=1)
 
+with assertRaises(TypeError, "twoargs() missing required argument 'x' (pos 1)"):
+    twoargs()
 with assertRaises(TypeError, "twoargs() missing required argument 'y' (pos 2)"):
     twoargs(1)
 with assertRaises(TypeError, 'twoargs() takes at most 2 arguments (3 given)'):