Make the fused dispatch function safe w.r.t. user defined wraparound/boundscheck (GH-3493)

The directives are currently inherited from user code, which is not correct, but it seemed difficult to reliably change the directives applied to the `__pyx_fused_cpdef` dispatch function, so instead the code was changed to be independent.

Fixes https://github.com/cython/cython/issues/3492

Cython/Compiler/FusedNode.py

@@ -594,13 +594,13 @@ class FusedCFuncDefNode(StatListNode):
                 if not _fused_sigindex:
                     for sig in <dict>signatures:
                         sigindex_node = _fused_sigindex
-                        sig_series = sig.strip('()').split('|')
-                        for sig_type in sig_series[:-1]:
+                        *sig_series, last_type = sig.strip('()').split('|')
+                        for sig_type in sig_series:
                             if sig_type not in sigindex_node:
                                 sigindex_node[sig_type] = sigindex_node = {}
                             else:
                                 sigindex_node = sigindex_node[sig_type]
-                        sigindex_node[sig_series[-1]] = sig
+                        sigindex_node[last_type] = sig
             """
         )
 

tests/compile/fused_wraparound.pyx

+# mode: compile
+# tag: fused, werror
+
+"""
+Very short test for https://github.com/cython/cython/issues/3492
+(Needs its own file since werror fails on the main fused-test files)
+wraparound and boundscheck directives shouldn't break the fused
+dispatcher function
+"""
+
+cimport cython
+
+ctypedef fused fused_t:
+    str
+    int
+    long
+    complex
+
+@cython.wraparound(False)
+@cython.boundscheck(False)
+def func(fused_t a, cython.floating b):
+    return a, b