Repair indexing syntax in generated Pythran code: must use "(...)" for...

Repair indexing syntax in generated Pythran code: must use "(...)" for multiple indices, but "[...]" for a single index.
Also refactor indexing code to avoid duplication.
Closes #1946.

Cython/Compiler/ExprNodes.py

@@ -4193,7 +4193,7 @@ class BufferIndexNode(_IndexingBaseNode):
             # case.
             code.putln("__Pyx_call_destructor(%s);" % obj)
             code.putln("new (&%s) decltype(%s){%s};" % (obj, obj, self.base.pythran_result()))
-            code.putln("%s(%s) %s= %s;" % (
+            code.putln("%s%s %s= %s;" % (
                 obj,
                 pythran_indexing_code(self.indices),
                 op,
@@ -4224,7 +4224,7 @@ class BufferIndexNode(_IndexingBaseNode):
         if is_pythran_expr(self.base.type):
             res = self.result()
             code.putln("__Pyx_call_destructor(%s);" % res)
-            code.putln("new (&%s) decltype(%s){%s[%s]};" % (
+            code.putln("new (&%s) decltype(%s){%s%s};" % (
                 res,
                 res,
                 self.base.pythran_result(),

Cython/Compiler/Pythran.py

@@ -58,8 +58,13 @@ def pythran_unaryop_type(op, type_):
         op, pythran_type(type_))
 
 
-def pythran_indexing_type(type_, indices):
-    def index_code(idx):
+@cython.ccall
+def _index_access(index_code, indices):
+    indexing = ",".join([index_code(idx) for idx in indices])
+    return ('[%s]' if len(indices) == 1 else '(%s)') % indexing
+
+
+def _index_type_code(idx):
     if idx.is_slice:
         if idx.step.is_none:
             func = "contiguous_slice"
@@ -75,12 +80,8 @@ def pythran_indexing_type(type_, indices):
         return "std::declval<%s>()" % idx.type.pythran_type
     raise ValueError("unsupported indexing type %s!" % idx.type)
 
-    indexing = ",".join(index_code(idx) for idx in indices)
-    return type_remove_ref("decltype(std::declval<%s>()[%s])" % (pythran_type(type_), indexing))
-
 
-def pythran_indexing_code(indices):
-    def index_code(idx):
+def _index_code(idx):
     if idx.is_slice:
         values = idx.start, idx.stop, idx.step
         if idx.step.is_none:
@@ -96,7 +97,16 @@ def pythran_indexing_code(indices):
         return idx.pythran_result()
     raise ValueError("unsupported indexing type %s" % idx.type)
 
-    return ",".join(index_code(idx) for idx in indices)
+
+def pythran_indexing_type(type_, indices):
+    return type_remove_ref("decltype(std::declval<%s>()%s)" % (
+        pythran_type(type_),
+        _index_access(_index_type_code, indices),
+    ))
+
+
+def pythran_indexing_code(indices):
+    return _index_access(_index_code, indices)
 
 
 def pythran_func_type(func, args):