Remove ban on assignment to reference (GH-3987)

Fixes https://github.com/cython/cython/issues/1863

Cython/Compiler/ExprNodes.py

@@ -2136,8 +2136,6 @@ class NameNode(AtomicExprNode):
 
         if self.type.is_const:
             error(self.pos, "Assignment to const '%s'" % self.name)
-        if self.type.is_reference:
-            error(self.pos, "Assignment to reference '%s'" % self.name)
         if not self.is_lvalue():
             error(self.pos, "Assignment to non-lvalue '%s'" % self.name)
             self.type = PyrexTypes.error_type

tests/run/lvalue_refs.pyx

@@ -29,6 +29,18 @@ def test_lvalue_ref_assignment():
     assert bar[0] == &baz[0][0]
     assert bar[0][0] == bongle
 
+cdef void assign_to_basic_reference(int& ref):
+    ref = 123
+
+def test_assign_to_basic_ref():
+    """
+    >>> test_assign_to_basic_ref()
+    123
+    """
+    cdef int x=0
+    assign_to_basic_reference(x)
+    print x
+
 # not *strictly* lvalue refs but this file seems the closest applicable place for it.
 # GH 3754 - std::vector operator[] returns a reference, and this causes problems if
 # the reference is passed into Cython __Pyx_GetItemInt