Fix memoryview casts involving fused types (GH-3882)

I think this approach is more satisfactory than the old way it used to "work", where "fused_to_specific" was permanently added to the module scope containing the fused type (in this case the Cython scope), was applied in "Scope.lookup_type", but continued to have an effect on the scope forever.

Closes https://github.com/cython/cython/issues/3881

Cython/Compiler/Nodes.py

@@ -1048,6 +1048,8 @@ class CSimpleBaseTypeNode(CBaseTypeNode):
                         type = PyrexTypes.TemplatePlaceholderType(self.name)
                     else:
                         error(self.pos, "'%s' is not a type identifier" % self.name)
+        if type and type.is_fused and env.fused_to_specific:
+            type = type.specialize(env.fused_to_specific)
         if self.complex:
             if not type.is_numeric or type.is_complex:
                 error(self.pos, "can only complexify c numeric types")

tests/run/fused_types.pyx

@@ -326,6 +326,8 @@ def test_fused_memslice_dtype(cython.floating[:] array):
     cdef cython.floating[:] otherarray = array[0:100:1]
     print cython.typeof(array), cython.typeof(otherarray), \
           array[5], otherarray[6]
+    cdef cython.floating value;
+    cdef cython.floating[:] test_cast = <cython.floating[:1:1]>&value
 
 def test_fused_memslice_dtype_repeated(cython.floating[:] array1, cython.floating[:] array2):
     """