Allow const declarations in nested template arguments (GH-3886)

Closes https://github.com/cython/cython/issues/1355

Cython/Compiler/Parsing.py

@@ -2678,7 +2678,7 @@ def looking_at_expr(s):
             s.put_back(*saved)
         elif s.sy == '[':
             s.next()
-            is_type = s.sy == ']'
+            is_type = s.sy == ']' or not looking_at_expr(s)  # could be a nested template type
             s.put_back(*saved)
 
         dotted_path.reverse()

tests/compile/cpp_templates_nested.pyx

+# tag: cpp
+# mode: compile
+
+from libcpp.vector cimport vector
+
+cdef extern from *:
+    cdef cppclass Foo[T]:
+        pass
+
+    cdef cppclass Bar:
+        pass
+
+cdef vector[vector[int]] a
+cdef vector[vector[const int]] b
+cdef vector[vector[vector[int]]] c
+cdef vector[vector[vector[const int]]] d
+cdef Foo[Foo[Bar]] e
+cdef Foo[Foo[const Bar]] f