Remove _canonical_fn<queries<>> specialization

This diff adds support for `erase` and `operator[]` to
`__static_vector<T, 0>`, which allows me to delete the specialization of
`_canonical_fn<queries<>>` because empty query lists can be sorted and
uniqued uniformly.

Author: ispeters

include/exec/function.hpp

@@ -304,15 +304,6 @@ namespace experimental::execution
       }
     };
 
-    template <>
-    struct _canonical_fn<queries<>>
-    {
-      consteval auto operator()() const noexcept
-      {
-        return queries<>();
-      }
-    };
-
     template <class Sigs>
     inline constexpr _canonical_fn<Sigs> _canonical{};
 

include/stdexec/__detail/__static_vector.hpp

@@ -159,6 +159,18 @@ namespace STDEXEC
 
     __static_vector() = default;
 
+    [[nodiscard]]
+    constexpr auto operator[](std::size_t) noexcept -> value_type &
+    {
+      STDEXEC_ASSERT(false);
+    }
+
+    [[nodiscard]]
+    constexpr auto operator[](std::size_t) const noexcept -> value_type const &
+    {
+      STDEXEC_ASSERT(false);
+    }
+
     [[nodiscard]]
     constexpr auto begin() noexcept -> iterator
     {
@@ -194,6 +206,14 @@ namespace STDEXEC
     {
       return 0;
     }
+
+    constexpr auto erase(const_iterator __first, const_iterator __last) noexcept -> iterator
+    {
+      STDEXEC_ASSERT(__first == __last);
+      STDEXEC_ASSERT(__first == nullptr);
+
+      return end();
+    }
   };
 
   template <class _First, __same_as<_First>... _Rest>