Reorder conditions in uses-allocator construction helper

The erased_type condition is only true for code using the Library
Fundamentals TS, so assume it's less common and only check it after
checking for convertibility.

This does mean for types using erased_type the more expensive
convertibility check is done first, but such types are rare.

	* include/bits/uses_allocator.h (__is_erased_or_convertible): Reorder
	conditions. Add comments.
	* testsuite/20_util/uses_allocator/69293_neg.cc: Adjust dg-error line.
	* testsuite/20_util/uses_allocator/cons_neg.cc: Likewise.
	* testsuite/20_util/scoped_allocator/69293_neg.cc: Likewise.

From-SVN: r262945

libstdc++-v3/ChangeLog

 2018-07-24  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/uses_allocator.h (__is_erased_or_convertible): Reorder
+	conditions. Add comments.
+	* testsuite/20_util/uses_allocator/69293_neg.cc: Adjust dg-error line.
+	* testsuite/20_util/uses_allocator/cons_neg.cc: Likewise.
+	* testsuite/20_util/scoped_allocator/69293_neg.cc: Likewise.
+
+	* include/bits/uses_allocator.h (__is_erased_or_convertible): Remove.
+	(__uses_allocator_helper): Check conditions directly instead of
+	using __is_erased_or_convertible.
+
 	* include/experimental/memory_resource: Adjust comments and
 	whitespace.
 	(__resource_adaptor_imp): Add second template parameter for type of

libstdc++-v3/include/bits/uses_allocator.h

@@ -36,11 +36,15 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+  // This is used for std::experimental::erased_type from Library Fundamentals.
   struct __erased_type { };
 
+  // This also supports the "type-erased allocator" protocol from the
+  // Library Fundamentals TS, where allocator_type is erased_type.
+  // The second condition will always be false for types not using the TS.
   template<typename _Alloc, typename _Tp>
     using __is_erased_or_convertible
-      = __or_<is_same<_Tp, __erased_type>, is_convertible<_Alloc, _Tp>>;
+      = __or_<is_convertible<_Alloc, _Tp>, is_same<_Tp, __erased_type>>;
 
   /// [allocator.tag]
   struct allocator_arg_t { explicit allocator_arg_t() = default; };

libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc

@@ -46,5 +46,5 @@ test01()
   scoped_alloc sa;
   auto p = sa.allocate(1);
   sa.construct(p);  // this is required to be ill-formed
-  // { dg-error "static assertion failed" "" { target *-*-* } 90 }
+  // { dg-error "static assertion failed" "" { target *-*-* } 94 }
 }

libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc

@@ -44,5 +44,5 @@ test01()
 {
   alloc_type a;
   std::tuple<X> t(std::allocator_arg, a); // this is required to be ill-formed
-  // { dg-error "static assertion failed" "" { target *-*-* } 90 }
+  // { dg-error "static assertion failed" "" { target *-*-* } 94 }
 }

libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc

@@ -43,4 +43,4 @@ void test01()
 
   tuple<Type> t(allocator_arg, a, 1);
 }
-// { dg-error "static assertion failed" "" { target *-*-* } 90 }
+// { dg-error "static assertion failed" "" { target *-*-* } 94 }