Restore enable_if lost during original import of pstl

        * include/pstl/glue_numeric_defs.h: Restore enable_if lost
        during original import of pstl.
        * include/pstl/glue_numeric_impl.h: Likewise.

From-SVN: r279212

libstdc++-v3/ChangeLog

+2019-12-10  Thomas Rodgers  <trodgers@redhat.com>
+
+	* include/pstl/glue_numeric_defs.h: Restore enable_if lost during original
+	import of pstl.
+	* include/pstl/glue_numeric_impl.h: Likewise.
+
 2019-12-10  Jonathan Wakely  <jwakely@redhat.com>
 
 	(span(ContiguousIterator, Sentinel)): Add conditional noexcept.

libstdc++-v3/include/pstl/glue_numeric_defs.h

@@ -55,7 +55,7 @@ exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIte
                _ForwardIterator2 __result, _Tp __init);
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation>
-_ForwardIterator2
+__pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result, _Tp __init, _BinaryOperation __binary_op);
 

libstdc++-v3/include/pstl/glue_numeric_impl.h

@@ -100,7 +100,7 @@ exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIte
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation>
-_ForwardIterator2
+__pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result, _Tp __init, _BinaryOperation __binary_op)
 {