[utilities] Use {} rather than = T() to default-construct

piecewise_construct and allocator_arg.

Fixes #250.

Author: zygoloid

source/utilities.tex

@@ -130,7 +130,7 @@
 
   // \ref{pair.piecewise}, pair piecewise construction
   struct piecewise_construct_t { };
-  constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
+  constexpr piecewise_construct_t piecewise_construct{};
   template <class... Types> class tuple;  // defined in \tcode{<tuple>}
 @
 \indexlibrary{\idxcode{index_sequence}}%
@@ -965,7 +965,7 @@
 \indexlibrary{\idxcode{piecewise_construct}}%
 \begin{itemdecl}
 struct piecewise_construct_t { };
-constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
+constexpr piecewise_construct_t piecewise_construct{};
 \end{itemdecl}
 
 \pnum
@@ -2983,7 +2983,7 @@
 
   // \ref{allocator.tag}, allocator argument tag
   struct allocator_arg_t { };
-  constexpr allocator_arg_t allocator_arg = allocator_arg_t();
+  constexpr allocator_arg_t allocator_arg{};
 
   // \ref{allocator.uses}, \tcode{uses_allocator}
   template <class T, class Alloc> struct uses_allocator;
@@ -3462,7 +3462,7 @@
 \begin{itemdecl}
 namespace std {
   struct allocator_arg_t { };
-  constexpr allocator_arg_t allocator_arg = allocator_arg_t();
+  constexpr allocator_arg_t allocator_arg{};
 }
 \end{itemdecl}