flat_buffer shrink_to_fit never throws

fixes #1831
closes #1835

CHANGELOG.md

@@ -1,5 +1,6 @@
 Version XXX:
 
+* flat_buffer shrink_to_fit is noexcept
 * moved-from dynamic buffers do not clear if different allocator
 * fix erase field
 

include/boost/beast/core/flat_buffer.hpp

@@ -358,17 +358,17 @@ public:
     void
     reserve(std::size_t n);
 
-    /** Reallocate the buffer to fit the readable bytes exactly.
+    /** Request the removal of unused capacity.
 
-        Buffer sequences previously obtained using @ref data or
-        @ref prepare become invalid.
+        This function attempts to reduce @ref capacity()
+        to @ref size(), which may not succeed.
 
         @esafe
 
-        Strong guarantee.
+        No-throw guarantee.
     */
     void
-    shrink_to_fit();
+    shrink_to_fit() noexcept;
 
     /** Set the size of the readable and writable bytes to zero.
 

include/boost/beast/core/impl/flat_buffer.hpp

@@ -260,17 +260,32 @@ reserve(std::size_t n)
 template<class Allocator>
 void
 basic_flat_buffer<Allocator>::
-shrink_to_fit()
+shrink_to_fit() noexcept
 {
     auto const len = size();
+
     if(len == capacity())
         return;
+
     char* p;
     if(len > 0)
     {
         BOOST_ASSERT(begin_);
         BOOST_ASSERT(in_);
-        p = alloc(len);
+#ifndef BOOST_NO_EXCEPTIONS
+        try
+        {
+#endif
+            p = alloc(len);
+#ifndef BOOST_NO_EXCEPTIONS
+        }
+        catch(std::exception const&)
+        {
+            // request could not be fulfilled,
+            // squelch the exception
+            return;
+        }
+#endif
         std::memcpy(p, in_, len);
     }
     else