From 21cdb11346ac01313dcdde4beeaf9bceb8f97b67 Mon Sep 17 00:00:00 2001
From: Richard Hodges <hodges.r@gmail.com>
Date: Tue, 17 Nov 2020 07:08:03 +0100
Subject: [PATCH] Fix value_to and value_from for MSVC-14.0:

close #465

value_to and value_from was incorrectly deducing that std::string was
"array-like" rather than "string-like", but only on msvc-14.0

Original test:

template<class T, typename std::enable_if<
    std::is_constructible<remove_cvref<T>, const char*, std::size_t>::value &&
    std::is_convertible<decltype(std::declval<T&>().data()), const char*>::value &&
    std::is_convertible<decltype(std::declval<T&>().size()), std::size_t>::value
>::type* = nullptr>

Which works for all compilers except msvc-14.0

New test:

template<class T, typename std::enable_if<
    std::is_constructible<remove_cvref<T>, const char*, std::size_t>::value &&
    std::is_convertible<decltype(std::declval<T&>().data()), const char*>::value &&
    std::is_integral<decltype(std::declval<T&>().size())>::value
>::type* = nullptr>

Note that each individual test works on all compilers. It seems to be
the conjuction of tests that caused msvc-14 to trip up.
---
 include/boost/json/detail/value_from.hpp | 8 +++++---
 include/boost/json/detail/value_to.hpp   | 8 +++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/boost/json/detail/value_from.hpp b/include/boost/json/detail/value_from.hpp
index 7f2e77c9..c14178bc 100644
--- a/include/boost/json/detail/value_from.hpp
+++ b/include/boost/json/detail/value_from.hpp
@@ -83,11 +83,13 @@ tag_invoke(
 // Generic conversions
 
 // string-like types
+// NOTE: original check for size used is_convertible but 
+// MSVC-140 selects wrong specialisation if used
 template<class T, typename std::enable_if<
     std::is_constructible<remove_cvref<T>, const char*, std::size_t>::value &&
-        std::is_convertible<decltype(std::declval<T&>().data()), const char*>::value && 
-    std::is_convertible<decltype(std::declval<T&>().size()),
-        std::size_t>::value>::type* = nullptr>
+    std::is_convertible<decltype(std::declval<T&>().data()), const char*>::value && 
+    std::is_integral<decltype(std::declval<T&>().size())>::value
+>::type* = nullptr>
 void 
 value_from_generic(
     value& jv,
diff --git a/include/boost/json/detail/value_to.hpp b/include/boost/json/detail/value_to.hpp
index 2708451a..bdf5401e 100644
--- a/include/boost/json/detail/value_to.hpp
+++ b/include/boost/json/detail/value_to.hpp
@@ -100,11 +100,13 @@ tag_invoke(
 // Use generic conversion
 
 // string-like types
+// NOTE: original check for size used is_convertible but 
+// MSVC-140 selects wrong specialisation if used
 template<class T, typename std::enable_if<
     std::is_constructible<T, const char*, std::size_t>::value &&
-        std::is_convertible<decltype(std::declval<T&>().data()), const char*>::value && 
-    std::is_convertible<decltype(std::declval<T&>().size()),
-        std::size_t>::value>::type* = nullptr>
+    std::is_convertible<decltype(std::declval<T&>().data()), const char*>::value && 
+    std::is_integral<decltype(std::declval<T&>().size())>::value
+>::type* = nullptr>
 T
 value_to_generic(
     const value& jv,