From 68149a3629c09f8e24ad7847cda51f0cb1b20078 Mon Sep 17 00:00:00 2001
From: Vladimir Panteleev <git@thecybershadow.net>
Date: Mon, 26 Jun 2017 12:28:04 +0000
Subject: [PATCH] Fix Issue 17553 - std.json should not do UTF decoding when
 encoding JSON

---
 std/json.d | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/std/json.d b/std/json.d
index 34ca5e8ad46..600ca9a091f 100644
--- a/std/json.d
+++ b/std/json.d
@@ -707,7 +707,8 @@ if (isInputRange!T && !isInfinite!T && isSomeChar!(ElementEncodingType!T))
     JSONValue root;
     root.type_tag = JSON_TYPE.NULL;
 
-    // UTF decoding is unnecessary when parsing JSON.
+    // Avoid UTF decoding when possible, as it is unnecessary when
+    // processing JSON.
     static if (is(T : const(char)[]))
         alias Char = char;
     else
@@ -1142,11 +1143,11 @@ string toJSON(const ref JSONValue root, in bool pretty = false, in JSONOptions o
 {
     auto json = appender!string();
 
-    void toString(string str) @safe
+    void toStringImpl(Char)(string str) @safe
     {
         json.put('"');
 
-        foreach (dchar c; str)
+        foreach (Char c; str)
         {
             switch (c)
             {
@@ -1160,7 +1161,7 @@ string toJSON(const ref JSONValue root, in bool pretty = false, in JSONOptions o
                 case '\t':      json.put("\\t");        break;
                 default:
                 {
-                    import std.uni : isControl;
+                    import std.ascii : isControl;
                     import std.utf : encode;
 
                     with (JSONOptions) if (isControl(c) ||
@@ -1192,6 +1193,16 @@ string toJSON(const ref JSONValue root, in bool pretty = false, in JSONOptions o
         json.put('"');
     }
 
+    void toString(string str) @safe
+    {
+        // Avoid UTF decoding when possible, as it is unnecessary when
+        // processing JSON.
+        if (options & JSONOptions.escapeNonAsciiChars)
+            toStringImpl!dchar(str);
+        else
+            toStringImpl!char(str);
+    }
+
     void toValue(ref in JSONValue value, ulong indentLevel) @safe
     {
         void putTabs(ulong additionalIndent = 0)
@@ -1813,3 +1824,9 @@ pure nothrow @safe unittest // issue 15884
     assert(parseJSON("\"\xFF\"").str == "\xFF");
     assert(parseJSON("\"\U0001D11E\"").str == "\U0001D11E");
 }
+
+@safe unittest // issue 17553
+{
+    auto v = JSONValue("\xFF");
+    assert(toJSON(v) == "\"\xFF\"");
+}