From 9a87d24bc7d1c75c2080bcdef99b97cd000c2b7e Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Mon, 22 Jan 2018 11:07:52 -0800
Subject: [PATCH] [string] Workaround fail emptySingleton check.

This is a temporary workaround for some situations where the empty
singleton is not being formed correctly (and this seems to be highly
configuration dependent). Work around that for now for also checking
for empty non-storage-backed Strings.

This is probably too expensive a check for us to do long-term, but it
works for now.
---
 stdlib/public/core/String.swift     | 19 +------------------
 stdlib/public/core/StringGuts.swift |  8 +++++++-
 2 files changed, 8 insertions(+), 19 deletions(-)

diff --git a/stdlib/public/core/String.swift b/stdlib/public/core/String.swift
index 016ad6af73e..9fc177a8ce9 100644
--- a/stdlib/public/core/String.swift
+++ b/stdlib/public/core/String.swift
@@ -1011,24 +1011,7 @@ extension String {
   /// - Parameter other: Another string.
   @_inlineable // FIXME(sil-serialize-all)
   public mutating func append(_ other: String) {
-    if self._guts._isEmptySingleton {
-      // We must be careful not to discard any capacity that
-      // may have been reserved for the append -- this is why
-      // we check for the empty string singleton rather than
-      // a zero `count` above.
-      self = other
-      return
-    }
-    defer { _fixLifetime(other) }
-    if _slowPath(other._guts._isOpaque) {
-      self._guts.append(other._guts._asOpaque())
-      return
-    }
-    if other._guts.isASCII {
-      self._guts.append(other._guts._unmanagedASCIIView)
-      return
-    }
-    self._guts.append(other._guts._unmanagedUTF16View)
+    self._guts.append(other._guts)
   }
 
   /// Appends the given Unicode scalar to the string.
diff --git a/stdlib/public/core/StringGuts.swift b/stdlib/public/core/StringGuts.swift
index 96804d57883..5d8000ba6d2 100644
--- a/stdlib/public/core/StringGuts.swift
+++ b/stdlib/public/core/StringGuts.swift
@@ -1006,10 +1006,16 @@ extension _StringGuts {
   @_inlineable
   public // TODO(StringGuts): for testing only
   mutating func append(_ other: _StringGuts) {
-    if _isEmptySingleton {
+    // FIXME(TODO: JIRA): shouldn't _isEmptySingleton be sufficient?
+    if _isEmptySingleton || self.count == 0 && !_object.isNative {
+      // We must be careful not to discard any capacity that
+      // may have been reserved for the append -- this is why
+      // we check for the empty string singleton rather than
+      // a zero `count` above.
       self = other
       return
     }
+
     defer { _fixLifetime(other) }
     if _slowPath(other._isOpaque) {
       self.append(other._asOpaque())