From 998e9c10943ef361785e6f15cab1fa61172b1c24 Mon Sep 17 00:00:00 2001
From: Stephen Kelly <steveire@gmail.com>
Date: Thu, 1 Jan 2015 17:56:01 +0100
Subject: WCDH: Add feature portability for thread_local.

AppleClang does not support the cxx_thread_local feature, even
though it is based on a Clang version which does support the
feature.

 http://stackoverflow.com/a/23850891/2428389

A possible reason for that is that thread_local might be used as
a variable in existing Apple SDK headers.

Extend the WriteCompilerDetectionHeader module to generate a define
for that feature with portability fallbacks.  For the avoidance of
making it easy to write code which looks correct but which has odd
runtime behavior, don't set the define symbol at all if no
equivalent keyword is known.
---
 Help/release/dev/WCDH-thread_local.rst     |  7 +++++++
 Modules/WriteCompilerDetectionHeader.cmake | 15 +++++++++++++++
 2 files changed, 22 insertions(+)
 create mode 100644 Help/release/dev/WCDH-thread_local.rst

diff --git a/Help/release/dev/WCDH-thread_local.rst b/Help/release/dev/WCDH-thread_local.rst
new file mode 100644
index 0000000..44516a7
--- /dev/null
+++ b/Help/release/dev/WCDH-thread_local.rst
@@ -0,0 +1,7 @@
+WriteCompilerDetectionHeader thread_local portability
+-----------------------------------------------------
+
+* The :module:`WriteCompilerDetectionHeader` module learned to
+  create a define for portability of the cxx_thread_local feature. The define
+  expands to either the C++11 ``thread_local`` keyword, or a
+  pre-standardization compiler-specific equivalent, as appropriate.
diff --git a/Modules/WriteCompilerDetectionHeader.cmake b/Modules/WriteCompilerDetectionHeader.cmake
index 36b9706..d61358f 100644
--- a/Modules/WriteCompilerDetectionHeader.cmake
+++ b/Modules/WriteCompilerDetectionHeader.cmake
@@ -194,6 +194,7 @@
 # ``cxx_static_assert``          ``<PREFIX>_STATIC_ASSERT_MSG``   ``static_assert``
 # ``cxx_attribute_deprecated``   ``<PREFIX>_DEPRECATED``          ``[[deprecated]]``
 # ``cxx_attribute_deprecated``   ``<PREFIX>_DEPRECATED_MSG``      ``[[deprecated]]``
+# ``cxx_thread_local``           ``<PREFIX>_THREAD_LOCAL``        ``thread_local``
 # ============================= ================================ =====================
 #
 # A use-case which arises with such deprecation macros is the deprecation
@@ -573,6 +574,20 @@ function(write_compiler_detection_header
 #  endif
 \n")
       endif()
+      if (feature STREQUAL cxx_thread_local)
+        set(def_value "${prefix_arg}_THREAD_LOCAL")
+        set(file_content "${file_content}
+#  if ${def_name}
+#    define ${def_value} thread_local
+#  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
+#    define ${def_value} __thread
+#  elif ${prefix_arg}_COMPILER_IS_MSVC
+#    define ${def_value} __declspec(thread)
+#  else
+// ${def_value} not defined for this configuration.
+#  endif
+\n")
+      endif()
       if (feature STREQUAL cxx_attribute_deprecated)
         set(def_name ${prefix_arg}_${feature_PP})
         set(def_value "${prefix_arg}_DEPRECATED")
-- 
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