deleted file mode 100644
--- a/build/build-clang/clang-4.0-linux64.json
+++ /dev/null
@@ -1,12 +0,0 @@
-{
-    "stages": "3",
-    "build_libcxx": true,
-    "build_type": "Release",
-    "assertions": false,
-    "python_path": "/usr/bin/python2.7",
-    "gcc_dir": "{MOZ_FETCHES_DIR}/gcc",
-    "cc": "{MOZ_FETCHES_DIR}/gcc/bin/gcc",
-    "cxx": "{MOZ_FETCHES_DIR}/gcc/bin/g++",
-    "as": "{MOZ_FETCHES_DIR}/gcc/bin/gcc",
-    "patches": []
-}

new file mode 100644
--- /dev/null
+++ b/build/build-clang/clang-5.0-linux64.json
@@ -0,0 +1,12 @@
+{
+    "stages": "3",
+    "build_libcxx": true,
+    "build_type": "Release",
+    "assertions": false,
+    "python_path": "/usr/bin/python2.7",
+    "gcc_dir": "{MOZ_FETCHES_DIR}/gcc",
+    "cc": "{MOZ_FETCHES_DIR}/gcc/bin/gcc",
+    "cxx": "{MOZ_FETCHES_DIR}/gcc/bin/g++",
+    "as": "{MOZ_FETCHES_DIR}/gcc/bin/gcc",
+    "patches": []
+}

--- a/build/moz.configure/toolchain.configure
+++ b/build/moz.configure/toolchain.configure
@@ -392,32 +392,27 @@ def get_compiler_info(compiler, language
     - the compiler version
     - the compiler supported language
     - the compiler supported language version
     '''
     # Note: We'd normally do a version check for clang, but versions of clang
     # in Xcode have a completely different versioning scheme despite exposing
     # the version with the same defines.
     # So instead, we make things such that the version is missing when the
-    # clang used is below the minimum supported version (currently clang 4.0,
-    # or 5.0 on mac).
+    # clang used is below the minimum supported version (currently clang 5.0).
     # We then only include the version information when the compiler matches
     # the feature check, so that an unsupported version of clang would have
     # no version number.
     check = dedent('''\
         #if defined(_MSC_VER) && defined(__clang__) && defined(_MT)
         %COMPILER "clang-cl"
         %VERSION __clang_major__.__clang_minor__.__clang_patchlevel__
         #elif defined(__clang__)
         %COMPILER "clang"
-        #  if defined(__APPLE__)
-        #    if __has_warning("-Wunguarded-availability")
-        %VERSION __clang_major__.__clang_minor__.__clang_patchlevel__
-        #    endif
-        #  elif __has_attribute(diagnose_if)
+        #  if __has_warning("-Wunguarded-availability")
         %VERSION __clang_major__.__clang_minor__.__clang_patchlevel__
         #  endif
         #elif defined(__GNUC__)
         %COMPILER "gcc"
         %VERSION __GNUC__.__GNUC_MINOR__.__GNUC_PATCHLEVEL__
         #endif
 
         #if __cplusplus
@@ -1029,21 +1024,18 @@ def compiler(language, host_or_target, c
             if info.version < '8.0.0':
                 raise FatalCheckError(
                     'Only clang-cl 8.0 or newer is supported (found version %s)'
                     % info.version)
 
         # If you want to bump the version check here search for
         # diagnose_if above, and see the associated comment.
         if info.type == 'clang' and not info.version:
-            if host_or_target.os == 'OSX':
-                raise FatalCheckError(
-                    'Only clang/llvm 5.0 or newer is supported.')
             raise FatalCheckError(
-                'Only clang/llvm 4.0 or newer is supported.')
+                'Only clang/llvm 5.0 or newer is supported.')
 
         if info.flags:
             raise FatalCheckError(
                 'Unknown compiler or compiler not supported.')
 
         return namespace(
             wrapper=wrapper,
             compiler=compiler,

--- a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
+++ b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
@@ -228,18 +228,18 @@ CLANGXX_4_0 = CLANGXX('4.0.2') + SUPPORT
 CLANG_5_0 = CLANG('5.0.1') + DEFAULT_C11 + {
     '__has_attribute(diagnose_if)': '1',
     '__has_warning("-Wunguarded-availability")': '1',
 }
 CLANGXX_5_0 = CLANGXX('5.0.1') + SUPPORTS_GNUXX17 + {
     '__has_attribute(diagnose_if)': '1',
     '__has_warning("-Wunguarded-availability")': '1',
 }
-DEFAULT_CLANG = CLANG_4_0
-DEFAULT_CLANGXX = CLANGXX_4_0
+DEFAULT_CLANG = CLANG_5_0
+DEFAULT_CLANGXX = CLANGXX_5_0
 
 
 def CLANG_PLATFORM(gcc_platform):
     base = {
         '--target=x86_64-linux-gnu': GCC_PLATFORM_X86_64_LINUX[None],
         '--target=x86_64-apple-darwin11.2.0': GCC_PLATFORM_X86_64_OSX[None],
         '--target=i686-linux-gnu': GCC_PLATFORM_X86_LINUX[None],
         '--target=i686-apple-darwin11.2.0': GCC_PLATFORM_X86_OSX[None],
@@ -474,16 +474,18 @@ class LinuxToolchainTest(BaseToolchainTe
         '/usr/bin/gcc-5': GCC_5 + GCC_PLATFORM_X86_64_LINUX,
         '/usr/bin/g++-5': GXX_5 + GCC_PLATFORM_X86_64_LINUX,
         '/usr/bin/gcc-6': GCC_6 + GCC_PLATFORM_X86_64_LINUX,
         '/usr/bin/g++-6': GXX_6 + GCC_PLATFORM_X86_64_LINUX,
         '/usr/bin/gcc-7': GCC_7 + GCC_PLATFORM_X86_64_LINUX,
         '/usr/bin/g++-7': GXX_7 + GCC_PLATFORM_X86_64_LINUX,
         '/usr/bin/clang': DEFAULT_CLANG + CLANG_PLATFORM_X86_64_LINUX,
         '/usr/bin/clang++': DEFAULT_CLANGXX + CLANG_PLATFORM_X86_64_LINUX,
+        '/usr/bin/clang-5.0': CLANG_5_0 + CLANG_PLATFORM_X86_64_LINUX,
+        '/usr/bin/clang++-5.0': CLANGXX_5_0 + CLANG_PLATFORM_X86_64_LINUX,
         '/usr/bin/clang-4.0': CLANG_4_0 + CLANG_PLATFORM_X86_64_LINUX,
         '/usr/bin/clang++-4.0': CLANGXX_4_0 + CLANG_PLATFORM_X86_64_LINUX,
         '/usr/bin/clang-3.3': CLANG_3_3 + CLANG_PLATFORM_X86_64_LINUX,
         '/usr/bin/clang++-3.3': CLANGXX_3_3 + CLANG_PLATFORM_X86_64_LINUX,
     }
 
     GCC_4_7_RESULT = old_gcc_message('4.7.3')
     GXX_4_7_RESULT = GCC_4_7_RESULT
@@ -517,34 +519,36 @@ class LinuxToolchainTest(BaseToolchainTe
         version='7.3.0',
         type='gcc',
         compiler='/usr/bin/g++-7',
         language='C++',
     )
     DEFAULT_GCC_RESULT = GCC_6_RESULT + {'compiler': '/usr/bin/gcc'}
     DEFAULT_GXX_RESULT = GXX_6_RESULT + {'compiler': '/usr/bin/g++'}
 
-    CLANG_3_3_RESULT = 'Only clang/llvm 4.0 or newer is supported.'
-    CLANGXX_3_3_RESULT = 'Only clang/llvm 4.0 or newer is supported.'
-    CLANG_4_0_RESULT = CompilerResult(
+    CLANG_3_3_RESULT = 'Only clang/llvm 5.0 or newer is supported.'
+    CLANGXX_3_3_RESULT = 'Only clang/llvm 5.0 or newer is supported.'
+    CLANG_4_0_RESULT = 'Only clang/llvm 5.0 or newer is supported.'
+    CLANGXX_4_0_RESULT = 'Only clang/llvm 5.0 or newer is supported.'
+    CLANG_5_0_RESULT = CompilerResult(
         flags=['-std=gnu99'],
-        version='4.0.2',
+        version='5.0.1',
         type='clang',
-        compiler='/usr/bin/clang-4.0',
+        compiler='/usr/bin/clang-5.0',
         language='C',
     )
-    CLANGXX_4_0_RESULT = CompilerResult(
+    CLANGXX_5_0_RESULT = CompilerResult(
         flags=['-std=gnu++14'],
-        version='4.0.2',
+        version='5.0.1',
         type='clang',
-        compiler='/usr/bin/clang++-4.0',
+        compiler='/usr/bin/clang++-5.0',
         language='C++',
     )
-    DEFAULT_CLANG_RESULT = CLANG_4_0_RESULT + {'compiler': '/usr/bin/clang'}
-    DEFAULT_CLANGXX_RESULT = CLANGXX_4_0_RESULT + {'compiler': '/usr/bin/clang++'}
+    DEFAULT_CLANG_RESULT = CLANG_5_0_RESULT + {'compiler': '/usr/bin/clang'}
+    DEFAULT_CLANGXX_RESULT = CLANGXX_5_0_RESULT + {'compiler': '/usr/bin/clang++'}
 
     def test_default(self):
         # We'll try clang and gcc, and find clang first.
         self.do_toolchain_test(self.PATHS, {
             'c_compiler': self.DEFAULT_CLANG_RESULT,
             'cxx_compiler': self.DEFAULT_CLANGXX_RESULT,
         })
 
@@ -663,30 +667,37 @@ class LinuxToolchainTest(BaseToolchainTe
         self.do_toolchain_test(paths, {
             'c_compiler': self.DEFAULT_CLANG_RESULT,
             'cxx_compiler': self.DEFAULT_CLANGXX_RESULT,
         })
 
     def test_guess_cxx_clang(self):
         # When CXX is not set, we guess it from CC.
         self.do_toolchain_test(self.PATHS, {
-            'c_compiler': self.CLANG_4_0_RESULT,
-            'cxx_compiler': self.CLANGXX_4_0_RESULT,
+            'c_compiler': self.CLANG_5_0_RESULT,
+            'cxx_compiler': self.CLANGXX_5_0_RESULT,
         }, environ={
-            'CC': 'clang-4.0',
+            'CC': 'clang-5.0',
         })
 
     def test_unsupported_clang(self):
         self.do_toolchain_test(self.PATHS, {
             'c_compiler': self.CLANG_3_3_RESULT,
             'cxx_compiler': self.CLANGXX_3_3_RESULT,
         }, environ={
             'CC': 'clang-3.3',
             'CXX': 'clang++-3.3',
         })
+        self.do_toolchain_test(self.PATHS, {
+            'c_compiler': self.CLANG_4_0_RESULT,
+            'cxx_compiler': self.CLANGXX_4_0_RESULT,
+        }, environ={
+            'CC': 'clang-4.0',
+            'CXX': 'clang++-4.0',
+        })
 
     def test_no_supported_compiler(self):
         # Even if there are gcc-x.y or clang-x.y compilers available, we
         # don't try them. This could be considered something to improve.
         paths = {
             k: v for k, v in self.PATHS.iteritems()
             if os.path.basename(k) not in ('gcc', 'g++', 'clang', 'clang++')
         }
@@ -949,16 +960,18 @@ class WindowsToolchainTest(BaseToolchain
         '/usr/bin/gcc-4.9': GCC_4_9 + GCC_PLATFORM_X86_WIN,
         '/usr/bin/g++-4.9': GXX_4_9 + GCC_PLATFORM_X86_WIN,
         '/usr/bin/gcc-5': GCC_5 + GCC_PLATFORM_X86_WIN,
         '/usr/bin/g++-5': GXX_5 + GCC_PLATFORM_X86_WIN,
         '/usr/bin/gcc-6': GCC_6 + GCC_PLATFORM_X86_WIN,
         '/usr/bin/g++-6': GXX_6 + GCC_PLATFORM_X86_WIN,
         '/usr/bin/clang': DEFAULT_CLANG + CLANG_PLATFORM_X86_WIN,
         '/usr/bin/clang++': DEFAULT_CLANGXX + CLANG_PLATFORM_X86_WIN,
+        '/usr/bin/clang-5.0': CLANG_5_0 + CLANG_PLATFORM_X86_WIN,
+        '/usr/bin/clang++-5.0': CLANGXX_5_0 + CLANG_PLATFORM_X86_WIN,
         '/usr/bin/clang-4.0': CLANG_4_0 + CLANG_PLATFORM_X86_WIN,
         '/usr/bin/clang++-4.0': CLANGXX_4_0 + CLANG_PLATFORM_X86_WIN,
         '/usr/bin/clang-3.3': CLANG_3_3 + CLANG_PLATFORM_X86_WIN,
         '/usr/bin/clang++-3.3': CLANGXX_3_3 + CLANG_PLATFORM_X86_WIN,
     }
 
     CLANG_CL_3_9_RESULT = 'Only clang-cl 8.0 or newer is supported (found version 3.9.0)'
     CLANG_CL_8_0_RESULT = CompilerResult(
@@ -973,16 +986,18 @@ class WindowsToolchainTest(BaseToolchain
         version='8.0.0',
         flags=['-Xclang', '-std=c++14'],
         type='clang-cl',
         compiler='/usr/bin/clang-cl',
         language='C++',
     )
     CLANG_3_3_RESULT = LinuxToolchainTest.CLANG_3_3_RESULT
     CLANGXX_3_3_RESULT = LinuxToolchainTest.CLANGXX_3_3_RESULT
+    CLANG_4_0_RESULT = LinuxToolchainTest.CLANG_4_0_RESULT
+    CLANGXX_4_0_RESULT = LinuxToolchainTest.CLANGXX_4_0_RESULT
     DEFAULT_CLANG_RESULT = LinuxToolchainTest.DEFAULT_CLANG_RESULT
     DEFAULT_CLANGXX_RESULT = LinuxToolchainTest.DEFAULT_CLANGXX_RESULT
     GCC_4_9_RESULT = LinuxToolchainTest.GCC_4_9_RESULT
     GXX_4_9_RESULT = LinuxToolchainTest.GXX_4_9_RESULT
     GCC_5_RESULT = LinuxToolchainTest.GCC_5_RESULT
     GXX_5_RESULT = LinuxToolchainTest.GXX_5_RESULT
     GCC_6_RESULT = LinuxToolchainTest.GCC_6_RESULT
     GXX_6_RESULT = LinuxToolchainTest.GXX_6_RESULT
@@ -1066,16 +1081,18 @@ class Windows64ToolchainTest(WindowsTool
         '/usr/bin/gcc-5': GCC_5 + GCC_PLATFORM_X86_64_WIN,
         '/usr/bin/g++-5': GXX_5 + GCC_PLATFORM_X86_64_WIN,
         '/usr/bin/gcc-6': GCC_6 + GCC_PLATFORM_X86_64_WIN,
         '/usr/bin/g++-6': GXX_6 + GCC_PLATFORM_X86_64_WIN,
         '/usr/bin/gcc-7': GCC_7 + GCC_PLATFORM_X86_64_WIN,
         '/usr/bin/g++-7': GXX_7 + GCC_PLATFORM_X86_64_WIN,
         '/usr/bin/clang': DEFAULT_CLANG + CLANG_PLATFORM_X86_64_WIN,
         '/usr/bin/clang++': DEFAULT_CLANGXX + CLANG_PLATFORM_X86_64_WIN,
+        '/usr/bin/clang-5.0': CLANG_5_0 + CLANG_PLATFORM_X86_64_WIN,
+        '/usr/bin/clang++-5.0': CLANGXX_5_0 + CLANG_PLATFORM_X86_64_WIN,
         '/usr/bin/clang-4.0': CLANG_4_0 + CLANG_PLATFORM_X86_64_WIN,
         '/usr/bin/clang++-4.0': CLANGXX_4_0 + CLANG_PLATFORM_X86_64_WIN,
         '/usr/bin/clang-3.3': CLANG_3_3 + CLANG_PLATFORM_X86_64_WIN,
         '/usr/bin/clang++-3.3': CLANGXX_3_3 + CLANG_PLATFORM_X86_64_WIN,
     }
 
 
 class LinuxCrossCompileToolchainTest(BaseToolchainTest):

--- a/taskcluster/ci/build/linux.yml
+++ b/taskcluster/ci/build/linux.yml
@@ -247,17 +247,17 @@ linux64-base-toolchains/opt:
             - builds/releng_base_firefox.py
             - builds/releng_base_linux_64_builds.py
         script: "mozharness/scripts/fx_desktop_build.py"
         secrets: true
         tooltool-downloads: public
         need-xvfb: true
     fetches:
         toolchain:
-            - linux64-clang-4.0
+            - linux64-clang-5.0
             - linux64-gcc-6
             - linux64-rust-1.37
             - linux64-cbindgen
             - linux64-sccache
             - linux64-nasm-2.13.02
             - linux64-node
 
 linux64-base-toolchains/debug:
@@ -284,17 +284,17 @@ linux64-base-toolchains/debug:
         script: "mozharness/scripts/fx_desktop_build.py"
         secrets: true
         custom-build-variant-cfg: debug
         mozconfig-variant: debug
         tooltool-downloads: public
         need-xvfb: true
     fetches:
         toolchain:
-            - linux64-clang-4.0
+            - linux64-clang-5.0
             - linux64-gcc-6
             - linux64-rust-1.37
             - linux64-cbindgen
             - linux64-sccache
             - linux64-nasm-2.13.02
             - linux64-node
 
 linux64-base-toolchains-clang/opt:
@@ -320,17 +320,17 @@ linux64-base-toolchains-clang/opt:
             - builds/releng_base_linux_64_builds.py
         script: "mozharness/scripts/fx_desktop_build.py"
         secrets: true
         tooltool-downloads: public
         need-xvfb: true
     fetches:
         toolchain:
             - linux64-binutils
-            - linux64-clang-4.0
+            - linux64-clang-5.0
             - linux64-rust-1.37
             - linux64-cbindgen
             - linux64-nasm-2.13.02
             - linux64-node
 
 linux64-base-toolchains-clang/debug:
     description: "Linux64 base toolchains clang Debug"
     index:
@@ -356,17 +356,17 @@ linux64-base-toolchains-clang/debug:
         secrets: true
         custom-build-variant-cfg: debug
         mozconfig-variant: debug
         tooltool-downloads: public
         need-xvfb: true
     fetches:
         toolchain:
             - linux64-binutils
-            - linux64-clang-4.0
+            - linux64-clang-5.0
             - linux64-rust-1.37
             - linux64-cbindgen
             - linux64-nasm-2.13.02
             - linux64-node
 
 linux/opt:
     description: "Linux32 Opt"
     index:

--- a/taskcluster/ci/fetch/toolchains.yml
+++ b/taskcluster/ci/fetch/toolchains.yml
@@ -382,22 +382,22 @@ llvm-mingw:
 
 android-rs-glue:
     description: android-rs-glue source code
     fetch:
         type: git
         repo: https://github.com/rust-windowing/android-rs-glue
         revision: e3ac6edea5814e1faca0c31ea8fac6877cb929ea
 
-clang-4.0:
-    description: clang 4.0 source code
+clang-5.0:
+    description: clang 5.0.2 source code
     fetch:
         type: git
         repo: https://github.com/llvm/llvm-project
-        revision: 449c3ef93afc7a668eb35e67a83717453e28b25a
+        revision: 6a075b6de4cafebec9ca1ff9eec7229a617c93f6
 
 clang-7:
     description: clang 7 source code
     fetch:
         type: git
         repo: https://github.com/llvm/llvm-project
         revision: d0d8eb2e5415b8be29343e3c17a18e49e67b5551
 

--- a/taskcluster/ci/toolchain/clang.yml
+++ b/taskcluster/ci/toolchain/clang.yml
@@ -5,31 +5,31 @@
 job-defaults:
     worker-type: b-linux-xlarge
     worker:
         max-run-time: 7200
     run:
         resources:
             - 'build/build-clang/build-clang.py'
 
-linux64-clang-4.0:
-    description: "Clang 4.0 toolchain build"
+linux64-clang-5.0:
+    description: "Clang 5.0 toolchain build"
     treeherder:
         symbol: TL(clang4.0)
     worker-type: b-linux-large
     run:
         script: build-clang.sh
         arguments:
-            - 'build/build-clang/clang-4.0-linux64.json'
+            - 'build/build-clang/clang-5.0-linux64.json'
         resources:
-            - 'build/build-clang/clang-4.0-linux64.json'
+            - 'build/build-clang/clang-5.0-linux64.json'
         toolchain-artifact: public/build/clang.tar.xz
     fetches:
         fetch:
-            - clang-4.0
+            - clang-5.0
         toolchain:
             - linux64-gcc-7
 
 linux64-clang-7:
     description: "Clang 7 toolchain build"
     treeherder:
         symbol: TL(clang7)
     run: