--- a/build/moz.configure/toolchain.configure
+++ b/build/moz.configure/toolchain.configure
@@ -338,26 +338,30 @@ def check_compiler(compiler, language, t
     # example)
     if info.language == 'C' and info.language_version != 199901:
         if info.type in ('clang-cl', 'clang', 'gcc'):
             append_flag('-std=gnu99')
 
     # Note: MSVC, while supporting C++11, still reports 199711L for __cplusplus.
     # Note: this is a strict version check because we used to always add
     # -std=gnu++11.
-    if info.language == 'C++' and info.language_version != 201103:
-        if info.type in ('clang-cl', 'clang', 'gcc'):
+    if info.language == 'C++':
+        if info.type in ('clang', 'gcc') and info.language_version != 201103:
             append_flag('-std=gnu++11')
+        # MSVC 2015 headers include C++14 features, but don't guard them
+        # with appropriate checks.
+        if info.type == 'clang-cl' and info.language_version != 201402:
+            append_flag('-std=gnu++14')
 
-    # We force clang-cl to emulate Visual C++ 2013 Update 3 with fallback to
+    # We force clang-cl to emulate Visual C++ 2015 Update 2 with fallback to
     # cl.exe.
-    if info.type == 'clang-cl' and info.version != '18.00.30723':
+    if info.type == 'clang-cl' and info.version != '19.00.23918':
         # Those flags are direct clang-cl flags that don't need -Xclang, add
         # them directly.
-        flags.append('-fms-compatibility-version=18.00.30723')
+        flags.append('-fms-compatibility-version=19.00.23918')
         flags.append('-fallback')
 
     # Check compiler target
     # --------------------------------------------------------------------
     if not info.cpu or info.cpu != target.cpu:
         if info.type == 'clang':
             append_flag('--target=%s' % target.toolchain)
         elif info.type == 'gcc':
@@ -667,17 +671,17 @@ def compiler(language, host_or_target, c
                 '%s %s compiler target endianness (%s) does not match --%s '
                 'endianness (%s)'
                 % (host_or_target_str.capitalize(), language,
                    info.target_endianness or 'unknown', host_or_target_str,
                    host_or_target.endianness))
 
         if info.flags:
             raise FatalCheckError(
-                'Unknown compiler or compiler not supported.')
+                'Unknown compiler or compiler not supported. ' + str(info.flags))
 
         # Compiler version checks
         # ===================================================
         # Check the compiler version here instead of in `compiler_version` so
         # that the `checking` message doesn't pretend the compiler can be used
         # to then bail out one line later.
         if info.type == 'gcc' and info.version < '4.8.0':
             raise FatalCheckError(

--- a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
+++ b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
@@ -213,23 +213,23 @@ VS_PLATFORM_X86 = {
 VS_PLATFORM_X86_64 = {
     '_M_X64': 100,
     '_WIN32': 1,
     '_WIN64': 1,
 }
 
 # Note: In reality, the -std=gnu* options are only supported when preceded by
 # -Xclang.
-CLANG_CL_3_9 = (CLANG_BASE('3.9.0') + VS('18.00.00000') + DEFAULT_C11 +
+CLANG_CL_3_9 = (CLANG_BASE('3.9.0') + VS('19.00.00000') + DEFAULT_C11 +
                 SUPPORTS_GNU99 + SUPPORTS_GNUXX11) + {
     '*.cpp': {
         '__STDC_VERSION__': False,
         '__cplusplus': '201103L',
     },
-    '-fms-compatibility-version=18.00.30723': VS('18.00.30723')[None],
+    '-fms-compatibility-version=19.00.23918': VS('19.00.23918')[None],
 }
 
 CLANG_CL_PLATFORM_X86 = FakeCompiler(VS_PLATFORM_X86, GCC_PLATFORM_X86[None])
 CLANG_CL_PLATFORM_X86_64 = FakeCompiler(VS_PLATFORM_X86_64, GCC_PLATFORM_X86_64[None])
 
 
 class BaseToolchainTest(BaseConfigureTest):
     def setUp(self):
@@ -766,25 +766,25 @@ class WindowsToolchainTest(BaseToolchain
         flags=[],
         version='19.00.23918',
         type='msvc',
         compiler='/usr/bin/cl',
         language='C++',
     )
     CLANG_CL_3_9_RESULT = CompilerResult(
         flags=['-Xclang', '-std=gnu99',
-               '-fms-compatibility-version=18.00.30723', '-fallback'],
-        version='18.00.30723',
+               '-fms-compatibility-version=19.00.23918', '-fallback'],
+        version='19.00.23918',
         type='clang-cl',
         compiler='/usr/bin/clang-cl',
         language='C',
     )
     CLANGXX_CL_3_9_RESULT = CompilerResult(
-        flags=['-fms-compatibility-version=18.00.30723', '-fallback'],
-        version='18.00.30723',
+        flags=['-fms-compatibility-version=19.00.23918', '-fallback'],
+        version='19.00.23918',
         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_3_6_RESULT = LinuxToolchainTest.CLANG_3_6_RESULT
     CLANGXX_3_6_RESULT = LinuxToolchainTest.CLANGXX_3_6_RESULT