diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -120,6 +120,11 @@
 C++ Language Changes in Clang
 -----------------------------
 
+* Clang's default C++ dialect is now ``gnu++17`` instead of ``gnu++14``. This
+  means Clang will by default accept code using features from C++17 and
+  conforming GNU extensions. Projects incompatible with C++17 can add
+  ``-std=gnu++14`` to their build settings to restore the previous behaviour.
+
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Basic/LangStandards.cpp b/clang/lib/Basic/LangStandards.cpp
--- a/clang/lib/Basic/LangStandards.cpp
+++ b/clang/lib/Basic/LangStandards.cpp
@@ -78,7 +78,7 @@
     if (T.isDriverKit())
       return LangStandard::lang_gnucxx17;
     else
-      return LangStandard::lang_gnucxx14;
+      return LangStandard::lang_gnucxx17;
   case Language::RenderScript:
     return LangStandard::lang_c99;
   case Language::HIP:
diff --git a/clang/test/Preprocessor/lang-std.cpp b/clang/test/Preprocessor/lang-std.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/lang-std.cpp
@@ -0,0 +1,11 @@
+/// Test default dialects.
+// RUN: %clang_cc1 -dM -E %s | FileCheck --check-prefix=CXX17 %s
+// RUN: %clang_cc1 -dM -E -x cuda %s | FileCheck --check-prefix=CXX14 %s
+// RUN: %clang_cc1 -dM -E -x hip %s | FileCheck --check-prefix=CXX14 %s
+
+// RUN: %clang_cc1 -dM -E -x cuda -std=c++14 %s | FileCheck --check-prefix=CXX14 %s
+// RUN: %clang_cc1 -dM -E -x hip -std=c++98 %s | FileCheck --check-prefix=CXX98 %s
+
+// CXX98: #define __cplusplus 199711L
+// CXX14: #define __cplusplus 201402L
+// CXX17: #define __cplusplus 201703L
diff --git a/clang/test/Preprocessor/lang-std.cu b/clang/test/Preprocessor/lang-std.cu
deleted file mode 100644
--- a/clang/test/Preprocessor/lang-std.cu
+++ /dev/null
@@ -1,7 +0,0 @@
-// RUN: %clang_cc1 -dM -E -x hip %s | FileCheck -check-prefix=CXX14 %s
-// RUN: %clang_cc1 -dM -E %s | FileCheck -check-prefix=CXX14 %s
-// RUN: %clang_cc1 -dM -E -std=c++98 -x hip %s | FileCheck -check-prefix=CXX98 %s
-// RUN: %clang_cc1 -dM -E -std=c++98 %s | FileCheck -check-prefix=CXX98 %s
-
-// CXX98: #define __cplusplus 199711L
-// CXX14: #define __cplusplus 201402L