Update the minimum C++ version to C++17

Differential Revision: D62329462

Pull Request resolved: #5158

Author: dbort

CONTRIBUTING.md

@@ -131,9 +131,7 @@ for detailed advice.
 
 #### C++ language version
 
-**C++11.**
-
-NOTE: The code does not yet fully conform to this, and some files require C++17.
+**C++17.**
 
 Rationale: This is a compromise between being compatible with older, proprietary
 toolchains, and having access to relatively modern C++ features.

docs/source/getting-started-setup.md

@@ -59,13 +59,11 @@ also work in similar environments.
   - We recommend `conda` as it provides cross-language
     support and integrates smoothly with `pip` (Python's built-in package manager)
   - Otherwise, Python's built-in virtual environment manager `python venv` is a good alternative.
-* `g++` version 8 or higher, `clang++` version 8 or higher, or another
-  C++17-compatible toolchain that supports GNU C-style [statement
-  expressions](https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html) (`({ ...
-  })` syntax).
+* `g++` version 7 or higher, `clang++` version 5 or higher, or another
+  C++17-compatible toolchain.
 
 Note that the cross-compilable core runtime code supports a wider range of
-toolchains, down to C++11. See the [Runtime Overview](./runtime-overview.md) for
+toolchains, down to C++17. See the [Runtime Overview](./runtime-overview.md) for
 portability details.
 
 ## Quick Setup: Colab/Jupyter Notebook Prototype

docs/source/runtime-overview.md

@@ -96,7 +96,7 @@ can build it for a wide variety of target systems.
 
 #### C++ Language Considerations
 
-* The code is C++11-compatible to work with older toolchains.
+* The code is C++17-compatible to work with older toolchains.
 * The runtime does not use exceptions or RTTI, although it is not antagonistic
   to them.
 * The code is compatible with GCC and Clang, and has also been built with

runtime/platform/compiler.h

@@ -13,17 +13,32 @@
 
 #pragma once
 
-// Compiler support checks.
+/*
+ * Compiler support checks. Follows the logic used by pytorch/c10/util/C++17.h
+ * but may support older versions.
+ */
+
+// https://gcc.gnu.org/projects/cxx-status.html#cxx17
+#if !defined(__clang__) && !defined(_MSC_VER) && defined(__GNUC__) && \
+    __GNUC__ < 7
+#error \
+    "You're trying to build ExecuTorch with a too old version of GCC. We need GCC 7 or later."
+#endif
+
+// https://clang.llvm.org/cxx_status.html#cxx17
+#if defined(__clang__) && __clang_major__ < 5
+#error \
+    "You're trying to build ExecuTorch with a too old version of Clang. We need Clang 5 or later."
+#endif
 
-#if !defined(__cplusplus)
-#error ExecuTorch must be compiled using a C++ compiler.
+#if (defined(_MSC_VER) && (!defined(_MSVC_LANG) || _MSVC_LANG < 201703L)) || \
+    (!defined(_MSC_VER) && __cplusplus < 201703L)
+#error "You need C++17 to compile ExecuTorch"
 #endif
 
-#if __cplusplus < 201103L && (!defined(_MSC_VER) || _MSC_VER < 1600) && \
-    (!defined(__GNUC__) ||                                              \
-     (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__ < 40400))
-#error ExecuTorch must use a compiler supporting at least the C++11 standard.
-#error __cplusplus _MSC_VER __GNUC__  __GNUC_MINOR__  __GNUC_PATCHLEVEL__
+#if defined(_WIN32) && (defined(min) || defined(max))
+#error \
+    "Macro clash with min and max -- define NOMINMAX when compiling your program on Windows"
 #endif
 
 /*