Update to C++20

It's time to update to the new standard. It has nice features we can
make use of.

Signed-off-by: Philipp Schrader <philipp.schrader@gmail.com>
Change-Id: Idb125d2239771972f0c3bfde221f24b18ddfb41f
diff --git a/WORKSPACE b/WORKSPACE
index f98a2da..8db3b72 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -290,7 +290,7 @@
 ]
 
 llvm_cxxopts = [
-    "-std=gnu++17",
+    "-std=gnu++20",
 ]
 
 llvm_opt_copts = [
diff --git a/third_party/bazel-toolchain/toolchain/cc_toolchain_config.bzl b/third_party/bazel-toolchain/toolchain/cc_toolchain_config.bzl
index e429c40..59b542c 100644
--- a/third_party/bazel-toolchain/toolchain/cc_toolchain_config.bzl
+++ b/third_party/bazel-toolchain/toolchain/cc_toolchain_config.bzl
@@ -191,7 +191,7 @@
 
     # Flags related to C++ standard.
     cxx_flags = [
-        "-std=c++17",
+        "-std=c++20",
     ]
     compile_not_cxx_flags = []
 
diff --git a/y2020/vision/sift/fast_gaussian_halide_generator.sh b/y2020/vision/sift/fast_gaussian_halide_generator.sh
index e305f5b..793e56c 100755
--- a/y2020/vision/sift/fast_gaussian_halide_generator.sh
+++ b/y2020/vision/sift/fast_gaussian_halide_generator.sh
@@ -59,6 +59,6 @@
   -lstdc++ -lpthread -ldl -lm -lz \
   "${SOURCE}" \
   "${HALIDE}/share/Halide/tools/GenGen.cpp" \
-  --std=gnu++17 \
+  --std=gnu++20 \
   -ggdb3 \
   -o "${BINARY}"