Only build the c++20 test when the default toolchain is being used

tests/BUILD.bazel

@@ -192,5 +192,11 @@ cc_test(
     size = "small",
     srcs = ["test_cxx_standard_main.cc"],
     args = ["202002"],
+
+    # Since some platforms require special toolchains (e.g. llvm 13.0.0) this
+    # target won't build on those platforms unless we create a new toolchain per
+    # platform with c++20. So instead just only run this test on platforms that
+    # can use the default toolchain
+    tags = ["manual"],
     deps = [":test_cxx_standard_lib_transitioned"],
 )

tests/MODULE.bazel

@@ -111,11 +111,6 @@ llvm.toolchain(
         "darwin-aarch64": ["https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.7/clang+llvm-15.0.7-arm64-apple-darwin22.0.tar.xz"],
         "darwin-x86_64": ["https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.7/clang+llvm-15.0.7-x86_64-apple-darwin21.0.tar.xz"],
     },
-    cxx_standard = {"": "c++17"},
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_with_urls",
-    constraints = ["@//:cxx17"],
 )
 use_repo(llvm, "llvm_toolchain_with_urls")
 
@@ -126,11 +121,6 @@ use_repo(llvm, "llvm_toolchain_with_urls")
 llvm.toolchain(
     name = "llvm_toolchain_13_0_0",
     llvm_version = "13.0.0",
-    cxx_standard = {"": "c++17"},
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_13_0_0",
-    constraints = ["@//:cxx17"],
 )
 use_repo(llvm, "llvm_toolchain_13_0_0")
 
@@ -139,11 +129,6 @@ llvm.toolchain(
     name = "llvm_toolchain_with_absolute_paths",
     absolute_paths = True,
     llvm_versions = LLVM_VERSIONS,
-    cxx_standard = {"": "c++17"},
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_with_absolute_paths",
-    constraints = ["@//:cxx17"],
 )
 # We can share the downloaded LLVM distribution with the first configuration.
 llvm.toolchain_root(
@@ -156,11 +141,6 @@ use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
     llvm_versions = LLVM_VERSIONS,
-    cxx_standard = {"": "c++17"},
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_with_system_llvm",
-    constraints = ["@//:cxx17"],
 )
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
 llvm.toolchain_root(
@@ -173,11 +153,6 @@ use_repo(llvm, "llvm_toolchain_with_system_llvm")
 llvm.toolchain(
     name = "llvm_toolchain_with_sysroot",
     llvm_versions = LLVM_VERSIONS,
-    cxx_standard = {"": "c++17"},
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_with_sysroot",
-    constraints = ["@//:cxx17"],
 )
 # We can share the downloaded LLVM distribution with the first configuration.
 llvm.toolchain_root(
@@ -202,10 +177,5 @@ llvm.toolchain(
     # distribution = "clang+llvm-17.0.6-x86_64-linux-gnu-ubuntu-22.04.tar.xz",
     exec_os = "linux",
     exec_arch = "amd64",
-    cxx_standard = {"": "c++17"},
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_linux_exec",
-    constraints = ["@//:cxx17"],
 )
 use_repo(llvm, "llvm_toolchain_linux_exec")

tests/WORKSPACE

@@ -57,10 +57,6 @@ llvm_toolchain(
 # TODO(siddharthab): Add test.
 llvm_toolchain(
     name = "llvm_toolchain_with_urls",
-    cxx_standard = {"": "c++17"},
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
     llvm_versions = LLVM_VERSIONS,
     sha256 = {
         "": "38bc7f5563642e73e69ac5626724e206d6d539fbef653541b34cae0ba9c3f036",
@@ -85,10 +81,6 @@ llvm_toolchain(
 # image base-devel as of the time of this writing (23 May 2022).
 llvm_toolchain(
     name = "llvm_toolchain_13_0_0",
-    cxx_standard = {"": "c++17"},
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
     llvm_version = "13.0.0",
 )
 
@@ -104,10 +96,6 @@ llvm_register_toolchains_cxx20()
 llvm_toolchain(
     name = "llvm_toolchain_with_absolute_paths",
     absolute_paths = True,
-    cxx_standard = {"": "c++17"},
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
     llvm_versions = LLVM_VERSIONS,
     # We can share the downloaded LLVM distribution with the first configuration.
     toolchain_roots = {
@@ -118,10 +106,6 @@ llvm_toolchain(
 ## Toolchain example with system LLVM; tested in GitHub CI.
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    cxx_standard = {"": "c++17"},
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
     llvm_versions = LLVM_VERSIONS,
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
@@ -145,10 +129,6 @@ filegroup(
 
 llvm_toolchain(
     name = "llvm_toolchain_with_sysroot",
-    cxx_standard = {"": "c++17"},
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
     llvm_versions = LLVM_VERSIONS,
     sysroot = {
         "linux-x86_64": "@org_chromium_sysroot_linux_x64//:sysroot",
@@ -165,14 +145,10 @@ bazel_skylib_workspace()
 
 llvm_toolchain(
     name = "llvm_toolchain_linux_exec",
-    cxx_standard = {"": "c++17"},
     exec_arch = "amd64",
     # Option 2:
     # distribution = "clang+llvm-17.0.6-x86_64-linux-gnu-ubuntu-22.04.tar.xz",
     exec_os = "linux",
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
     llvm_version = "17.0.6",
     # Option 1:
     sha256 = {"": "884ee67d647d77e58740c1e645649e29ae9e8a6fe87c1376be0f3a30f3cc9ab3"},

tests/scripts/run_tests.sh

@@ -46,8 +46,17 @@ test_args=(
   "--linkopt=-Wl,-t"
 )
 
+# :test_cxx_standard_is_20 builds with a version of the default toolchain, if
+# we're trying to build with a different toolchain then it's likely the default
+# toolchain won't work so :test_cxx_standard_is_20 won't build.
+if [[ -z ${toolchain_name} ]]; then
+  run_cxx20_toolchain=":test_cxx_standard_is_20"
+else
+  run_cxx20_toolchain=""
+fi
+
 "${bazel}" ${TEST_MIGRATION:+"--strict"} --bazelrc=/dev/null test \
-  "${common_test_args[@]}" "${test_args[@]}" //:all
+  "${common_test_args[@]}" "${test_args[@]}" //:all "${run_cxx20_toolchain}"
 
 # Note that the following flags are currently known to cause issues in migration tests:
 # --incompatible_disallow_struct_provider_syntax # https://github.com/bazelbuild/bazel/issues/7347