From fd2e32323361f0eb65b86a863b0070a8daf97c03 Mon Sep 17 00:00:00 2001
From: Marcus Boerger <marcus.boerger@gmail.com>
Date: Sun, 26 Oct 2025 11:48:27 +0100
Subject: [PATCH 01/19] Update llvm versions for tests to at least 17.0.0.
 Allow attribute llvm_versions to accept version requirements.

---
 tests/MODULE.bazel                        | 6 +++---
 tests/WORKSPACE                           | 6 +++---
 toolchain/internal/llvm_distributions.bzl | 5 +++--
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 711f279f..f7b6f75f 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -63,10 +63,10 @@ llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
+# We use C++17 and the first LLVM version with full suppor is 16.0.0.
+# We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 LLVM_VERSIONS = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "first:>=17.0.0,<18",
 }
 
 llvm.toolchain(
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index 93ea6b38..aeea1dd9 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -48,10 +48,10 @@ load("@toolchains_llvm//toolchain:rules.bzl", "llvm_toolchain")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
+# We use C++17 and the first LLVM version with full suppor is 16.0.0.
+# We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 LLVM_VERSIONS = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "first:>=17.0.0,<18",
 }
 
 llvm_toolchain(
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index 2721b7ca..076d7988 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1160,10 +1160,11 @@ def _required_llvm_release_name(*, version_or_requirements, all_llvm_distributio
     return None, None, "ERROR: No matching distribution found."
 
 def required_llvm_release_name_rctx(rctx, llvm_version):
+    print("Determining required LLVM release for version/requirement: %s" % llvm_version)  # buildifier: disable=print
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version),
+        parsed_llvm_version = _parse_version(llvm_version) if not is_requirement(llvm_version) else None,
     )
     return _required_llvm_release_name(
         version_or_requirements = llvm_version,
@@ -1192,7 +1193,7 @@ def _distribution_urls(rctx):
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version),
+        parsed_llvm_version = _parse_version(llvm_version) if not is_requirement(llvm_version) else None,
     )
     _, sha256, strip_prefix, _ = _key_attrs(rctx)
 

From 3bf3c6cc583f77b9677fbcf1938b25df27d19448 Mon Sep 17 00:00:00 2001
From: Marcus Boerger <marcus.boerger@gmail.com>
Date: Sun, 26 Oct 2025 11:50:36 +0100
Subject: [PATCH 02/19] Update llvm version used for tests. Allow attribute
 llvm_versions to specify version requirements.

---
 tests/MODULE.bazel                        | 6 +++---
 tests/WORKSPACE                           | 6 +++---
 toolchain/internal/llvm_distributions.bzl | 5 +++--
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 711f279f..f7b6f75f 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -63,10 +63,10 @@ llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
+# We use C++17 and the first LLVM version with full suppor is 16.0.0.
+# We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 LLVM_VERSIONS = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "first:>=17.0.0,<18",
 }
 
 llvm.toolchain(
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index 93ea6b38..aeea1dd9 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -48,10 +48,10 @@ load("@toolchains_llvm//toolchain:rules.bzl", "llvm_toolchain")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
+# We use C++17 and the first LLVM version with full suppor is 16.0.0.
+# We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 LLVM_VERSIONS = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "first:>=17.0.0,<18",
 }
 
 llvm_toolchain(
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index 2721b7ca..076d7988 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1160,10 +1160,11 @@ def _required_llvm_release_name(*, version_or_requirements, all_llvm_distributio
     return None, None, "ERROR: No matching distribution found."
 
 def required_llvm_release_name_rctx(rctx, llvm_version):
+    print("Determining required LLVM release for version/requirement: %s" % llvm_version)  # buildifier: disable=print
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version),
+        parsed_llvm_version = _parse_version(llvm_version) if not is_requirement(llvm_version) else None,
     )
     return _required_llvm_release_name(
         version_or_requirements = llvm_version,
@@ -1192,7 +1193,7 @@ def _distribution_urls(rctx):
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version),
+        parsed_llvm_version = _parse_version(llvm_version) if not is_requirement(llvm_version) else None,
     )
     _, sha256, strip_prefix, _ = _key_attrs(rctx)
 

From ddfcaecec649c0ccd06485e33355911616a6f078 Mon Sep 17 00:00:00 2001
From: Marcus Boerger <marcus.boerger@gmail.com>
Date: Sun, 26 Oct 2025 12:07:44 +0100
Subject: [PATCH 03/19] Further bump LLVM versions for tests.

---
 tests/MODULE.bazel                        | 3 ++-
 tests/WORKSPACE                           | 3 ++-
 toolchain/internal/llvm_distributions.bzl | 1 -
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index f7b6f75f..024bad57 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -65,8 +65,9 @@ llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
 # through the `llvm_toolchain_with_sysroot` toolchain.
 # We use C++17 and the first LLVM version with full suppor is 16.0.0.
 # We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
+# MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
 LLVM_VERSIONS = {
-    "": "first:>=17.0.0,<18",
+    "": "first:>=17.0.0,<20",
 }
 
 llvm.toolchain(
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index aeea1dd9..0f514ec9 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -50,8 +50,9 @@ load("@toolchains_llvm//toolchain:rules.bzl", "llvm_toolchain")
 # through the `llvm_toolchain_with_sysroot` toolchain.
 # We use C++17 and the first LLVM version with full suppor is 16.0.0.
 # We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
+# MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
 LLVM_VERSIONS = {
-    "": "first:>=17.0.0,<18",
+    "": "first:>=17.0.0,<20",
 }
 
 llvm_toolchain(
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index 076d7988..46e2bf8d 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1160,7 +1160,6 @@ def _required_llvm_release_name(*, version_or_requirements, all_llvm_distributio
     return None, None, "ERROR: No matching distribution found."
 
 def required_llvm_release_name_rctx(rctx, llvm_version):
-    print("Determining required LLVM release for version/requirement: %s" % llvm_version)  # buildifier: disable=print
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,

From 3d086912986bdb9dd17219c2d9315a7309166a1b Mon Sep 17 00:00:00 2001
From: Marcus Boerger <marcus.boerger@gmail.com>
Date: Sun, 26 Oct 2025 15:09:22 +0100
Subject: [PATCH 04/19] Make some tests explicitly use llvm 16, as they do not
 yet work with llvm 17+.

---
 tests/MODULE.bazel                 | 28 +++++++++++++++++++++++++---
 tests/WORKSPACE                    | 21 +++++++++++++++++++--
 tests/scripts/ubuntu_20_04_test.sh |  2 +-
 3 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 024bad57..d33a195b 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -63,13 +63,21 @@ llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
-# We use C++17 and the first LLVM version with full suppor is 16.0.0.
+# We use C++17 and the first LLVM version with full support is 16.0.0.
 # We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 # MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
 LLVM_VERSIONS = {
     "": "first:>=17.0.0,<20",
 }
 
+# Some tests do not yet work with LLVM versions > 16, so we define a separate
+# version set.
+LLVM_VERSIONS_16 = {
+    "": "16.0.0",
+    "darwin-aarch64": "16.0.5",
+    "darwin-x86_64": "15.0.7",
+}
+
 llvm.toolchain(
     name = "llvm_toolchain",
     cxx_standard = {"": "c++17"},
@@ -83,6 +91,20 @@ use_repo(llvm, "llvm_toolchain", "llvm_toolchain_llvm")
 
 register_toolchains("@llvm_toolchain//:all")
 
+llvm.toolchain(
+    name = "llvm_toolchain_16",
+    cxx_standard = {"": "c++17"},
+    llvm_versions = LLVM_VERSIONS_16,
+)
+llvm.extra_target_compatible_with(
+    name = "llvm_toolchain_16",
+    constraints = ["@//:cxx17"],
+)
+use_repo(llvm, "llvm_toolchain_16")
+
+register_toolchains("@llvm_toolchain_16//:all")
+
+
 llvm.toolchain(
     name = "llvm_toolchain_cxx20",
     cxx_standard = {"": "c++20"},
@@ -150,7 +172,7 @@ use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 # Toolchain example with system LLVM; tested in GitHub CI.
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = LLVM_VERSIONS_16,
 )
 
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
@@ -163,7 +185,7 @@ use_repo(llvm, "llvm_toolchain_with_system_llvm")
 # Toolchain example with a sysroot.
 llvm.toolchain(
     name = "llvm_toolchain_with_sysroot",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = LLVM_VERSIONS_16,
 )
 
 # We can share the downloaded LLVM distribution with the first configuration.
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index 0f514ec9..6bd2de0c 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -55,6 +55,14 @@ LLVM_VERSIONS = {
     "": "first:>=17.0.0,<20",
 }
 
+# Some tests do not yet work with LLVM versions > 16, so we define a separate
+# version set.
+LLVM_VERSIONS_16 = {
+    "": "16.0.0",
+    "darwin-aarch64": "16.0.5",
+    "darwin-x86_64": "15.0.7",
+}
+
 llvm_toolchain(
     name = "llvm_toolchain",
     cxx_standard = {"": "c++17"},
@@ -64,6 +72,15 @@ llvm_toolchain(
     llvm_versions = LLVM_VERSIONS,
 )
 
+llvm_toolchain(
+    name = "llvm_toolchain_16",
+    cxx_standard = {"": "c++17"},
+    extra_target_compatible_with = {
+        "": ["@//:cxx17"],
+    },
+    llvm_versions = LLVM_VERSIONS_16,
+)
+
 llvm_toolchain(
     name = "llvm_toolchain_cxx20",
     cxx_standard = {"": "c++20"},
@@ -126,7 +143,7 @@ llvm_toolchain(
 ## Toolchain example with system LLVM; tested in GitHub CI.
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = LLVM_VERSIONS_16,
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )
@@ -144,7 +161,7 @@ sysroot(
 
 llvm_toolchain(
     name = "llvm_toolchain_with_sysroot",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = LLVM_VERSIONS_16,
     sysroot = {
         "linux-x86_64": "@org_chromium_sysroot_linux_x64//sysroot",
     },
diff --git a/tests/scripts/ubuntu_20_04_test.sh b/tests/scripts/ubuntu_20_04_test.sh
index 38e6d20e..028b7fc6 100755
--- a/tests/scripts/ubuntu_20_04_test.sh
+++ b/tests/scripts/ubuntu_20_04_test.sh
@@ -40,6 +40,6 @@ disable_wasm_tests='-W'
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh \${disable_wasm_tests}
+tests/scripts/run_tests.sh -t'@llvm_toolchain_16//:cc-toolchain-x86_64-linux' \${disable_wasm_tests}
 """
 done

From 4e404751cb9e3a50ef8c40ab329cc44773d617e4 Mon Sep 17 00:00:00 2001
From: Marcus Boerger <marcus.boerger@gmail.com>
Date: Mon, 27 Oct 2025 12:08:06 +0100
Subject: [PATCH 05/19] Provide omp dependency via toolchain _name argument in
 run_tests.sh.

---
 tests/BUILD.bazel                  | 23 ++++++++++++++++++++---
 tests/scripts/run_tests.sh         | 20 +++++++++++++++++---
 tests/scripts/ubuntu_20_04_test.sh |  4 +++-
 3 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index 2589ac76..4160369c 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -81,7 +81,7 @@ build_test(
 dwp_file(
     name = "stdlib.dwp",
     src = ":stdlib_bin",
-    # NOTE: we should eventually we able to drop this; see #109.
+    # NOTE: we should eventually be able to drop this; see #109.
     override_compilation_mode = "dbg",
     target_compatible_with = [
         "@platforms//os:linux",
@@ -95,13 +95,27 @@ build_test(
     ],
 )
 
+# Define a constraint setting and value for OpenMP testing.
+# Use: bazel ... --define omp=toolchain_16
+# The value is provided automatically by the run_tests.sh script and defaults to
+# the standard llvm_toolchain'. The script removes the '@' and any target.
+constraint_setting(name = "omp")
+
+constraint_value(
+    name = "llvm_toolchain_16",
+    constraint_setting = "omp",
+)
+
 # Simple test in C that depends on libomp.
 cc_test(
     name = "omp_test",
     srcs = ["omp_test.c"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
-    deps = ["@llvm_toolchain//:omp"],
+    deps = select({
+        ":llvm_toolchain_16": ["@llvm_toolchain_16//:omp"],
+        "//conditions:default": ["@llvm_toolchain//:omp"],
+    }),
 )
 
 # C++ variant of omp_test; needed to check that including the llvm toolchain
@@ -114,7 +128,10 @@ cc_test(
     srcs = ["omp_test.cc"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
-    deps = ["@llvm_toolchain//:omp"],
+    deps = select({
+        ":llvm_toolchain_16": ["@llvm_toolchain_16//:omp"],
+        "//conditions:default": ["@llvm_toolchain//:omp"],
+    }),
 )
 
 sh_test(
diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index abca1b20..2eedf479 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -18,9 +18,11 @@ set -euo pipefail
 toolchain_name=""
 disable_wasm_tests=""
 
-while getopts "t:hW" opt; do
+while getopts "t:hOW" opt; do
   case "${opt}" in
-  "t") toolchain_name="${OPTARG}" ;;
+  "t")
+    toolchain_name="${OPTARG}"
+    ;;
   "h")
     echo "Usage:"
     echo "-t - Toolchain name to use for testing; default is llvm_toolchain"
@@ -56,10 +58,22 @@ targets=(
 # :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 ${toolchain_name} ]]; then
+if [[ -z "${toolchain_name}" ]]; then
   targets+=("//:test_cxx_standard_is_20")
 fi
 
+# If a toolchain is specified, set up OpenMP dependency accordingly.
+# We could just rewrite the BUILD files to depend on the correct omp target, but
+# it is preferable to leave the BUILD file alone and instead pass the correct
+# target via command line so that the BUILD files can be used in other contexts.
+# We do so using a combination of 'constraint_setting' and 'constraint_value'
+# whose possible values must be predefined in the BUILD file. Using the defined
+# value is only possible by creating a custom rule which is overly complex here.
+if [[ -n "${toolchain_name}" ]]; then
+  omp_target="${toolchain_name/\/\/*/}"
+  test_args+=("--define omp=${omp_target/#@/}")
+fi
+
 "${bazel}" ${TEST_MIGRATION:+"--strict"} --bazelrc=/dev/null test \
   "${common_test_args[@]}" "${test_args[@]}" "${targets[@]}"
 
diff --git a/tests/scripts/ubuntu_20_04_test.sh b/tests/scripts/ubuntu_20_04_test.sh
index 028b7fc6..9ecadbc4 100755
--- a/tests/scripts/ubuntu_20_04_test.sh
+++ b/tests/scripts/ubuntu_20_04_test.sh
@@ -19,6 +19,8 @@ images=(
   "ubuntu:20.04"
 )
 
+toolchain="@llvm_toolchain_16//:cc-toolchain-x86_64-linux"
+
 git_root=$(git rev-parse --show-toplevel)
 readonly git_root
 
@@ -40,6 +42,6 @@ disable_wasm_tests='-W'
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t'@llvm_toolchain_16//:cc-toolchain-x86_64-linux' \${disable_wasm_tests}
+tests/scripts/run_tests.sh -t ${toolchain} \${disable_wasm_tests}
 """
 done

From 02bb7ad9496dad9d4d6dc665d44a882ac4ad0a89 Mon Sep 17 00:00:00 2001
From: Marcus Boerger <marcus.boerger@gmail.com>
Date: Mon, 27 Oct 2025 12:12:25 +0100
Subject: [PATCH 06/19] Must use two args to provide the define as on the
 intended machine (unlike mine) we end up getting quotes.

---
 tests/scripts/run_tests.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index 2eedf479..dc0ac473 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -71,7 +71,7 @@ fi
 # value is only possible by creating a custom rule which is overly complex here.
 if [[ -n "${toolchain_name}" ]]; then
   omp_target="${toolchain_name/\/\/*/}"
-  test_args+=("--define omp=${omp_target/#@/}")
+  test_args+=("--define" "omp=${omp_target/#@/}")
 fi
 
 "${bazel}" ${TEST_MIGRATION:+"--strict"} --bazelrc=/dev/null test \

From 8bdb19049f91e81600e4fe8b0e9e3ad1a53ec0ff Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 11:07:57 +0100
Subject: [PATCH 07/19] Ubuntu 20.04 does not have newer glibc as required by
 newer LLVM 16+. So we force version 15 for that. Possibly have to do so for
 some other tests as well.

---
 tests/BUILD.bazel                  |  9 ++++++---
 tests/MODULE.bazel                 | 28 +++++++++++++++-------------
 tests/WORKSPACE                    | 10 +++++-----
 tests/scripts/run_tests.sh         |  6 ++++++
 tests/scripts/ubuntu_20_04_test.sh |  2 +-
 5 files changed, 33 insertions(+), 22 deletions(-)

diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index 4160369c..6d8e7032 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -102,8 +102,9 @@ build_test(
 constraint_setting(name = "omp")
 
 constraint_value(
-    name = "llvm_toolchain_16",
+    name = "llvm_toolchain_15",
     constraint_setting = "omp",
+    visibility = ["//visibility:public"],
 )
 
 # Simple test in C that depends on libomp.
@@ -113,7 +114,8 @@ cc_test(
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
     deps = select({
-        ":llvm_toolchain_16": ["@llvm_toolchain_16//:omp"],
+        ":llvm_toolchain_15": ["@llvm_toolchain_15//:omp"],
+        #":llvm_toolchain_with_sysroot": ["@llvm_toolchain_with_sysroot//:omp"],
         "//conditions:default": ["@llvm_toolchain//:omp"],
     }),
 )
@@ -129,7 +131,8 @@ cc_test(
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
     deps = select({
-        ":llvm_toolchain_16": ["@llvm_toolchain_16//:omp"],
+        ":llvm_toolchain_15": ["@llvm_toolchain_15//:omp"],
+        #":llvm_toolchain_with_sysroot": ["@llvm_toolchain_with_sysroot//:omp"],
         "//conditions:default": ["@llvm_toolchain//:omp"],
     }),
 )
diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index d33a195b..3936e2b7 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -22,7 +22,7 @@ local_path_override(
 
 bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "platforms", version = "1.0.0")
-bazel_dep(name = "rules_cc", version = "0.0.9")
+bazel_dep(name = "rules_cc", version = "0.2.8")
 bazel_dep(name = "rules_go", version = "0.50.1", repo_name = "io_bazel_rules_go")
 bazel_dep(name = "rules_rust", version = "0.67.0")
 bazel_dep(name = "rules_foreign_cc", version = "0.15.0")
@@ -72,10 +72,9 @@ LLVM_VERSIONS = {
 
 # Some tests do not yet work with LLVM versions > 16, so we define a separate
 # version set.
-LLVM_VERSIONS_16 = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+LLVM_VERSIONS_15 = {
+    "": "latest:>15.0.0,<16",
+    "darwin-x86_64": "15.0.7",  # Verify this works as opposed to using one version.
 }
 
 llvm.toolchain(
@@ -92,17 +91,20 @@ use_repo(llvm, "llvm_toolchain", "llvm_toolchain_llvm")
 register_toolchains("@llvm_toolchain//:all")
 
 llvm.toolchain(
-    name = "llvm_toolchain_16",
+    name = "llvm_toolchain_15",
     cxx_standard = {"": "c++17"},
-    llvm_versions = LLVM_VERSIONS_16,
+    llvm_versions = LLVM_VERSIONS_15,
 )
 llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_16",
-    constraints = ["@//:cxx17"],
+    name = "llvm_toolchain_15",
+    constraints = [
+        "@//:cxx17",
+        "//:llvm_toolchain_15",
+    ],
 )
-use_repo(llvm, "llvm_toolchain_16")
+use_repo(llvm, "llvm_toolchain_15")
 
-register_toolchains("@llvm_toolchain_16//:all")
+register_toolchains("@llvm_toolchain_15//:all")
 
 
 llvm.toolchain(
@@ -172,7 +174,7 @@ use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 # Toolchain example with system LLVM; tested in GitHub CI.
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS_16,
+    llvm_versions = LLVM_VERSIONS_15,
 )
 
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
@@ -185,7 +187,7 @@ use_repo(llvm, "llvm_toolchain_with_system_llvm")
 # Toolchain example with a sysroot.
 llvm.toolchain(
     name = "llvm_toolchain_with_sysroot",
-    llvm_versions = LLVM_VERSIONS_16,
+    llvm_versions = LLVM_VERSIONS_15,
 )
 
 # We can share the downloaded LLVM distribution with the first configuration.
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index 6bd2de0c..aee2ad14 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -57,7 +57,7 @@ LLVM_VERSIONS = {
 
 # Some tests do not yet work with LLVM versions > 16, so we define a separate
 # version set.
-LLVM_VERSIONS_16 = {
+LLVM_VERSIONS_15 = {
     "": "16.0.0",
     "darwin-aarch64": "16.0.5",
     "darwin-x86_64": "15.0.7",
@@ -73,12 +73,12 @@ llvm_toolchain(
 )
 
 llvm_toolchain(
-    name = "llvm_toolchain_16",
+    name = "llvm_toolchain_15",
     cxx_standard = {"": "c++17"},
     extra_target_compatible_with = {
         "": ["@//:cxx17"],
     },
-    llvm_versions = LLVM_VERSIONS_16,
+    llvm_versions = LLVM_VERSIONS_15,
 )
 
 llvm_toolchain(
@@ -143,7 +143,7 @@ llvm_toolchain(
 ## Toolchain example with system LLVM; tested in GitHub CI.
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS_16,
+    llvm_versions = LLVM_VERSIONS_15,
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )
@@ -161,7 +161,7 @@ sysroot(
 
 llvm_toolchain(
     name = "llvm_toolchain_with_sysroot",
-    llvm_versions = LLVM_VERSIONS_16,
+    llvm_versions = LLVM_VERSIONS_15,
     sysroot = {
         "linux-x86_64": "@org_chromium_sysroot_linux_x64//sysroot",
     },
diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index dc0ac473..2a3021ad 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -72,11 +72,17 @@ fi
 if [[ -n "${toolchain_name}" ]]; then
   omp_target="${toolchain_name/\/\/*/}"
   test_args+=("--define" "omp=${omp_target/#@/}")
+  common_test_args+=(
+    "--platforms=@toolchains_llvm//platforms:linux-x86_64"
+  #  "--extra_execution_platforms=@toolchains_llvm//platforms:linux-x86_64"
+  )
 fi
 
 "${bazel}" ${TEST_MIGRATION:+"--strict"} --bazelrc=/dev/null test \
   "${common_test_args[@]}" "${test_args[@]}" "${targets[@]}"
 
+exit
+
 # 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
 # --incompatible_no_rule_outputs_param # from rules_rust
diff --git a/tests/scripts/ubuntu_20_04_test.sh b/tests/scripts/ubuntu_20_04_test.sh
index 9ecadbc4..1e1e380c 100755
--- a/tests/scripts/ubuntu_20_04_test.sh
+++ b/tests/scripts/ubuntu_20_04_test.sh
@@ -19,7 +19,7 @@ images=(
   "ubuntu:20.04"
 )
 
-toolchain="@llvm_toolchain_16//:cc-toolchain-x86_64-linux"
+toolchain="@llvm_toolchain_15//:cc-toolchain-x86_64-linux"
 
 git_root=$(git rev-parse --show-toplevel)
 readonly git_root

From 01c66d168f22ae6fb65040ab0e364107150c5a0f Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 11:14:01 +0100
Subject: [PATCH 08/19] Must be one newer...

---
 tests/MODULE.bazel | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 3936e2b7..2cf2b5fb 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -73,7 +73,7 @@ LLVM_VERSIONS = {
 # Some tests do not yet work with LLVM versions > 16, so we define a separate
 # version set.
 LLVM_VERSIONS_15 = {
-    "": "latest:>15.0.0,<16",
+    "": "latest:>15.0.0,<17",
     "darwin-x86_64": "15.0.7",  # Verify this works as opposed to using one version.
 }
 

From e7b145bf3eca53a67ab1a7bf89d7b5bc9de8e7b3 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 11:18:24 +0100
Subject: [PATCH 09/19] Missing version piece

---
 toolchain/internal/repo.bzl | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/toolchain/internal/repo.bzl b/toolchain/internal/repo.bzl
index 934b5373..3d226e77 100644
--- a/toolchain/internal/repo.bzl
+++ b/toolchain/internal/repo.bzl
@@ -21,6 +21,8 @@ load(
 load(
     "//toolchain/internal:llvm_distributions.bzl",
     _download_llvm = "download_llvm",
+    _is_requirement = "is_requirement",
+    _required_llvm_release_name_rctx = "required_llvm_release_name_rctx",
 )
 
 _target_pairs = ", ".join(_supported_os_arch_keys())
@@ -425,6 +427,16 @@ def llvm_repo_impl(rctx):
 
     _, llvm_version = _exec_os_arch_dict_value(rctx, "llvm_versions")
 
+    if _is_requirement(llvm_version):
+        llvm_version, distribution, error = _required_llvm_release_name_rctx(rctx, llvm_version)
+        if error:
+            fail(error)
+        if llvm_version:
+            print("\nINFO: Resolved latest LLVM version to {llvm_version}: {distribution}".format(
+                distribution = distribution,
+                llvm_version = llvm_version,
+            ))  # buildifier: disable=print
+
     major_llvm_version = int(llvm_version.split(".")[0])
 
     rctx.file(

From e382ab4a4a01cba3f08a367c97b659af523d84c0 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 16:27:20 +0100
Subject: [PATCH 10/19] Allow to override the LLVM_VERSION via an environment
 variable for testing.

---
 MODULE.bazel                              |  8 +--
 tests/BUILD.bazel                         | 24 +-------
 tests/MODULE.bazel                        | 32 ++---------
 tests/WORKSPACE                           | 25 ++-------
 tests/scripts/linux_sysroot_test.sh       |  4 +-
 tests/scripts/run_tests.sh                | 27 +++++----
 tests/scripts/ubuntu_20_04_test.sh        |  4 +-
 toolchain/internal/configure.bzl          | 16 ++----
 toolchain/internal/llvm_distributions.bzl | 68 ++++++++++++++++++-----
 toolchain/internal/repo.bzl               | 17 +-----
 10 files changed, 99 insertions(+), 126 deletions(-)

diff --git a/MODULE.bazel b/MODULE.bazel
index fa99a1da..0dc95afb 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -20,10 +20,10 @@ module(
 )
 
 bazel_dep(name = "bazel_features", version = "1.38.0")
-bazel_dep(name = "bazel_skylib", version = "1.5.0")
-bazel_dep(name = "aspect_bazel_lib", version = "2.0.0")
-bazel_dep(name = "rules_cc", version = "0.2.2")
-bazel_dep(name = "platforms", version = "0.0.8")
+bazel_dep(name = "bazel_skylib", version = "1.8.2")
+bazel_dep(name = "aspect_bazel_lib", version = "2.19.3")
+bazel_dep(name = "rules_cc", version = "0.2.13")
+bazel_dep(name = "platforms", version = "1.0.0")
 bazel_dep(name = "helly25_bzl", version = "0.3.1")
 
 bazel_dep(name = "tar.bzl", version = "0.6.0")
diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index 6d8e7032..a7b02659 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -95,29 +95,13 @@ build_test(
     ],
 )
 
-# Define a constraint setting and value for OpenMP testing.
-# Use: bazel ... --define omp=toolchain_16
-# The value is provided automatically by the run_tests.sh script and defaults to
-# the standard llvm_toolchain'. The script removes the '@' and any target.
-constraint_setting(name = "omp")
-
-constraint_value(
-    name = "llvm_toolchain_15",
-    constraint_setting = "omp",
-    visibility = ["//visibility:public"],
-)
-
 # Simple test in C that depends on libomp.
 cc_test(
     name = "omp_test",
     srcs = ["omp_test.c"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
-    deps = select({
-        ":llvm_toolchain_15": ["@llvm_toolchain_15//:omp"],
-        #":llvm_toolchain_with_sysroot": ["@llvm_toolchain_with_sysroot//:omp"],
-        "//conditions:default": ["@llvm_toolchain//:omp"],
-    }),
+    deps = ["@llvm_toolchain//:omp"],
 )
 
 # C++ variant of omp_test; needed to check that including the llvm toolchain
@@ -130,11 +114,7 @@ cc_test(
     srcs = ["omp_test.cc"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
-    deps = select({
-        ":llvm_toolchain_15": ["@llvm_toolchain_15//:omp"],
-        #":llvm_toolchain_with_sysroot": ["@llvm_toolchain_with_sysroot//:omp"],
-        "//conditions:default": ["@llvm_toolchain//:omp"],
-    }),
+    deps = ["@llvm_toolchain//:omp"],
 )
 
 sh_test(
diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 2cf2b5fb..69b20807 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -22,7 +22,7 @@ local_path_override(
 
 bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "platforms", version = "1.0.0")
-bazel_dep(name = "rules_cc", version = "0.2.8")
+bazel_dep(name = "rules_cc", version = "0.2.13")
 bazel_dep(name = "rules_go", version = "0.50.1", repo_name = "io_bazel_rules_go")
 bazel_dep(name = "rules_rust", version = "0.67.0")
 bazel_dep(name = "rules_foreign_cc", version = "0.15.0")
@@ -66,14 +66,9 @@ llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
 # We use C++17 and the first LLVM version with full support is 16.0.0.
 # We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 # MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
+# We also allow to override this with a environment LLVM_VERSION for testing.
 LLVM_VERSIONS = {
-    "": "first:>=17.0.0,<20",
-}
-
-# Some tests do not yet work with LLVM versions > 16, so we define a separate
-# version set.
-LLVM_VERSIONS_15 = {
-    "": "latest:>15.0.0,<17",
+    "": "getenv(LLVM_VERSION,latest:>=17.0.0,<20)",
     "darwin-x86_64": "15.0.7",  # Verify this works as opposed to using one version.
 }
 
@@ -90,23 +85,6 @@ use_repo(llvm, "llvm_toolchain", "llvm_toolchain_llvm")
 
 register_toolchains("@llvm_toolchain//:all")
 
-llvm.toolchain(
-    name = "llvm_toolchain_15",
-    cxx_standard = {"": "c++17"},
-    llvm_versions = LLVM_VERSIONS_15,
-)
-llvm.extra_target_compatible_with(
-    name = "llvm_toolchain_15",
-    constraints = [
-        "@//:cxx17",
-        "//:llvm_toolchain_15",
-    ],
-)
-use_repo(llvm, "llvm_toolchain_15")
-
-register_toolchains("@llvm_toolchain_15//:all")
-
-
 llvm.toolchain(
     name = "llvm_toolchain_cxx20",
     cxx_standard = {"": "c++20"},
@@ -174,7 +152,7 @@ use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 # Toolchain example with system LLVM; tested in GitHub CI.
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS_15,
+    llvm_versions = LLVM_VERSIONS,
 )
 
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
@@ -187,7 +165,7 @@ use_repo(llvm, "llvm_toolchain_with_system_llvm")
 # Toolchain example with a sysroot.
 llvm.toolchain(
     name = "llvm_toolchain_with_sysroot",
-    llvm_versions = LLVM_VERSIONS_15,
+    llvm_versions = LLVM_VERSIONS,
 )
 
 # We can share the downloaded LLVM distribution with the first configuration.
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index b0414b2f..cdaa0bee 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -62,16 +62,10 @@ load("@toolchains_llvm//toolchain:rules.bzl", "llvm_toolchain")
 # We use C++17 and the first LLVM version with full suppor is 16.0.0.
 # We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
 # MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
+# We also allow to override this with a environment LLVM_VERSION for testing.
 LLVM_VERSIONS = {
-    "": "first:>=17.0.0,<20",
-}
-
-# Some tests do not yet work with LLVM versions > 16, so we define a separate
-# version set.
-LLVM_VERSIONS_15 = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "getenv(LLVM_VERSION,latest:>=17.0.0,<20)",
+    "darwin-x86_64": "15.0.7",  # Verify this works as opposed to using one version.
 }
 
 llvm_toolchain(
@@ -83,15 +77,6 @@ llvm_toolchain(
     llvm_versions = LLVM_VERSIONS,
 )
 
-llvm_toolchain(
-    name = "llvm_toolchain_15",
-    cxx_standard = {"": "c++17"},
-    extra_target_compatible_with = {
-        "": ["@//:cxx17"],
-    },
-    llvm_versions = LLVM_VERSIONS_15,
-)
-
 llvm_toolchain(
     name = "llvm_toolchain_cxx20",
     cxx_standard = {"": "c++20"},
@@ -154,7 +139,7 @@ llvm_toolchain(
 ## Toolchain example with system LLVM; tested in GitHub CI.
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS_15,
+    llvm_versions = LLVM_VERSIONS,
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )
@@ -172,7 +157,7 @@ sysroot(
 
 llvm_toolchain(
     name = "llvm_toolchain_with_sysroot",
-    llvm_versions = LLVM_VERSIONS_15,
+    llvm_versions = LLVM_VERSIONS,
     sysroot = {
         "linux-x86_64": "@org_chromium_sysroot_linux_x64//sysroot",
     },
diff --git a/tests/scripts/linux_sysroot_test.sh b/tests/scripts/linux_sysroot_test.sh
index a2935c24..3cbd0d6b 100755
--- a/tests/scripts/linux_sysroot_test.sh
+++ b/tests/scripts/linux_sysroot_test.sh
@@ -19,6 +19,8 @@ images=(
   "ubuntu:22.04"
 )
 
+LLVM_VERSION="first:>=15.0.0,<17"
+
 git_root=$(git rev-parse --show-toplevel)
 readonly git_root
 
@@ -36,6 +38,6 @@ apt-get -qq -y install curl libtinfo5 libxml2 zlib1g-dev >/dev/null
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t '@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux'
+tests/scripts/run_tests.sh -t '@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux' -v '${LLVM_VERSION}'
 """
 done
diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index 2a3021ad..1fdfd9d2 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -17,17 +17,21 @@ set -euo pipefail
 
 toolchain_name=""
 disable_wasm_tests=""
+LLVM_VERSION=""
 
-while getopts "t:hOW" opt; do
+while getopts "ht:v:W" opt; do
   case "${opt}" in
-  "t")
-    toolchain_name="${OPTARG}"
-    ;;
   "h")
     echo "Usage:"
     echo "-t - Toolchain name to use for testing; default is llvm_toolchain"
     exit 2
     ;;
+  "t")
+    toolchain_name="${OPTARG}"
+    ;;
+  "v")
+    LLVM_VERSION="${OPTARG}"
+    ;;
   "W")
     disable_wasm_tests="yes"
     ;;
@@ -69,20 +73,21 @@ fi
 # We do so using a combination of 'constraint_setting' and 'constraint_value'
 # whose possible values must be predefined in the BUILD file. Using the defined
 # value is only possible by creating a custom rule which is overly complex here.
-if [[ -n "${toolchain_name}" ]]; then
-  omp_target="${toolchain_name/\/\/*/}"
-  test_args+=("--define" "omp=${omp_target/#@/}")
+#if [[ -n "${toolchain_name}" ]]; then
+#  omp_target="${toolchain_name/\/\/*/}"
+#  common_test_args+=("--define" "omp=${omp_target/#@/}")
+#fi
+
+if [[ -n "${LLVM_VERSION}" ]]; then
+  echo "LLVM_VERSION=${LLVM_VERSION}"
   common_test_args+=(
-    "--platforms=@toolchains_llvm//platforms:linux-x86_64"
-  #  "--extra_execution_platforms=@toolchains_llvm//platforms:linux-x86_64"
+    "--repo_env=LLVM_VERSION=${LLVM_VERSION}"
   )
 fi
 
 "${bazel}" ${TEST_MIGRATION:+"--strict"} --bazelrc=/dev/null test \
   "${common_test_args[@]}" "${test_args[@]}" "${targets[@]}"
 
-exit
-
 # 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
 # --incompatible_no_rule_outputs_param # from rules_rust
diff --git a/tests/scripts/ubuntu_20_04_test.sh b/tests/scripts/ubuntu_20_04_test.sh
index 1e1e380c..a8510a14 100755
--- a/tests/scripts/ubuntu_20_04_test.sh
+++ b/tests/scripts/ubuntu_20_04_test.sh
@@ -19,7 +19,7 @@ images=(
   "ubuntu:20.04"
 )
 
-toolchain="@llvm_toolchain_15//:cc-toolchain-x86_64-linux"
+LLVM_VERSION="first:>=15.0.0,<17"
 
 git_root=$(git rev-parse --show-toplevel)
 readonly git_root
@@ -42,6 +42,6 @@ disable_wasm_tests='-W'
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain} \${disable_wasm_tests}
+tests/scripts/run_tests.sh -v '${LLVM_VERSION}' \${disable_wasm_tests}
 """
 done
diff --git a/toolchain/internal/configure.bzl b/toolchain/internal/configure.bzl
index 279e1094..45d20556 100644
--- a/toolchain/internal/configure.bzl
+++ b/toolchain/internal/configure.bzl
@@ -38,7 +38,10 @@ load(
     _supported_targets = "SUPPORTED_TARGETS",
     _toolchain_tools = "toolchain_tools",
 )
-load("//toolchain/internal:llvm_distributions.bzl", "is_requirement", "required_llvm_release_name_rctx")
+load(
+    "//toolchain/internal:llvm_distributions.bzl",
+    _required_llvm_version_rctx = "required_llvm_version_rctx",
+)
 load(
     "//toolchain/internal:sysroot.bzl",
     _default_sysroot_path = "default_sysroot_path",
@@ -81,16 +84,7 @@ def llvm_config_impl(rctx):
 
     if not toolchain_root:
         fail("LLVM toolchain root missing for ({}, {})".format(os, arch))
-    _, llvm_version = _exec_os_arch_dict_value(rctx, "llvm_versions")
-    if is_requirement(llvm_version):
-        llvm_version, distribution, error = required_llvm_release_name_rctx(rctx, llvm_version)
-        if error:
-            fail(error)
-        if llvm_version:
-            print("\nINFO: Resolved latest LLVM version to {llvm_version}: {distribution}".format(
-                distribution = distribution,
-                llvm_version = llvm_version,
-            ))  # buildifier: disable=print
+    llvm_version = _required_llvm_version_rctx(rctx)
     if not llvm_version:
         # LLVM version missing for (os, arch)
         _empty_repository(rctx)
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index 076d7988..141bb5c8 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1125,12 +1125,21 @@ def _find_llvm_basename_or_error(llvm_version, all_llvm_distributions, host_info
 
     return basenames[0], None
 
+def _is_requirement(version_or_requirements):
+    """Return whether `version_or_requirements` is likely a requirement (True) or should be a version."""
+    if version_or_requirements.startswith("getenv("):
+        return True
+    for prefix in ["first:", "latest:"]:
+        if version_or_requirements.startswith(prefix) or version_or_requirements == prefix[:-1]:
+            return True
+    return False
+
 def _parse_version_or_requirements(version_or_requirements):
-    if version_or_requirements in ["latest", "first"]:
-        return None
     for prefix in ["latest:", "first:"]:
         if version_or_requirements.startswith(prefix):
             return versions.parse_requirements(version_or_requirements.removeprefix(prefix))
+    if _is_requirement(version_or_requirements):
+        return None
     fail("ERROR: Invalid version requirements: '{version_or_requirements}'.".format(
         version_or_requirements = version_or_requirements,
     ))
@@ -1159,12 +1168,37 @@ def _required_llvm_release_name(*, version_or_requirements, all_llvm_distributio
             return llvm_version, basenames[0], None
     return None, None, "ERROR: No matching distribution found."
 
-def required_llvm_release_name_rctx(rctx, llvm_version):
-    print("Determining required LLVM release for version/requirement: %s" % llvm_version)  # buildifier: disable=print
+def _resolve_llvm_version_rctx(rctx, llvm_version):
+    if llvm_version.startswith("getenv(") and llvm_version.endswith(")"):
+        env_var = llvm_version[len("getenv("):-1]
+        if env_var.find(",") >= 0:
+            env_name, env_default = env_var.split(",", 1)
+        else:
+            env_name = env_var
+            env_default = None
+
+        # We prefer 'repository_ctx.getenv' if it is available (~7.1+) and default
+        # to accessing the environment directly. The latter breaks "hermeticity".
+        if hasattr(rctx, "getenv"):
+            llvm_version = rctx.getenv(env_name, env_default)
+        elif env_name in rctx.os.environ:
+            llvm_version = rctx.os.environ[env_name]
+        else:
+            llvm_version = env_default
+
+        #print("\nINFO: Read environment variable '{env_var}' = '{llvm_version}'".format(
+        #    env_var = env_var,
+        #    llvm_version = llvm_version,
+        #))  # buildifier: disable=print
+
+    return llvm_version
+
+def _required_llvm_release_name_rctx(rctx, llvm_version):
+    llvm_version = _resolve_llvm_version_rctx(rctx, llvm_version)
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version) if not is_requirement(llvm_version) else None,
+        parsed_llvm_version = _parse_version(llvm_version) if not _is_requirement(llvm_version) else None,
     )
     return _required_llvm_release_name(
         version_or_requirements = llvm_version,
@@ -1172,12 +1206,19 @@ def required_llvm_release_name_rctx(rctx, llvm_version):
         host_info = host_info(rctx),
     )
 
-def is_requirement(version_or_requirement):
-    """Return whether `version_or_requirement` is likely a requirement (True) or should be a version."""
-    for prefix in ["first:", "latest:"]:
-        if version_or_requirement.startswith(prefix) or version_or_requirement == prefix[:-1]:
-            return True
-    return False
+def required_llvm_version_rctx(rctx):
+    _, llvm_version = exec_os_arch_dict_value(rctx, "llvm_versions")
+    if _is_requirement(llvm_version):
+        llvm_version, distribution, error = _required_llvm_release_name_rctx(rctx, llvm_version)
+        if error:
+            fail(error)
+        if llvm_version:
+            print("\nINFO: Resolved latest LLVM version for {name} to {llvm_version}: {distribution}".format(
+                name = rctx.attr.name,
+                distribution = distribution,
+                llvm_version = llvm_version,
+            ))  # buildifier: disable=print
+    return llvm_version
 
 def _filter_llvm_distributions(*, llvm_version, all_llvm_distributions):
     """Return (distribution: sha) entries from `all_llvm_distributions` that match `llvm_version`."""
@@ -1193,13 +1234,14 @@ def _distribution_urls(rctx):
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version) if not is_requirement(llvm_version) else None,
+        parsed_llvm_version = _parse_version(llvm_version) if not _is_requirement(llvm_version) else None,
     )
     _, sha256, strip_prefix, _ = _key_attrs(rctx)
 
     if rctx.attr.distribution == "auto":
         rctx_host_info = host_info(rctx)
-        if is_requirement(llvm_version):
+        if _is_requirement(llvm_version):
+            llvm_version = _resolve_llvm_version_rctx(rctx, llvm_version)
             llvm_version, basename, error = _required_llvm_release_name(
                 version_or_requirements = llvm_version,
                 all_llvm_distributions = all_llvm_distributions,
diff --git a/toolchain/internal/repo.bzl b/toolchain/internal/repo.bzl
index 3d226e77..99e2ee3d 100644
--- a/toolchain/internal/repo.bzl
+++ b/toolchain/internal/repo.bzl
@@ -14,15 +14,13 @@
 load(
     "//toolchain/internal:common.bzl",
     _attr_dict = "attr_dict",
-    _exec_os_arch_dict_value = "exec_os_arch_dict_value",
     _os = "os",
     _supported_os_arch_keys = "supported_os_arch_keys",
 )
 load(
     "//toolchain/internal:llvm_distributions.bzl",
     _download_llvm = "download_llvm",
-    _is_requirement = "is_requirement",
-    _required_llvm_release_name_rctx = "required_llvm_release_name_rctx",
+    _required_llvm_version_rctx = "required_llvm_version_rctx",
 )
 
 _target_pairs = ", ".join(_supported_os_arch_keys())
@@ -425,18 +423,7 @@ def llvm_repo_impl(rctx):
         rctx.file("BUILD.bazel", executable = False)
         return None
 
-    _, llvm_version = _exec_os_arch_dict_value(rctx, "llvm_versions")
-
-    if _is_requirement(llvm_version):
-        llvm_version, distribution, error = _required_llvm_release_name_rctx(rctx, llvm_version)
-        if error:
-            fail(error)
-        if llvm_version:
-            print("\nINFO: Resolved latest LLVM version to {llvm_version}: {distribution}".format(
-                distribution = distribution,
-                llvm_version = llvm_version,
-            ))  # buildifier: disable=print
-
+    llvm_version = _required_llvm_version_rctx(rctx)
     major_llvm_version = int(llvm_version.split(".")[0])
 
     rctx.file(

From b4f88242b527fc3b3ccceacf2bac517d2c52fb5c Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 16:37:38 +0100
Subject: [PATCH 11/19] Make omp targets manual and cc back to 2.8

---
 tests/BUILD.bazel  | 2 ++
 tests/MODULE.bazel | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index a7b02659..b5c9fc23 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -101,6 +101,7 @@ cc_test(
     srcs = ["omp_test.c"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
+    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
@@ -114,6 +115,7 @@ cc_test(
     srcs = ["omp_test.cc"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
+    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 69b20807..5dff3113 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -22,7 +22,7 @@ local_path_override(
 
 bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "platforms", version = "1.0.0")
-bazel_dep(name = "rules_cc", version = "0.2.13")
+bazel_dep(name = "rules_cc", version = "0.2.8")
 bazel_dep(name = "rules_go", version = "0.50.1", repo_name = "io_bazel_rules_go")
 bazel_dep(name = "rules_rust", version = "0.67.0")
 bazel_dep(name = "rules_foreign_cc", version = "0.15.0")

From 6c54cdb742682031fbc6b6d4fea4300ff7509101 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 18:44:38 +0100
Subject: [PATCH 12/19] Downgrade.

---
 MODULE.bazel | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/MODULE.bazel b/MODULE.bazel
index 0dc95afb..bc2fafbe 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -22,7 +22,7 @@ module(
 bazel_dep(name = "bazel_features", version = "1.38.0")
 bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "aspect_bazel_lib", version = "2.19.3")
-bazel_dep(name = "rules_cc", version = "0.2.13")
+bazel_dep(name = "rules_cc", version = "0.2.8")
 bazel_dep(name = "platforms", version = "1.0.0")
 bazel_dep(name = "helly25_bzl", version = "0.3.1")
 

From 8e45e0a07743d01395d38af1ac0da3c480ef0287 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 19:09:13 +0100
Subject: [PATCH 13/19] Sync sys version back to 16.0.0

---
 .github/workflows/tests.yml | 1 +
 tests/MODULE.bazel          | 3 ++-
 tests/WORKSPACE             | 3 ++-
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index bc49d9c2..dfa643ca 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -134,6 +134,7 @@ jobs:
       - uses: actions/checkout@v5
       - run: tests/scripts/ubuntu_install_libtinfo.sh
       - name: Download and Extract LLVM distribution
+        # The downloaded version here must match the llvm_version for llvm_toolchain_with_system_llvm in tests/{MODULE.bazel,WORKSPACE}
         env:
           release: llvmorg-16.0.0
           archive: clang+llvm-16.0.0-x86_64-linux-gnu-ubuntu-18.04
diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 5dff3113..fcac3a78 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -150,9 +150,10 @@ llvm.toolchain_root(
 use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 
 # Toolchain example with system LLVM; tested in GitHub CI.
+# The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = "16.0.0", 
 )
 
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index cdaa0bee..d4be18ed 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -137,9 +137,10 @@ llvm_toolchain(
 )
 
 ## Toolchain example with system LLVM; tested in GitHub CI.
+# The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = "16.0.0",
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )

From 093df5fa4a54800e970b7ba2112e217593f72b1f Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Fri, 31 Oct 2025 19:15:09 +0100
Subject: [PATCH 14/19] Oops, versions require a dict. This was not a version
 attribute.

---
 tests/MODULE.bazel | 2 +-
 tests/WORKSPACE    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index fcac3a78..ab7af41f 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -153,7 +153,7 @@ use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 # The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = "16.0.0", 
+    llvm_versions = {"": "16.0.0"},
 )
 
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index d4be18ed..d770103f 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -140,7 +140,7 @@ llvm_toolchain(
 # The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = "16.0.0",
+    llvm_versions = {"": "16.0.0"},
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )

From 54ed33bcc6f1164cc3d4e8a1547867aeb7cef7f5 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Sat, 1 Nov 2025 11:48:13 +0100
Subject: [PATCH 15/19] Address comments. Check whether all targets support omp
 again. If that fails we have to add some flags and other tag managment and
 determine why that is necessary.

---
 .github/workflows/tests.yml               |  4 ++--
 tests/BUILD.bazel                         |  2 --
 tests/WORKSPACE                           |  2 +-
 tests/scripts/run_tests.sh                | 12 ------------
 toolchain/internal/llvm_distributions.bzl | 14 ++++++++++----
 5 files changed, 13 insertions(+), 21 deletions(-)

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index dfa643ca..a96ed91a 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -134,7 +134,7 @@ jobs:
       - uses: actions/checkout@v5
       - run: tests/scripts/ubuntu_install_libtinfo.sh
       - name: Download and Extract LLVM distribution
-        # The downloaded version here must match the llvm_version for llvm_toolchain_with_system_llvm in tests/{MODULE.bazel,WORKSPACE}
+        # The downloaded version here must match version specified as '-v' arg to run_tests.sh below.
         env:
           release: llvmorg-16.0.0
           archive: clang+llvm-16.0.0-x86_64-linux-gnu-ubuntu-18.04
@@ -144,4 +144,4 @@ jobs:
       - name: Test
         env:
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_system_llvm//:cc-toolchain-x86_64-linux
+        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_system_llvm//:cc-toolchain-x86_64-linux -v 16.0.0
diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index b5c9fc23..a7b02659 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -101,7 +101,6 @@ cc_test(
     srcs = ["omp_test.c"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
-    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
@@ -115,7 +114,6 @@ cc_test(
     srcs = ["omp_test.cc"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
-    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index d770103f..dd2d2bdb 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -140,7 +140,7 @@ llvm_toolchain(
 # The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = {"": "16.0.0"},
+    llvm_version = "getenv(LLVM_VERSION)",
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )
diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index 1fdfd9d2..3f51dedd 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -66,18 +66,6 @@ if [[ -z "${toolchain_name}" ]]; then
   targets+=("//:test_cxx_standard_is_20")
 fi
 
-# If a toolchain is specified, set up OpenMP dependency accordingly.
-# We could just rewrite the BUILD files to depend on the correct omp target, but
-# it is preferable to leave the BUILD file alone and instead pass the correct
-# target via command line so that the BUILD files can be used in other contexts.
-# We do so using a combination of 'constraint_setting' and 'constraint_value'
-# whose possible values must be predefined in the BUILD file. Using the defined
-# value is only possible by creating a custom rule which is overly complex here.
-#if [[ -n "${toolchain_name}" ]]; then
-#  omp_target="${toolchain_name/\/\/*/}"
-#  common_test_args+=("--define" "omp=${omp_target/#@/}")
-#fi
-
 if [[ -n "${LLVM_VERSION}" ]]; then
   echo "LLVM_VERSION=${LLVM_VERSION}"
   common_test_args+=(
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index 141bb5c8..d9eb5421 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1177,6 +1177,12 @@ def _resolve_llvm_version_rctx(rctx, llvm_version):
             env_name = env_var
             env_default = None
 
+        env_name = env_name.strip(" ")
+        if not env_var:
+            fail("ERROR: Bad getenv parameter '{env_var}'.".format(
+                env_var = env_var,
+            ))
+
         # We prefer 'repository_ctx.getenv' if it is available (~7.1+) and default
         # to accessing the environment directly. The latter breaks "hermeticity".
         if hasattr(rctx, "getenv"):
@@ -1186,10 +1192,10 @@ def _resolve_llvm_version_rctx(rctx, llvm_version):
         else:
             llvm_version = env_default
 
-        #print("\nINFO: Read environment variable '{env_var}' = '{llvm_version}'".format(
-        #    env_var = env_var,
-        #    llvm_version = llvm_version,
-        #))  # buildifier: disable=print
+        if not llvm_version:
+            fail("ERROR: Empty getenv lookup for '{env_var}'.".format(
+                env_var = env_var,
+            ))
 
     return llvm_version
 

From 3cfd97fc13580bb6874f7afba91d91320c83b68b Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Sat, 1 Nov 2025 12:18:55 +0100
Subject: [PATCH 16/19] Conditionally run omp_targets

---
 .github/workflows/tests.yml               |  6 +++---
 tests/BUILD.bazel                         | 10 ++++++++++
 tests/scripts/run_tests.sh                | 14 +++++++++++---
 toolchain/internal/llvm_distributions.bzl |  6 +++---
 4 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index a96ed91a..e5f1b8a4 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -41,7 +41,7 @@ jobs:
         env:
           USE_BAZEL_VERSION: ${{ matrix.bazel_version }}
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/run_tests.sh
+        run: tests/scripts/run_tests.sh -O
   toolchain_test:
     strategy:
       fail-fast: false
@@ -98,7 +98,7 @@ jobs:
       - name: Test
         env:
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/${{ matrix.script }}_test.sh
+        run: tests/scripts/${{ matrix.script }}_test.sh ${{ matrix.script == 'ubuntu_20_04' && '' || '-O' }}
   xcompile_test:
     strategy:
       fail-fast: false
@@ -123,7 +123,7 @@ jobs:
       - name: Test
         env:
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_absolute_paths//:cc-toolchain-x86_64-linux
+        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_absolute_paths//:cc-toolchain-x86_64-linux -O
   sys_paths_test:
     strategy:
       fail-fast: false
diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index a7b02659..836b0f42 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -101,6 +101,7 @@ cc_test(
     srcs = ["omp_test.c"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
+    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
@@ -114,9 +115,18 @@ cc_test(
     srcs = ["omp_test.cc"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
+    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
+test_suite(
+    name = "omp_tests",
+    tests = [
+        ":omp_test",
+        ":omp_test_cc",
+    ],
+)
+
 sh_test(
     name = "file_dependency_test",
     srcs = ["file_dependency_test.sh"],
diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index 3f51dedd..adcd477d 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -16,7 +16,8 @@
 set -euo pipefail
 
 toolchain_name=""
-disable_wasm_tests=""
+enable_omp_targets="1"
+enable_wasm_tests="1"
 LLVM_VERSION=""
 
 while getopts "ht:v:W" opt; do
@@ -26,6 +27,9 @@ while getopts "ht:v:W" opt; do
     echo "-t - Toolchain name to use for testing; default is llvm_toolchain"
     exit 2
     ;;
+  "O")
+    enable_omp_targets=""
+    ;;
   "t")
     toolchain_name="${OPTARG}"
     ;;
@@ -33,7 +37,7 @@ while getopts "ht:v:W" opt; do
     LLVM_VERSION="${OPTARG}"
     ;;
   "W")
-    disable_wasm_tests="yes"
+    enable_wasm_tests=""
     ;;
   *)
     echo "invalid option: -${OPTARG}"
@@ -66,6 +70,10 @@ if [[ -z "${toolchain_name}" ]]; then
   targets+=("//:test_cxx_standard_is_20")
 fi
 
+if [[ -n "${enable_omp_targets}" ]]; then
+  targets+=("//:omp_tests")
+fi
+
 if [[ -n "${LLVM_VERSION}" ]]; then
   echo "LLVM_VERSION=${LLVM_VERSION}"
   common_test_args+=(
@@ -85,7 +93,7 @@ fi
 # to run out of disk space.
 #
 # Mitigate this by expunging the workspace before trying to build Wasm targets.
-if [[ -z ${toolchain_name} && -z ${disable_wasm_tests} ]]; then
+if [[ -z "${toolchain_name}" ]] && [[ -n "${enable_wasm_tests}" ]]; then
   # Redefine `test_args` without `--linkopt=-Wl,-v`, which breaks `wasm-ld`.
   #
   # https://github.com/llvm/llvm-project/issues/112836
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index d9eb5421..e060459d 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1168,7 +1168,7 @@ def _required_llvm_release_name(*, version_or_requirements, all_llvm_distributio
             return llvm_version, basenames[0], None
     return None, None, "ERROR: No matching distribution found."
 
-def _resolve_llvm_version_rctx(rctx, llvm_version):
+def _resolve_llvm_version_rctx_env(rctx, llvm_version):
     if llvm_version.startswith("getenv(") and llvm_version.endswith(")"):
         env_var = llvm_version[len("getenv("):-1]
         if env_var.find(",") >= 0:
@@ -1200,7 +1200,7 @@ def _resolve_llvm_version_rctx(rctx, llvm_version):
     return llvm_version
 
 def _required_llvm_release_name_rctx(rctx, llvm_version):
-    llvm_version = _resolve_llvm_version_rctx(rctx, llvm_version)
+    llvm_version = _resolve_llvm_version_rctx_env(rctx, llvm_version)
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
@@ -1246,8 +1246,8 @@ def _distribution_urls(rctx):
 
     if rctx.attr.distribution == "auto":
         rctx_host_info = host_info(rctx)
+        llvm_version = _resolve_llvm_version_rctx_env(rctx, llvm_version)
         if _is_requirement(llvm_version):
-            llvm_version = _resolve_llvm_version_rctx(rctx, llvm_version)
             llvm_version, basename, error = _required_llvm_release_name(
                 version_or_requirements = llvm_version,
                 all_llvm_distributions = all_llvm_distributions,

From 0f9ce9d63e0a21fefd741d4cf83381bad92ffeb4 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Sat, 1 Nov 2025 12:22:12 +0100
Subject: [PATCH 17/19] Oversight: Missed adding the O flag in the last commit

---
 tests/scripts/run_tests.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index adcd477d..48c50742 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -20,7 +20,7 @@ enable_omp_targets="1"
 enable_wasm_tests="1"
 LLVM_VERSION=""
 
-while getopts "ht:v:W" opt; do
+while getopts "hOt:v:W" opt; do
   case "${opt}" in
   "h")
     echo "Usage:"

From 2c1cba4620544639c67881ef4c963d95e0f0e797 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Sat, 1 Nov 2025 12:29:25 +0100
Subject: [PATCH 18/19] Another manual tag is needed

---
 tests/BUILD.bazel | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index 836b0f42..d34b1308 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -121,6 +121,7 @@ cc_test(
 
 test_suite(
     name = "omp_tests",
+    tags = ["manual"],
     tests = [
         ":omp_test",
         ":omp_test_cc",

From 5c8dd698a7550788957811e5d506b90011739510 Mon Sep 17 00:00:00 2001
From: helly25 <marcus.boerger@gmail.com>
Date: Sat, 1 Nov 2025 12:49:22 +0100
Subject: [PATCH 19/19] Manual disabling omp tests and version handling
 improvements.

---
 .github/workflows/tests.yml               | 2 +-
 tests/scripts/archlinux_test.sh           | 2 +-
 tests/scripts/debian_test.sh              | 2 +-
 tests/scripts/fedora_test.sh              | 2 +-
 tests/scripts/suse_leap_test.sh           | 2 +-
 tests/scripts/suse_tumbleweed_test.sh     | 2 +-
 tests/scripts/ubuntu_22_04_test.sh        | 2 +-
 toolchain/internal/llvm_distributions.bzl | 4 +++-
 8 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index e5f1b8a4..876da978 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -98,7 +98,7 @@ jobs:
       - name: Test
         env:
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/${{ matrix.script }}_test.sh ${{ matrix.script == 'ubuntu_20_04' && '' || '-O' }}
+        run: tests/scripts/${{ matrix.script }}_test.sh
   xcompile_test:
     strategy:
       fail-fast: false
diff --git a/tests/scripts/archlinux_test.sh b/tests/scripts/archlinux_test.sh
index dd3d4bf1..eb244f4e 100755
--- a/tests/scripts/archlinux_test.sh
+++ b/tests/scripts/archlinux_test.sh
@@ -32,6 +32,6 @@ set -exuo pipefail
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain}
+tests/scripts/run_tests.sh -O -t ${toolchain}
 """
 done
diff --git a/tests/scripts/debian_test.sh b/tests/scripts/debian_test.sh
index c0de9666..0fb2858e 100755
--- a/tests/scripts/debian_test.sh
+++ b/tests/scripts/debian_test.sh
@@ -36,6 +36,6 @@ apt-get -qq -y install curl libtinfo5 libxml2 zlib1g-dev >/dev/null
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh
+tests/scripts/run_tests.sh -O
 """
 done
diff --git a/tests/scripts/fedora_test.sh b/tests/scripts/fedora_test.sh
index 1cc4cce5..20e31994 100755
--- a/tests/scripts/fedora_test.sh
+++ b/tests/scripts/fedora_test.sh
@@ -32,6 +32,6 @@ dnf install -qy glibc-headers ncurses-compat-libs
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh
+tests/scripts/run_tests.sh -O
 """
 done
diff --git a/tests/scripts/suse_leap_test.sh b/tests/scripts/suse_leap_test.sh
index 7ba31974..3e3308d5 100755
--- a/tests/scripts/suse_leap_test.sh
+++ b/tests/scripts/suse_leap_test.sh
@@ -38,6 +38,6 @@ zypper -n install curl gcc libc++1
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain}
+tests/scripts/run_tests.sh -O -t ${toolchain}
 """
 done
diff --git a/tests/scripts/suse_tumbleweed_test.sh b/tests/scripts/suse_tumbleweed_test.sh
index e5d6cd82..07db78a7 100755
--- a/tests/scripts/suse_tumbleweed_test.sh
+++ b/tests/scripts/suse_tumbleweed_test.sh
@@ -38,6 +38,6 @@ zypper -n install curl gcc libc++1
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain}
+tests/scripts/run_tests.sh -O -t ${toolchain}
 """
 done
diff --git a/tests/scripts/ubuntu_22_04_test.sh b/tests/scripts/ubuntu_22_04_test.sh
index f5341fd8..2e5ff013 100755
--- a/tests/scripts/ubuntu_22_04_test.sh
+++ b/tests/scripts/ubuntu_22_04_test.sh
@@ -36,6 +36,6 @@ apt-get -qq -y install curl libtinfo5 libxml2 zlib1g-dev >/dev/null
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh
+tests/scripts/run_tests.sh -O
 """
 done
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index e060459d..6d2f6e79 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1138,7 +1138,9 @@ def _parse_version_or_requirements(version_or_requirements):
     for prefix in ["latest:", "first:"]:
         if version_or_requirements.startswith(prefix):
             return versions.parse_requirements(version_or_requirements.removeprefix(prefix))
-    if _is_requirement(version_or_requirements):
+    if version_or_requirements in ["latest", "first"]:
+        return None
+    if not _is_requirement(version_or_requirements):
         return None
     fail("ERROR: Invalid version requirements: '{version_or_requirements}'.".format(
         version_or_requirements = version_or_requirements,