From f6049f0df6b8e8993e031657501338f885d22fd5 Mon Sep 17 00:00:00 2001
From: muk2 <mukundchanchlani@gmail.com>
Date: Sun, 16 Aug 2026 13:13:38 -0400
Subject: [PATCH] Add LLVM Flang CI

Add a Linux CI job that builds libamrex and its Fortran interfaces with
LLVM Flang (and matching Clang). This catches LLVM-Flang-specific Fortran
compile errors such as the duplicate `public` attribute fixed in #5604,
which gfortran-based CI does not flag.

Closes #5605.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 .../dependencies/dependencies_flang.sh        | 32 ++++++++++++
 .github/workflows/flang.yml                   | 49 +++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100755 .github/workflows/dependencies/dependencies_flang.sh
 create mode 100644 .github/workflows/flang.yml

diff --git a/.github/workflows/dependencies/dependencies_flang.sh b/.github/workflows/dependencies/dependencies_flang.sh
new file mode 100755
index 00000000000..07aa25049d2
--- /dev/null
+++ b/.github/workflows/dependencies/dependencies_flang.sh
@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+#
+# Copyright 2020-2026 The AMReX Community
+#
+# License: BSD-3-Clause-LBNL
+
+set -eu -o pipefail
+
+# `man apt.conf`:
+#   Number of retries to perform. If this is non-zero APT will retry
+#   failed files the given number of times.
+echo 'Acquire::Retries "3";' | sudo tee /etc/apt/apt.conf.d/80-retries
+
+# Install a recent LLVM Flang (and the matching Clang) from apt.llvm.org.
+# The desired LLVM major version is passed as the first argument.
+if [[ ! -f /etc/apt/trusted.gpg.d/apt.llvm.org.asc ]]; then
+    wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo tee /etc/apt/trusted.gpg.d/apt.llvm.org.asc
+fi
+
+source /etc/os-release # set UBUNTU_CODENAME
+
+sudo add-apt-repository "deb http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME} main"
+sudo add-apt-repository "deb http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-$1 main"
+
+sudo apt-get update
+
+sudo apt-get install -y --no-install-recommends \
+    build-essential      \
+    libfftw3-dev         \
+    clang-$1             \
+    flang-$1             \
+    libomp-$1-dev
diff --git a/.github/workflows/flang.yml b/.github/workflows/flang.yml
new file mode 100644
index 00000000000..0efe0de56d9
--- /dev/null
+++ b/.github/workflows/flang.yml
@@ -0,0 +1,49 @@
+name: LinuxFlang
+
+on:
+  push:
+    branches-ignore:
+      - 'dependabot/**'
+  pull_request:
+
+concurrency:
+  group: ${{ github.ref }}-${{ github.head_ref }}-linux-flang
+  cancel-in-progress: true
+
+jobs:
+  check_changes:
+    uses: ./.github/workflows/check_changes.yml
+
+  # Build libamrex and its Fortran interfaces with LLVM Flang.
+  # This guards against LLVM-Flang-specific Fortran errors such as #5603.
+  library_flang:
+    name: Flang@20 DP NOMPI Fortran Interfaces [lib]
+    runs-on: ubuntu-24.04
+    needs: check_changes
+    if: needs.check_changes.outputs.has_non_docs_changes == 'true'
+    steps:
+    - uses: actions/checkout@v7
+    - name: Dependencies
+      run: .github/workflows/dependencies/dependencies_flang.sh 20
+    - name: Build & Install
+      run: |
+        # The apt.llvm.org packages ship the driver as either flang-<ver>
+        # or flang-new-<ver> depending on the LLVM release.
+        FLANG=$(which flang-20 || which flang-new-20)
+
+        mkdir build
+        cd build
+        cmake ..                                       \
+            -DCMAKE_BUILD_TYPE=Debug                   \
+            -DCMAKE_VERBOSE_MAKEFILE=ON                \
+            -DCMAKE_INSTALL_PREFIX=/tmp/my-amrex       \
+            -DAMReX_EB=ON                              \
+            -DAMReX_FORTRAN=ON                         \
+            -DAMReX_FORTRAN_INTERFACES=ON              \
+            -DAMReX_MPI=OFF                            \
+            -DCMAKE_C_COMPILER=$(which clang-20)       \
+            -DCMAKE_CXX_COMPILER=$(which clang++-20)   \
+            -DCMAKE_Fortran_COMPILER=${FLANG}
+        make -j 4
+        make install
+        make test_install