From 55a23c53fe1e47b6d12d32a0fd5a9f68700c67e0 Mon Sep 17 00:00:00 2001
From: Jay Berkenbilt <ejb@ql.org>
Date: Sun, 30 Nov 2025 14:26:59 -0500
Subject: [PATCH] Check for C++-17 header compatibility in CI (fixes #1567)

---
 CMakeLists.txt               |  1 +
 build-scripts/build-linux    |  2 ++
 build-scripts/check-headers  | 41 +++++++++++++++++++++++++++++++++++++++++
 include/qpdf/Pl_StdioFile.hh |  2 --
 job.sums                     |  2 +-
 manual/release-notes.rst     |  4 ++++
 6 files changed, 49 insertions(+), 3 deletions(-)
 create mode 100755 build-scripts/check-headers

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1dfb234..f97313e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -169,6 +169,7 @@ endif()
 if(CXX_NEXT)
   set(CMAKE_CXX_STANDARD 23)
 else()
+  # See also build-scripts/check-headers
   set(CMAKE_CXX_STANDARD 20)
 endif()
 set(CMAKE_CXX_EXTENSIONS OFF)
diff --git a/build-scripts/build-linux b/build-scripts/build-linux
index 3c0de7f..a1216b9 100755
--- a/build-scripts/build-linux
+++ b/build-scripts/build-linux
@@ -18,6 +18,8 @@ for i in $(./qpdf/qpdf --show-crypto | tail -n +2); do
     env QPDF_CRYPTO_PROVIDER=$i ctest --verbose -R '^qpdf$'
 done
 cd ..
+# Perform additional tests on header files.
+./build-scripts/check-headers
 export TMPDIR=$PWD/dist-tmp
 rm -rf $TMPDIR
 ./make_dist --ci
diff --git a/build-scripts/check-headers b/build-scripts/check-headers
new file mode 100755
index 0000000..5557c65
--- /dev/null
+++ b/build-scripts/check-headers
@@ -0,0 +1,41 @@
+#!/bin/bash
+set -eo pipefail
+cd $(dirname $0)/..
+
+trap "rm -f a.cc" EXIT
+declare -a headers
+cd include
+for i in qpdf/*.hh; do
+    if [[ ! $i =~ .*auto_.* ]] && ! grep -q >/dev/null 2>&1 QPDFOBJECT_OLD_HH $i; then
+        headers+=($i)
+    fi
+done
+cd ..
+# Make sure each header file can be included in isolation and that the
+# result can be compiled with the intended version of the C++
+# standard, which may be older than one we build with internally.
+declare -a errors
+for i in "${headers[@]}"; do
+    rm -f a.cc
+    cat > a.cc <<EOF
+#include "$i"
+int main() { return 0; }
+EOF
+    echo "Checking $i"
+    if ! g++ -std=c++17 -pedantic-errors -c -Iinclude a.cc -o /dev/null; then
+        errors+=("$i doesn't compile")
+    fi
+    # Fail if any C++20 headers are included. Modern g++/clang treat
+    # these as empty if the compiler standard is too old.
+    if grep -q >/dev/null 2>&1 -E "#\s*include\s*[<\"](concepts|coroutine|compare|ranges|format|source_location|version|span|bit|numbers|barrier|latch|semaphore|stop_token|syncstream)[>\"]" "include/$i"; then
+        errors+=("$i includes a non-C++-17 standard header")
+    fi
+done
+if [[ ${#errors[@]} -gt 0 ]]; then
+    echo ""
+    echo "Some header files had errors"
+    for i in "${errors[@]}"; do
+        echo "$i"
+    done
+    exit 2
+fi
diff --git a/include/qpdf/Pl_StdioFile.hh b/include/qpdf/Pl_StdioFile.hh
index 47e6fc9..9e0ad54 100644
--- a/include/qpdf/Pl_StdioFile.hh
+++ b/include/qpdf/Pl_StdioFile.hh
@@ -45,8 +45,6 @@ class QPDF_DLL_CLASS Pl_StdioFile: public Pipeline
 
   private:
     class Members;
-    ;
-
     std::unique_ptr<Members> m;
 };
 
diff --git a/job.sums b/job.sums
index 3fc3b9b..5502dfb 100644
--- a/job.sums
+++ b/job.sums
@@ -1,5 +1,5 @@
 # Generated by generate_auto_job
-CMakeLists.txt 18214e276670dc8beb2ab83f789c6d94941bc92b199b353f3943024cfd41d3bc
+CMakeLists.txt 5b8566f49d7b8b96cd6f7404670f65a03851826364e52697c3a7a2206f0f3069
 generate_auto_job 8e3175a515aa8837d8a01bba0346b04b3d777d70330ba5b7d52f691316054a34
 include/qpdf/auto_job_c_att.hh 4c2b171ea00531db54720bf49a43f8b34481586ae7fb6cbf225099ee42bc5bb4
 include/qpdf/auto_job_c_copy_att.hh 50609012bff14fd82f0649185940d617d05d530cdc522185c7f3920a561ccb42
diff --git a/manual/release-notes.rst b/manual/release-notes.rst
index b2d6da0..27c030e 100644
--- a/manual/release-notes.rst
+++ b/manual/release-notes.rst
@@ -52,6 +52,10 @@ more detail.
       than ``unsigned char``) container and facilitate the efficient moving
       of its content into a `std::string``.
 
+  - Build fixes
+
+    - Attempt to detect if any > C++-17 changes snuck into any public
+      headers.
 
   - CLI Enhancements
 
--
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