From 23d2246e0f6ebfc69232ab29b6c6325d8009d1f7 Mon Sep 17 00:00:00 2001
From: Robert Edmonds <edmonds@users.noreply.github.com>
Date: Sun, 2 Jul 2023 19:48:17 -0400
Subject: [PATCH] configure.ac: Require C++17 when building against protobuf >=
 4.22.0

It's unclear from looking at the online protobuf documentation whether
C++14 or C++17 is now required when building against newer versions of
protobuf (or perhaps I'm looking in the wrong place), but experimentally
it did not work for me with C++14.

This commit rewrites the versioned PKG_CHECK_MODULES checks in
configure.ac to enforce the following policies:

 * Require protobuf >= 2.6.0.
 * On protobuf >= 3.0.0, require C++11.
 * On protobuf >= 4.22.0, require C++17.
---
 configure.ac | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/configure.ac b/configure.ac
index 94d01dc..fa024b6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -76,12 +76,26 @@ AC_ARG_ENABLE([protoc],
 if test "x$enable_protoc" != "xno"; then
   AC_LANG_PUSH([C++])
 
-  AX_CXX_COMPILE_STDCXX(11, noext, mandatory)
-
-  PKG_CHECK_MODULES([protobuf], [protobuf >= 3.0.0],
-    [proto3_supported=yes],
-    [PKG_CHECK_MODULES([protobuf], [protobuf >= 2.6.0])]
-  )
+  # PKG_CHECK_MODULES(prefix, list-of-modules, action-if-found, action-if-not-found)
+  PKG_CHECK_MODULES(
+    [protobuf],
+    [protobuf >= 4.22.0],
+    [
+      proto3_supported=yes
+      AX_CXX_COMPILE_STDCXX(17, noext, mandatory)
+    ],
+    [
+      PKG_CHECK_MODULES(
+        [protobuf],
+        [protobuf >= 3.0.0],
+        [
+          proto3_supported=yes
+          AX_CXX_COMPILE_STDCXX(11, noext, mandatory)
+        ],
+        [
+          PKG_CHECK_MODULES([protobuf], [protobuf >= 2.6.0])
+        ])
+    ])
 
   save_CPPFLAGS="$CPPFLAGS"
   CPPFLAGS="$save_CPPFLAGS $protobuf_CFLAGS"