tests: Add a test C++ extension module

While we already test that our headers are valid C++ using
headerscheck, it turns out that the macros we define might still
expand to invalid C++ code.  This adds a minimal test extension that
is compiled using C++ to test that it's actually possible to build and
run extensions written in C++.  Future commits will improve C++
compatibility of some of our macros and add usage of them to this
extension make sure that they don't regress in the future.

The test module is for the moment disabled when using MSVC.  In
particular, the use of designated initializers in PG_MODULE_MAGIC
would require C++20, for which we are currently not set up.  (GCC and
Clang support it as extensions.)  It is planned to fix this.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg@mail.gmail.com

configure

@@ -760,6 +760,7 @@ CLANG
 LLVM_CONFIG
 AWK
 with_llvm
+have_cxx
 ac_ct_CXX
 CXXFLAGS
 CXX
@@ -4769,6 +4770,13 @@ ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
+if test -n "$ac_ct_CXX"; then
+  have_cxx=yes
+else
+  have_cxx=no
+fi
+
+
 # Check if it's Intel's compiler, which (usually) pretends to be gcc,
 # but has idiosyncrasies of its own.  We assume icc will define
 # __INTEL_COMPILER regardless of CFLAGS.

configure.ac

@@ -387,6 +387,13 @@ fi
 
 AC_PROG_CXX([$pgac_cxx_list])
 
+if test -n "$ac_ct_CXX"; then
+  have_cxx=yes
+else
+  have_cxx=no
+fi
+AC_SUBST(have_cxx)
+
 # Check if it's Intel's compiler, which (usually) pretends to be gcc,
 # but has idiosyncrasies of its own.  We assume icc will define
 # __INTEL_COMPILER regardless of CFLAGS.

src/Makefile.global.in

@@ -280,6 +280,8 @@ PERMIT_DECLARATION_AFTER_STATEMENT = @PERMIT_DECLARATION_AFTER_STATEMENT@
 PERMIT_MISSING_VARIABLE_DECLARATIONS = @PERMIT_MISSING_VARIABLE_DECLARATIONS@
 CXXFLAGS = @CXXFLAGS@
 
+have_cxx = @have_cxx@
+
 LLVM_CPPFLAGS = @LLVM_CPPFLAGS@
 LLVM_CFLAGS = @LLVM_CFLAGS@
 LLVM_CXXFLAGS = @LLVM_CXXFLAGS@

src/makefiles/meson.build

@@ -118,6 +118,8 @@ pgxs_kv = {
   'FLEXFLAGS': ' '.join(flex_flags),
 
   'LIBS': var_libs,
+
+  'have_cxx': have_cxx ? 'yes' : 'no',
 }
 
 if llvm.found()

src/test/modules/Makefile

@@ -75,5 +75,11 @@ else
 ALWAYS_SUBDIRS += ldap_password_func
 endif
 
+ifeq ($(have_cxx),yes)
+SUBDIRS += test_cplusplusext
+else
+ALWAYS_SUBDIRS += test_cplusplusext
+endif
+
 $(recurse)
 $(recurse_always)

src/test/modules/meson.build

@@ -21,6 +21,7 @@ subdir('test_bitmapset')
 subdir('test_bloomfilter')
 subdir('test_cloexec')
 subdir('test_copy_callbacks')
+subdir('test_cplusplusext')
 subdir('test_custom_rmgrs')
 subdir('test_custom_stats')
 subdir('test_ddl_deparse')

src/test/modules/test_cplusplusext/.gitignore

@@ -0,0 +1,3 @@
+/log/
+/results/
+/tmp_check/

src/test/modules/test_cplusplusext/Makefile

@@ -0,0 +1,26 @@
+# src/test/modules/test_cplusplusext/Makefile
+
+MODULE_big = test_cplusplusext
+OBJS = \
+	$(WIN32RES) \
+	test_cplusplusext.o
+PGFILEDESC = "test_cplusplusext - test C++ compatibility of PostgreSQL headers"
+
+EXTENSION = test_cplusplusext
+DATA = test_cplusplusext--1.0.sql
+
+REGRESS = test_cplusplusext
+
+# Use C++ compiler for linking because this module includes C++ files
+override COMPILER = $(CXX) $(CXXFLAGS)
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_cplusplusext
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif

src/test/modules/test_cplusplusext/README

@@ -0,0 +1,10 @@
+test_cplusplusext - Test C++ Extension Compatibility
+====================================================
+
+This test module verifies that PostgreSQL headers and macros work
+correctly when compiled with a C++ compiler.
+
+While PostgreSQL already tests that headers are syntactically valid
+C++ using headerscheck, the macros defined in those headers might
+still expand to invalid C++ code.  This module catches such issues by
+actually compiling and running an extension that's written in C++.

src/test/modules/test_cplusplusext/expected/test_cplusplusext.out

@@ -0,0 +1,7 @@
+CREATE EXTENSION test_cplusplusext;
+SELECT test_cplusplus_add(1, 2);
+ test_cplusplus_add 
+--------------------
+                  3
+(1 row)
+

src/test/modules/test_cplusplusext/meson.build

@@ -0,0 +1,42 @@
+# Copyright (c) 2025-2026, PostgreSQL Global Development Group
+
+if not have_cxx
+  subdir_done()
+endif
+
+# Currently not supported, to be fixed.
+if cc.get_id() == 'msvc'
+  subdir_done()
+endif
+
+test_cplusplusext_sources = files(
+  'test_cplusplusext.cpp',
+)
+
+if host_system == 'windows'
+  test_cplusplusext_sources += rc_lib_gen.process(win32ver_rc, extra_args: [
+    '--NAME', 'test_cplusplusext',
+    '--FILEDESC', 'test_cplusplusext - test C++ compatibility of PostgreSQL headers',])
+endif
+
+test_cplusplusext = shared_module('test_cplusplusext',
+  test_cplusplusext_sources,
+  kwargs: pg_test_mod_args,
+)
+test_install_libs += test_cplusplusext
+
+test_install_data += files(
+  'test_cplusplusext.control',
+  'test_cplusplusext--1.0.sql',
+)
+
+tests += {
+  'name': 'test_cplusplusext',
+  'sd': meson.current_source_dir(),
+  'bd': meson.current_build_dir(),
+  'regress': {
+    'sql': [
+      'test_cplusplusext',
+    ],
+  },
+}

src/test/modules/test_cplusplusext/sql/test_cplusplusext.sql

@@ -0,0 +1,3 @@
+CREATE EXTENSION test_cplusplusext;
+
+SELECT test_cplusplus_add(1, 2);

src/test/modules/test_cplusplusext/test_cplusplusext--1.0.sql

@@ -0,0 +1,8 @@
+/* src/test/modules/test_cplusplusext/test_cplusplusext--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_cplusplusext" to load this file. \quit
+
+CREATE FUNCTION test_cplusplus_add(int4, int4) RETURNS int4
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;

src/test/modules/test_cplusplusext/test_cplusplusext.control

@@ -0,0 +1,4 @@
+comment = 'Test module for C++ extension compatibility'
+default_version = '1.0'
+module_pathname = '$libdir/test_cplusplusext'
+relocatable = true

src/test/modules/test_cplusplusext/test_cplusplusext.cpp

@@ -0,0 +1,37 @@
+/*--------------------------------------------------------------------------
+ *
+ * test_cplusplusext.cpp
+ *		Test that PostgreSQL headers compile with a C++ compiler.
+ *
+ * This file is compiled with a C++ compiler to verify that PostgreSQL
+ * headers remain compatible with C++ extensions.
+ *
+ * Copyright (c) 2025-2026, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		src/test/modules/test_cplusplusext/test_cplusplusext.cpp
+ *
+ * -------------------------------------------------------------------------
+ */
+
+extern "C" {
+#include "postgres.h"
+#include "fmgr.h"
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(test_cplusplus_add);
+}
+
+/*
+ * Simple function that returns the sum of two integers.  This verifies that
+ * C++ extension modules can be loaded and called correctly at runtime.
+ */
+extern "C" Datum
+test_cplusplus_add(PG_FUNCTION_ARGS)
+{
+	int32		a = PG_GETARG_INT32(0);
+	int32		b = PG_GETARG_INT32(1);
+
+	PG_RETURN_INT32(a + b);
+}