diff --git a/.cirrus.tasks.yml b/.cirrus.tasks.yml
index 0f32827952f..a22cef063f3 100644
--- a/.cirrus.tasks.yml
+++ b/.cirrus.tasks.yml
@@ -782,6 +782,7 @@ task:
     CIRRUS_WINDOWS_ERROR_MODE: 0x8001
 
     MESON_FEATURES:
+      -Dcpp_args=/std:c++20
       -Dauto_features=disabled
       -Dldap=enabled
       -Dssl=openssl
diff --git a/meson.build b/meson.build
index c4b4cd4af06..8b134b28a69 100644
--- a/meson.build
+++ b/meson.build
@@ -2176,6 +2176,20 @@ choke me
 endif
 
 
+# Check whether the C++ compiler supports designated initializers.
+# These are used by PG_MODULE_MAGIC, and we use the result of this
+# test to decide whether to enable the test_cplusplusext test module.
+# Designated initializers only got standardized in C++20.  In GCC and
+# Clang they also work when using earlier C++ versions, but MSVC
+# really only supports them when its configured to be in C++20 mode or
+# higher.
+if have_cxx
+  have_cxx_desinit = cxx.compiles('struct S { int x; } s = { .x = 1 };', name: 'C++ designated initializers')
+else
+  have_cxx_desinit = false
+endif
+
+
 
 ###############################################################
 # Compiler flags
diff --git a/src/test/modules/test_cplusplusext/meson.build b/src/test/modules/test_cplusplusext/meson.build
index d13210ca593..5860464a503 100644
--- a/src/test/modules/test_cplusplusext/meson.build
+++ b/src/test/modules/test_cplusplusext/meson.build
@@ -1,11 +1,6 @@
 # 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'
+if not have_cxx or not have_cxx_desinit
   subdir_done()
 endif