diff --git a/config.mak.dev b/config.mak.dev
index 0fd8cc4d35..3142363816 100644
--- a/config.mak.dev
+++ b/config.mak.dev
@@ -40,6 +40,10 @@ DEVELOPER_CFLAGS += -Wvla
 DEVELOPER_CFLAGS += -Wwrite-strings
 DEVELOPER_CFLAGS += -fno-common
 
+ifneq ($(filter clang9,$(COMPILER_FEATURES)),)
+DEVELOPER_CFLAGS += -Wcomma
+endif
+
 ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare
 endif
diff --git a/meson.build b/meson.build
index efe2871c9d..fd8c05dec9 100644
--- a/meson.build
+++ b/meson.build
@@ -715,6 +715,7 @@ libgit_dependencies = [ ]
 # Makefile.
 if get_option('warning_level') in ['2','3', 'everything'] and compiler.get_argument_syntax() == 'gcc'
   foreach cflag : [
+    '-Wcomma',
     '-Wdeclaration-after-statement',
     '-Wformat-security',
     '-Wold-style-definition',