clang.mk: paper over new macOS and upstream defaults.

As soon as 2024Q1 branches, we should:

1. Do a bulk build with all these -Wno-error tweaks removed, so we can
   see how much is broken.
2. If it's "too much" breakage and we'll have to keep overriding these
   compiler defaults for "a while", find a way to accomplish them with
   fewer compiler invocations. (Some ideas: define a default
   FORCE_C_STD, or apply overrides keyed on CC_VERSION or similar.) Do
   a bulk build to make sure things continue to work as before.
3. Otherwise, fix as much as we can before 2024Q2. This will help with
   gcc 14 (which has many similar new defaults) as well.

Author: schmonz

mk/compiler/clang.mk

@@ -1,4 +1,4 @@
-# $NetBSD: clang.mk,v 1.42 2023/10/18 08:48:51 jperkin Exp $
+# $NetBSD: clang.mk,v 1.43 2024/03/19 21:12:47 schmonz Exp $
 #
 # This is the compiler definition for the clang compiler.
 #
@@ -80,15 +80,44 @@ CWRAPPERS_PREPEND.cc+=	-Qunused-arguments
 _WRAP_EXTRA_ARGS.CXX+=	-Qunused-arguments
 CWRAPPERS_APPEND.cxx+=	-Qunused-arguments
 
-# Xcode 12 has a zealous new default. Turn it off until we're ready,
-# while allowing callers (or users, via CFLAGS/CPPFLAGS) to override.
-.if ${OPSYS} == "Darwin"
-_NOERROR_IMPLICIT_cmd=	${CCPATH} -\#\#\# -E -x c /dev/null 2>&1 \
+# Xcode 12 and upstream Clang 16 have a zealous new default that the
+# wide world of random third-party software isn't ready for. Turn it
+# back off by default.
+#
+# Packages and users can override via CFLAGS/CPPFLAGS.
+_NOERROR_IMPLICIT_cmd=	${CCPATH} -\#\#\# -E -x c /dev/null \
+			2>&1 \
 			| ${GREP} -q Werror=implicit-function-declaration \
 			&& ${ECHO} -Wno-error=implicit-function-declaration \
 			|| ${TRUE}
 CWRAPPERS_PREPEND.cc+=	${_NOERROR_IMPLICIT_cmd:sh}
-.endif
+
+# Xcode 15 (Apple clang-1500.3.9.4) and upstream Clang 16 have several
+# more zealous new defaults. We can't default them off as precisely
+# (they don't show up in `clang -###` output), so we simply attempt to
+# turn them off for any clang that doesn't complain when we try.
+#
+# Packages and users can override via CFLAGS/CPPFLAGS.
+#
+# As soon as 2024Q1 branches, we should:
+#
+# 1. Do a bulk build with all these -Wno-error tweaks removed, so we can
+#    see how much is broken.
+# 2. If it's "too much" breakage and we'll have to keep overriding these
+#    compiler defaults for "a while", find a way to accomplish them with
+#    fewer compiler invocations. (Some ideas: define a default
+#    FORCE_C_STD, or apply overrides keyed on CC_VERSION or similar.) Do
+#    a bulk build to make sure things continue to work as before.
+# 3. Otherwise, fix as much as we can before 2024Q2. This will help with
+#    gcc 14 (which has many similar new defaults) as well.
+_WERROR_CLANG16=	implicit-int int-conversion incompatible-pointer-types
+_NOERROR_CLANG16_cmd=	for _warn_ in ${_WERROR_CLANG16}; do \
+			${CCPATH} -\#\#\# -Wno-error=$${_warn_} -x c /dev/null \
+			>/dev/null 2>&1 \
+			&& ${ECHO} -Wno-error=$${_warn_} \
+			|| ${TRUE}; \
+			done
+CWRAPPERS_PREPEND.cc+=	${_NOERROR_CLANG16_cmd:sh}
 
 .for _version_ in ${_CXX_STD_VERSIONS}
 _CXX_STD_FLAG.${_version_}?=	-std=${_version_}