Problem: clang 6 warns about comparisons on 64 bit

Solution: ignore tautological-constant-compare warnings, as they
might be useless on 64 bit but they are not on 32 bit where sizeof
size_t != sizeof uint64_t

CMakeLists.txt

@@ -35,6 +35,18 @@ if(COMPILER_SUPPORTS_C11)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu11")
 endif()
 
+if(NOT MSVC)
+    # clang 6 has a warning that does not make sense on multi-platform code
+    CHECK_CXX_COMPILER_FLAG("-Wno-tautological-constant-compare" CXX_HAS_TAUT_WARNING)
+    if(CXX_HAS_TAUT_WARNING)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-tautological-constant-compare")
+    endif()
+    CHECK_C_COMPILER_FLAG("-Wno-tautological-constant-compare" CC_HAS_TAUT_WARNING)
+    if(CC_HAS_TAUT_WARNING)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-tautological-constant-compare")
+    endif()
+endif()
+
 # Will be used to add flags to pkg-config useful when apps want to statically link
 set (pkg_config_libs_private "")
 

configure.ac

@@ -812,6 +812,12 @@ AM_CONDITIONAL([WITH_CLANG_FORMAT], [$WITH_CLANG_FORMAT])
 # unittests will not build without the static libzmq.a
 AM_CONDITIONAL(ENABLE_STATIC, test "x$enable_static" = "xyes")
 
+# clang 6 has a warning that does not make sense on multi-platform code
+AX_CHECK_COMPILE_FLAG([-Wno-tautological-constant-compare],
+    [CFLAGS+=" -Wno-tautological-constant-compare" CXXFLAGS+=" -Wno-tautological-constant-compare"],
+    [],
+    [-Werror])
+
 # Subst LIBZMQ_EXTRA_CFLAGS & CXXFLAGS & LDFLAGS
 AC_SUBST(LIBZMQ_EXTRA_CFLAGS)
 AC_SUBST(LIBZMQ_EXTRA_CXXFLAGS)