diff --git a/posix.cc b/posix.cc
index 2511f24d..302c78ad 100644
--- a/posix.cc
+++ b/posix.cc
@@ -54,20 +54,6 @@
 
 #endif  // _WIN32
 
-#if FMT_GCC_VERSION >= 407
-# define FMT_UNUSED __attribute__((unused))
-#else
-# define FMT_UNUSED
-#endif
-
-#if FMT_USE_STATIC_ASSERT
-# define FMT_STATIC_ASSERT(cond, message) static_assert(cond, message)
-#else
-# define FMT_CONCAT_(a, b) FMT_CONCAT(a, b)
-# define FMT_STATIC_ASSERT(cond, message) \
-  typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
-#endif
-
 namespace {
 #ifdef _WIN32
 // Return type of read and write functions.
diff --git a/posix.h b/posix.h
index 96a4e973..6a48adb6 100644
--- a/posix.h
+++ b/posix.h
@@ -62,6 +62,20 @@
 # endif
 #endif
 
+#if FMT_GCC_VERSION >= 407
+# define FMT_UNUSED __attribute__((unused))
+#else
+# define FMT_UNUSED
+#endif
+
+#if FMT_USE_STATIC_ASSERT
+# define FMT_STATIC_ASSERT(cond, message) static_assert(cond, message)
+#else
+# define FMT_CONCAT_(a, b) FMT_CONCAT(a, b)
+# define FMT_STATIC_ASSERT(cond, message) \
+  typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
+#endif
+
 // Retries the expression while it evaluates to error_result and errno
 // equals to EINTR.
 #ifndef _WIN32
diff --git a/test/compile-test/CMakeLists.txt b/test/compile-test/CMakeLists.txt
index c67441b5..f8f9e04a 100644
--- a/test/compile-test/CMakeLists.txt
+++ b/test/compile-test/CMakeLists.txt
@@ -8,6 +8,7 @@ set(CMAKE_REQUIRED_INCLUDES ${CMAKE_CURRENT_SOURCE_DIR}/../..)
 function (expect_compile_error code)
   check_cxx_source_compiles("
   #include \"format.cc\"
+  #include \"posix.h\"
   int main() {
     ${code}
   }
@@ -44,3 +45,5 @@ expect_compile_error("fmt::Writer() << fmt::pad(42, 5, L' ');")
 
 # Formatting a wide character with a narrow format string is forbidden.
 expect_compile_error("fmt::format(\"{}\", L'a';")
+
+expect_compile_error("FMT_STATIC_ASSERT(0 > 1, \"oops\");")
diff --git a/test/posix-test.cc b/test/posix-test.cc
index c995de06..5d5bdeb0 100644
--- a/test/posix-test.cc
+++ b/test/posix-test.cc
@@ -56,14 +56,6 @@ int fileno_count;
 std::size_t read_nbyte;
 std::size_t write_nbyte;
 bool increase_file_size;
-
-std::string TEST_FILE_CONTENT = "top secret, destroy before reading";
-
-void WriteTestFile() {
-  BufferedFile bf("test", "w");
-  bf.print(TEST_FILE_CONTENT);
-  bf.close();
-}
 }
 
 #define EMULATE_EINTR(func, error_result) \
@@ -183,7 +175,13 @@ int test::fileno(FILE *stream) {
 
 void write_file(fmt::StringRef filename, fmt::StringRef content) {
   fmt::BufferedFile f(filename, "w");
-  fmt::print(f.get(), "{}", content);
+  f.print("{}", content);
+}
+
+TEST(UtilTest, StaticAssert) {
+  FMT_STATIC_ASSERT(true, "success");
+  // Static assertion failure is tested in compile-test because it causes
+  // a compile-time error.
 }
 
 TEST(FileTest, OpenRetry) {
@@ -222,15 +220,16 @@ TEST(FileTest, CloseNoRetry) {
 }
 
 TEST(FileTest, Size) {
-  WriteTestFile();
+  std::string content = "top secret, destroy before reading";
+  write_file("test", content);
   File f("test", File::RDONLY);
-  EXPECT_EQ(TEST_FILE_CONTENT.size(), f.size());
+  EXPECT_EQ(content.size(), f.size());
   f.close();
   EXPECT_SYSTEM_ERROR(f.size(), EBADF, "cannot get file attributes");
 }
 
 TEST(FileTest, MaxSize) {
-  WriteTestFile();
+  write_file("test", "");
   File f("test", File::RDONLY);
   increase_file_size = true;
   EXPECT_GE(f.size(), 0);
@@ -238,8 +237,6 @@ TEST(FileTest, MaxSize) {
   increase_file_size = false;
 }
 
-// TODO: test FMT_STATIC_ASSERT
-
 TEST(FileTest, ReadRetry) {
   File read_end, write_end;
   File::pipe(read_end, write_end);