Basics of formatting at compile-time based on compile-time API (#2019)

test/compile-test.cc

@@ -7,6 +7,9 @@
 
 #include <string>
 #include <type_traits>
+#if __cplusplus >= 202002L
+#  include <string_view>
+#endif
 
 // Check that fmt/compile.h compiles with windows.h included before it.
 #ifdef _WIN32
@@ -171,3 +174,69 @@ TEST(CompileTest, TextAndArg) {
   EXPECT_EQ("42!", fmt::format(FMT_COMPILE("{}!"), 42));
 }
 #endif
+
+#if __cplusplus >= 202002L
+template <size_t max_string_length> struct test_string {
+  template <typename T> constexpr bool operator==(const T& rhs) const noexcept {
+    return (std::string_view(rhs).compare(buffer.data()) == 0);
+  }
+
+  std::array<char, max_string_length> buffer{};
+};
+
+template <size_t max_string_length, typename... Args>
+consteval auto test_format(auto format, const Args&... args) {
+  test_string<max_string_length> string{};
+  fmt::format_to(string.buffer.data(), format, args...);
+  return string;
+}
+
+TEST(CompileTimeFormattingTest, Bool) {
+  {
+    constexpr auto result = test_format<5>(FMT_COMPILE("{}"), true);
+    EXPECT_EQ(result, "true");
+  }
+  {
+    constexpr auto result = test_format<6>(FMT_COMPILE("{}"), false);
+    EXPECT_EQ(result, "false");
+  }
+}
+
+TEST(CompileTimeFormattingTest, Integer) {
+  {
+    constexpr auto result = test_format<3>(FMT_COMPILE("{}"), 42);
+    EXPECT_EQ(result, "42");
+  }
+  {
+    constexpr auto result = test_format<4>(FMT_COMPILE("{}"), 420);
+    EXPECT_EQ(result, "420");
+  }
+  {
+    constexpr auto result = test_format<6>(FMT_COMPILE("{} {}"), 42, 42);
+    EXPECT_EQ(result, "42 42");
+  }
+  {
+    constexpr auto result =
+        test_format<6>(FMT_COMPILE("{} {}"), uint32_t{42}, uint64_t{42});
+    EXPECT_EQ(result, "42 42");
+  }
+}
+
+TEST(CompileTimeFormattingTest, String) {
+  {
+    constexpr auto result = test_format<3>(FMT_COMPILE("{}"), "42");
+    EXPECT_EQ(result, "42");
+  }
+  {
+    constexpr auto result =
+        test_format<17>(FMT_COMPILE("{} is {}"), "The answer", "42");
+    EXPECT_EQ(result, "The answer is 42");
+  }
+}
+
+TEST(CompileTimeFormattingTest, Combination) {
+  constexpr auto result =
+      test_format<18>(FMT_COMPILE("{}, {}, {}"), 420, true, "answer");
+  EXPECT_EQ(result, "420, true, answer");
+}
+#endif