Add formatter for std::exception

Co-authored-by: fekir <[email protected]>
Co-authored-by: Alexey Ochapov <[email protected]>

include/fmt/std.h

@@ -8,6 +8,7 @@
 #ifndef FMT_STD_H_
 #define FMT_STD_H_
 
+#include <exception>
 #include <thread>
 #include <type_traits>
 #include <utility>
@@ -166,6 +167,21 @@ struct formatter<
   }
 };
 FMT_END_NAMESPACE
-#endif
+
+#endif  // __cpp_lib_variant
+
+FMT_BEGIN_NAMESPACE
+template <typename T, typename Char>
+struct formatter<
+    T, Char,
+    typename std::enable_if<std::is_base_of<std::exception, T>::value>::type>
+    : formatter<std::string> {
+  template <typename FormatContext>
+  auto format(const std::exception& ex, FormatContext& ctx) const ->
+      typename FormatContext::iterator {
+    return fmt::formatter<std::string>::format(ex.what(), ctx);
+  }
+};
+FMT_END_NAMESPACE
 
 #endif  // FMT_STD_H_

test/std-test.cc

@@ -6,11 +6,11 @@
 // For the license information refer to format.h.
 
 #include "fmt/std.h"
-#include "fmt/ranges.h"
 
 #include <string>
 #include <vector>
 
+#include "fmt/ranges.h"
 #include "gtest/gtest.h"
 
 TEST(std_test, path) {
@@ -77,3 +77,21 @@ TEST(std_test, variant) {
   EXPECT_EQ(fmt::format("{}", v5), "variant(\"yes, this is variant\")");
 #endif
 }
+
+TEST(std_test, exception) {
+  try {
+    std::vector<int> vec;
+    std::ignore = vec.at(42);
+  } catch (const std::exception& ex) {
+    EXPECT_EQ(fmt::format("{}", ex), "invalid vector subscript");
+    EXPECT_EQ(fmt::format("{:?}", ex), "\"invalid vector subscript\"");
+  }
+
+  try {
+    std::vector<int> vec;
+    std::ignore = vec.at(42);
+  } catch (const std::out_of_range& ex) {
+    EXPECT_EQ(fmt::format("{}", ex), "invalid vector subscript");
+    EXPECT_EQ(fmt::format("{:?}", ex), "\"invalid vector subscript\"");
+  }
+}