Support width and alignment in std::exception formatter

include/fmt/std.h

@@ -620,14 +620,23 @@ struct formatter<
     T, char,
     typename std::enable_if<std::is_base_of<std::exception, T>::value>::type> {
  private:
+  format_specs specs_;
+  detail::arg_ref<char> width_ref_;
   bool with_typename_ = false;
 
  public:
   FMT_CONSTEXPR auto parse(parse_context<>& ctx) -> const char* {
     auto it = ctx.begin();
     auto end = ctx.end();
     if (it == end || *it == '}') return it;
-    if (*it == 't') {
+
+    it = detail::parse_align(it, end, specs_);
+    if (it == end) return it;
+
+    char c = *it;
+    if ((c >= '0' && c <= '9') || c == '{')
+      it = detail::parse_width(it, end, specs_, width_ref_, ctx);
+    if (it != end && *it == 't') {
       ++it;
       with_typename_ = FMT_USE_RTTI != 0;
     }
@@ -637,7 +646,20 @@ struct formatter<
   template <typename Context>
   auto format(const std::exception& ex, Context& ctx) const
       -> decltype(ctx.out()) {
-    return write(ctx.out(), ex);
+    auto buf = memory_buffer();
+    write(appender(buf), ex);
+    return write_padded(ctx, string_view(buf.data(), buf.size()));
+  }
+
+ protected:
+  // Applies the parsed fill/align/width to an already-formatted message.
+  template <typename Context>
+  auto write_padded(Context& ctx, string_view message) const
+      -> decltype(ctx.out()) {
+    auto specs = specs_;
+    detail::handle_dynamic_spec(specs.dynamic_width(), specs.width, width_ref_,
+                                ctx);
+    return detail::write(ctx.out(), message, specs);
   }
 
  private:
@@ -675,13 +697,13 @@ template <> struct formatter<std::exception_ptr> : formatter<std::exception> {
   template <typename FormatContext>
   auto format(const std::exception_ptr& ep, FormatContext& ctx) const
       -> decltype(ctx.out()) {
-    if (!ep) return detail::write(ctx.out(), string_view("none"));
+    if (!ep) return this->write_padded(ctx, string_view("none"));
     try {
       std::rethrow_exception(ep);
     } catch (const std::exception& e) {
       return formatter<std::exception>::format(e, ctx);
     } catch (...) {
-      return detail::write(ctx.out(), string_view("unknown exception"));
+      return this->write_padded(ctx, string_view("unknown exception"));
     }
   }
 };

test/std-test.cc

@@ -465,6 +465,43 @@ TEST(std_test, exception_ptr) {
 #endif  // FMT_USE_RTTI
 }
 
+TEST(std_test, exception_align) {
+  // The exception formatter supports standard width, fill and alignment.
+  auto ex = std::runtime_error("boom");
+  EXPECT_EQ(fmt::format("{:>8}", ex), "    boom");
+  EXPECT_EQ(fmt::format("{:<8}", ex), "boom    ");
+  EXPECT_EQ(fmt::format("{:^8}", ex), "  boom  ");
+  EXPECT_EQ(fmt::format("{:*>8}", ex), "****boom");
+  EXPECT_EQ(fmt::format("{:{}}", ex, 8), "boom    ");
+  // A width smaller than the message leaves the message untouched.
+  EXPECT_EQ(fmt::format("{:>2}", ex), "boom");
+
+#if FMT_USE_RTTI
+  // Alignment combines with the 't' (type name) presentation type.
+  try {
+    using namespace my_ns1::my_ns2;
+    throw my_exception("oops");
+  } catch (const std::exception& e) {
+    auto base = fmt::format("{:t}", e);
+    auto padded = fmt::format("{:*<40t}", e);
+    EXPECT_EQ(padded.size(), 40u);
+    EXPECT_EQ(padded.substr(0, base.size()), base);
+    EXPECT_EQ(padded.substr(base.size()), std::string(40 - base.size(), '*'));
+  }
+#endif  // FMT_USE_RTTI
+
+  // exception_ptr honors the same specifiers, for both the empty ("none")
+  // and the non-empty case.
+  std::exception_ptr enull = nullptr;
+  EXPECT_EQ(fmt::format("{:>8}", enull), "    none");
+  std::exception_ptr ep;
+  try {
+    throw std::runtime_error("bang");
+  } catch (...) {
+    ep = std::current_exception();
+  }
+  EXPECT_EQ(fmt::format("{:>8}", ep), "    bang");
+}
 #if FMT_USE_RTTI
 TEST(std_test, type_info) {
   EXPECT_EQ(fmt::format("{}", typeid(std::runtime_error)),