Transitions between faint and bold go through normal

include/terminalpp/detail/element_difference.hpp

@@ -160,11 +160,26 @@ void change_effect(
     bool &change_appended,
     WriteContinuation &&wc)
 {
+  static byte_storage const separator = {ansi::ps};
+
   if (source != dest)
   {
+    if constexpr (effect_has_normal<EffectType>::value)
+    {
+      if (source.value_ != EffectType::normal
+          && dest.value_ != EffectType::normal)
+      {
+        if (std::exchange(change_appended, true))
+        {
+          wc(separator);
+        }
+
+        wc(to_bytes(fmt::format("{}", int(EffectType::normal))));
+      }
+    }
+
     if (std::exchange(change_appended, true))
     {
-      static byte_storage const separator = {ansi::ps};
       wc(separator);
     }
 

include/terminalpp/effect.hpp

@@ -4,6 +4,7 @@
 #include <boost/container_hash/hash.hpp>
 #include <boost/operators.hpp>
 #include <iostream>
+#include <type_traits>
 
 namespace terminalpp {
 
@@ -117,6 +118,15 @@ using underlining = effect<terminalpp::graphics::underlining>;
 using polarity = effect<terminalpp::graphics::polarity>;
 using blinking = effect<terminalpp::graphics::blinking>;
 
+template <typename EffectType>
+struct effect_has_normal : std::bool_constant<false>
+{};
+
+template <>
+struct effect_has_normal<terminalpp::graphics::intensity>
+    : std::bool_constant<true>
+{};
+
 //* =========================================================================
 /// \brief Streaming output operator for intensities.  Prints the text
 /// equivalent of the intensity (e.g. "normal", "bold", "faint").

test/terminal_string_test.cpp

@@ -78,6 +78,12 @@ static streaming_text_data const streaming_text_data_table[] = {
     streaming_text_data{ "\\i>abc"_ets, "\\i=de"_ets,    "\x1B[0mde"_tb },
     streaming_text_data{ "\\i>abc"_ets, "\\ixde"_ets,    "\x1B[0mde"_tb },
 
+    // Bold-faint transitions must go through an intermediate normal state
+    // because certain terminals have a "bold-and-faint" combination that is
+    // not useful.
+    streaming_text_data{ "\\i<a"_ets,   "\\i>b"_ets,     "\x1B[22;1mb"_tb },
+    streaming_text_data{ "\\i>a"_ets,   "\\i<b"_ets,     "\x1B[22;2mb"_tb },
+
     // Test polarity changes
     streaming_text_data{ ""_ets,        "\\p+abcde"_ets, "abcde"_tb },
     streaming_text_data{ ""_ets,        "\\p-abcde"_ets, "\x1B[7mabcde"_tb },