fix: text output of 8-bit integers fixed (resolves #470)

src/core-io/include/mp-units/ostream.h

@@ -34,7 +34,11 @@ namespace detail {
 template<typename CharT, class Traits, auto R, typename Rep>
 void to_stream(std::basic_ostream<CharT, Traits>& os, const quantity<R, Rep>& q)
 {
-  os << q.number();
+  if constexpr (is_same_v<Rep, std::uint8_t> || is_same_v<Rep, std::int8_t>)
+    // promote the value to int
+    os << +q.number();
+  else
+    os << q.number();
   if constexpr (has_unit_symbol(get_unit(R))) {
     os << " ";
     unit_symbol_to<CharT>(std::ostream_iterator<CharT>(os), get_unit(R));

test/unit_test/runtime/fmt_test.cpp

@@ -197,7 +197,6 @@ TEST_CASE("operator<< on a quantity", "[text][ostream][fmt]")
     }
   }
 
-
   SECTION("dimensionless quantity")
   {
     SECTION("one with ratio == 1")
@@ -236,6 +235,45 @@ TEST_CASE("operator<< on a quantity", "[text][ostream][fmt]")
       SECTION("fmt with format {:%Q %q} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{:%Q %q}", q) == os.str()); }
     }
   }
+
+  SECTION("8-bit integers")
+  {
+    SECTION("signed positive")
+    {
+      const auto q = std::int8_t{42} * si::second;
+      os << q;
+
+      SECTION("iostream") { CHECK(os.str() == "42 s"); }
+
+      SECTION("fmt with default format {} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{}", q) == os.str()); }
+
+      SECTION("fmt with format {:%Q %q} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{:%Q %q}", q) == os.str()); }
+    }
+
+    SECTION("signed negative")
+    {
+      const auto q = std::int8_t{-42} * si::second;
+      os << q;
+
+      SECTION("iostream") { CHECK(os.str() == "-42 s"); }
+
+      SECTION("fmt with default format {} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{}", q) == os.str()); }
+
+      SECTION("fmt with format {:%Q %q} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{:%Q %q}", q) == os.str()); }
+    }
+
+    SECTION("unsigned")
+    {
+      const auto q = std::uint8_t{42} * si::second;
+      os << q;
+
+      SECTION("iostream") { CHECK(os.str() == "42 s"); }
+
+      SECTION("fmt with default format {} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{}", q) == os.str()); }
+
+      SECTION("fmt with format {:%Q %q} on a quantity") { CHECK(MP_UNITS_STD_FMT::format("{:%Q %q}", q) == os.str()); }
+    }
+  }
 }
 
 TEST_CASE("format string with only %Q should print quantity value only", "[text][fmt]")