feat: added an option to disable space before unit symbol in the text output

Resolves #387

docs/users_guide/framework_basics/text_output.md

@@ -13,7 +13,36 @@ any quantity in the most user-friendly way.
     a much better solution, but the library does not have enough information to print it that way by itself.
 
 
-## Output Streams
+## Customization point
+
+The [SI Brochure](../appendix/references.md#SIBrochure) says:
+
+!!! quote "SI Brochure"
+
+    The numerical value always precedes the unit and a space is always used to separate the unit from
+    the number. ... The only exceptions to this rule are for the unit symbols for degree, minute and
+    second for plane angle, `°`, `′` and `″`, respectively, for which no space is left between the
+    numerical value and the unit symbol.
+
+To support the above, the library exposes `space_before_unit_symbol` customization point. By default,
+its value is `true` for all the units, so the space between a number and a unit will be present in the
+output text. To change this behavior, we have to provide a partial specialization for a specific unit:
+
+```cpp
+template<>
+inline constexpr bool space_before_unit_symbol<non_si::degree> = false;
+```
+
+!!! note
+
+    The above works only for [the default formatting](#default-formatting). In case we provide our own
+    format specification (i.e. `std::format("{:%Q %q}", q)`), the library will always obey this
+    specification for all the units (no matter of what is the actual value of the
+    `space_before_unit_symbol` customization point) and the separating space will always be present
+    in this case.
+
+
+## Output streams
 
 !!! tip
 

src/core-fmt/include/mp-units/format.h

@@ -398,7 +398,7 @@ private:
       // default format should print value followed by the unit separated with 1 space
       out = mp_units::detail::format_units_quantity_value<CharT>(out, q.numerical_value(), specs.rep, ctx.locale());
       if constexpr (mp_units::detail::has_unit_symbol(get_unit(Reference))) {
-        *out++ = CharT(' ');
+        if constexpr (mp_units::space_before_unit_symbol<get_unit(Reference)>) *out++ = CharT(' ');
         out = unit_symbol_to<CharT>(out, get_unit(Reference));
       }
     } else {

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

@@ -40,7 +40,7 @@ void to_stream(std::basic_ostream<CharT, Traits>& os, const quantity<R, Rep>& q)
   else
     os << q.numerical_value();
   if constexpr (has_unit_symbol(get_unit(R))) {
-    os << " ";
+    if constexpr (space_before_unit_symbol<get_unit(R)>) os << " ";
     unit_symbol_to<CharT>(std::ostream_iterator<CharT>(os), get_unit(R));
   }
 }

src/core/include/mp-units/unit.h

@@ -807,6 +807,15 @@ constexpr Out unit_symbol_impl(Out out, const derived_unit<Expr...>&, unit_symbo
 
 }  // namespace detail
 
+/**
+ * @brief Puts a space ' ' sign before a unit symbol
+ *
+ * Quantities of some units (e.g. degree, arcminute, arcsecond) should not be printed with the
+ * space between a number and a unit. For those a partial specialization with the value `false` should
+ * be provided.
+ */
+template<Unit auto U>
+inline constexpr bool space_before_unit_symbol = true;
 
 template<typename CharT = char, std::output_iterator<CharT> Out, Unit U>
 constexpr Out unit_symbol_to(Out out, U u, unit_symbol_formatting fmt = unit_symbol_formatting{})

src/systems/si/include/mp-units/systems/si/units.h

@@ -123,4 +123,11 @@ inline constexpr bool unit_can_be_prefixed<non_si::hour> = false;
 template<>
 inline constexpr bool unit_can_be_prefixed<non_si::day> = false;
 
+template<>
+inline constexpr bool space_before_unit_symbol<non_si::degree> = false;
+template<>
+inline constexpr bool space_before_unit_symbol<non_si::arcminute> = false;
+template<>
+inline constexpr bool space_before_unit_symbol<non_si::arcsecond> = false;
+
 }  // namespace mp_units

test/unit_test/runtime/fmt_test.cpp

@@ -236,6 +236,45 @@ TEST_CASE("operator<< on a quantity", "[text][ostream][fmt]")
     }
   }
 
+  SECTION("no space before unit symbol")
+  {
+    SECTION("degree")
+    {
+      const auto q = 42 * deg;
+      os << q;
+
+      SECTION("iostream") { CHECK(os.str() == "42°"); }
+
+      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) == "42 °"); }
+    }
+
+    SECTION("arcminute")
+    {
+      const auto q = 42 * arcmin;
+      os << q;
+
+      SECTION("iostream") { CHECK(os.str() == "42′"); }
+
+      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) == "42 ′"); }
+    }
+
+    SECTION("arcsecond")
+    {
+      const auto q = 42 * arcsec;
+      os << q;
+
+      SECTION("iostream") { CHECK(os.str() == "42″"); }
+
+      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) == "42 ″"); }
+    }
+  }
+
   SECTION("8-bit integers")
   {
     SECTION("signed positive")