Correctly stream out quantities with a char Rep (#218)

Quantities that have a `char` `Rep` will have the resulting raw number
streamed as a `char`.  Given that a Quantity is a number, users would
expect that a the value would be streamed as the integral number, not
interpreted as a character code.

au/io.hh

@@ -25,7 +25,12 @@ namespace au {
 // Streaming output support for Quantity types.
 template <typename U, typename R>
 std::ostream &operator<<(std::ostream &out, const Quantity<U, R> &q) {
-    out << q.in(U{}) << " " << unit_label(U{});
+    // In the case that the Rep is a type that resolves to 'char' (e.g. int8_t),
+    // the << operator will match the implementation that takes a character
+    // literal.  Using the unary + operator will trigger an integer promotion on
+    // the operand, which will then match an appropriate << operator that will
+    // output the integer representation.
+    out << +q.in(U{}) << " " << unit_label(U{});
     return out;
 }
 

au/io_test.cc

@@ -14,6 +14,8 @@
 
 #include "au/io.hh"
 
+#include <cstdint>
+
 #include "au/prefix.hh"
 #include "au/quantity.hh"
 #include "gtest/gtest.h"
@@ -61,6 +63,14 @@ TEST(StreamingOutput, PrintsValueAndUnitLabel) {
     EXPECT_EQ(stream_to_string((feet / milli(second))(1.25)), "1.25 ft / ms");
 }
 
+TEST(StreamingOutput, PrintValueRepChar) {
+    // If the Rep resolves to a char, we sill want the number '65' to be output,
+    // not the character literal that corresponds to 65 ('A').
+    static_assert(std::is_same<int8_t, signed char>::value,
+                  "Expected 'int8_t' to resolve to 'char'");
+    EXPECT_EQ(stream_to_string(feet(int8_t{65})), "65 ft");
+}
+
 TEST(StreamingOutput, DistinguishesPointFromQuantityByAtSign) {
     EXPECT_EQ(stream_to_string(celsius_qty(20)), "20 deg C");
     EXPECT_EQ(stream_to_string(celsius_pt(20)), "@(20 deg C)");