Fixes #80.

c++/src/kj/common-test.c++

@@ -286,6 +286,10 @@ TEST(Common, MinMaxValue) {
 
   f = nan();
   EXPECT_FALSE(f == f);
+
+  // `char`'s signedness is platform-specific.
+  EXPECT_LE(char(minValue), '\0');
+  EXPECT_GE(char(maxValue), '\x7f');
 }
 
 TEST(Common, Defer) {

c++/src/kj/common.h

@@ -416,6 +416,13 @@ public:
   _kJ_HANDLE_TYPE(long)
   _kJ_HANDLE_TYPE(long long)
 #undef _kJ_HANDLE_TYPE
+
+  inline constexpr operator char() const {
+    // `char` is different from both `signed char` and `unsigned char`, and may be signed or
+    // unsigned on different platforms.  Ugh.
+    return char(-1) < 0 ? MaxValue_::maxSigned<char>()
+                        : MaxValue_::maxUnsigned<char>();
+  }
 };
 
 class MinValue_ {
@@ -439,6 +446,13 @@ public:
   _kJ_HANDLE_TYPE(long)
   _kJ_HANDLE_TYPE(long long)
 #undef _kJ_HANDLE_TYPE
+
+  inline constexpr operator char() const {
+    // `char` is different from both `signed char` and `unsigned char`, and may be signed or
+    // unsigned on different platforms.  Ugh.
+    return char(-1) < 0 ? MinValue_::minSigned<char>()
+                        : MinValue_::minUnsigned<char>();
+  }
 };
 
 static constexpr MaxValue_ maxValue = MaxValue_();