fix: encode_one signed char right-shift

Author: alandefreitas

include/boost/url/detail/impl/format_args.hpp

@@ -212,8 +212,9 @@ encode_one(
         return;
     }
     *out++ = '%';
-    *out++ = urls::detail::hexdigs[0][c>>4];
-    *out++ = urls::detail::hexdigs[0][c&0xf];
+    auto uc = static_cast<unsigned char>(c);
+    *out++ = urls::detail::hexdigs[0][uc>>4];
+    *out++ = urls::detail::hexdigs[0][uc&0xf];
 }
 
 // get an unsigned value from format_args

test/unit/format.cpp

@@ -954,6 +954,19 @@ struct format_test
         }
     }
 
+    void
+    testHighByteEncode()
+    {
+        // signed char shift UB regression: encoding
+        // a byte with the high bit set (e.g. 0x80)
+        {
+            url u = urls::format("/{}/",
+                std::string(1, '\x80'));
+            auto p = u.encoded_path();
+            BOOST_TEST(p.find("%80") != core::string_view::npos);
+        }
+    }
+
     void
     run()
     {
@@ -965,6 +978,7 @@ struct format_test
         testLLONGMIN();
         testCenterAlignPad();
         testColonInFirstSegment();
+        testHighByteEncode();
 #endif
     }
 };