libmain: fix UB in verbosity assignment

This was generating an out-of-range verbosity value. We should just
process it as an int and then convert to verbosity with a clamping
function, which trivially avoids any domain type violations.

Change-Id: I0ed20da8e1496a1225ff3008b76827d99265d404

src/libmain/common-args.cc

@@ -1,5 +1,6 @@
 #include "common-args.hh"
 #include "args/root.hh"
+#include "error.hh"
 #include "globals.hh"
 #include "loggers.hh"
 #include "logging.hh"
@@ -14,14 +15,14 @@ MixCommonArgs::MixCommonArgs(const std::string & programName)
         .shortName = 'v',
         .description = "Increase the logging verbosity level.",
         .category = loggingCategory,
-        .handler = {[]() { verbosity = (Verbosity) (verbosity + 1); }},
+        .handler = {[]() { verbosity = verbosityFromIntClamped(int(verbosity) + 1); }},
     });
 
     addFlag({
         .longName = "quiet",
         .description = "Decrease the logging verbosity level.",
         .category = loggingCategory,
-        .handler = {[]() { verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError; }},
+        .handler = {[]() { verbosity = verbosityFromIntClamped(int(verbosity) - 1); }},
     });
 
     addFlag({

src/libutil/error.hh

@@ -45,6 +45,8 @@ typedef enum {
     lvlVomit
 } Verbosity;
 
+Verbosity verbosityFromIntClamped(int val);
+
 /**
  * The lines of code surrounding an error.
  */

src/libutil/logging.cc

@@ -5,6 +5,7 @@
 #include "position.hh"
 #include "terminal.hh"
 
+#include <algorithm>
 #include <atomic>
 #include <sstream>
 #include <nlohmann/json.hpp>
@@ -110,6 +111,12 @@ public:
 
 Verbosity verbosity = lvlInfo;
 
+Verbosity verbosityFromIntClamped(int val)
+{
+    int clamped = std::clamp(val, int(lvlError), int(lvlVomit));
+    return static_cast<Verbosity>(clamped);
+}
+
 void writeToStderr(std::string_view s)
 {
     try {