Generate random rng seed if user did not specify one

Closes #2161

src/catch2/catch_config.cpp

@@ -72,7 +72,7 @@ namespace Catch {
     ShowDurations Config::showDurations() const        { return m_data.showDurations; }
     double Config::minDuration() const                 { return m_data.minDuration; }
     TestRunOrder Config::runOrder() const              { return m_data.runOrder; }
-    unsigned int Config::rngSeed() const               { return m_data.rngSeed; }
+    unsigned int Config::rngSeed() const               { return *m_data.rngSeed; }
     UseColour Config::useColour() const                { return m_data.useColour; }
     bool Config::shouldDebugBreak() const              { return m_data.shouldDebugBreak; }
     int Config::abortAfter() const                     { return m_data.abortAfter; }

src/catch2/catch_config.hpp

@@ -11,6 +11,7 @@
 #include <catch2/catch_test_spec.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/internal/catch_unique_ptr.hpp>
+#include <catch2/internal/catch_optional.hpp>
 
 #include <vector>
 #include <string>
@@ -33,7 +34,7 @@ namespace Catch {
         bool libIdentify = false;
 
         int abortAfter = -1;
-        unsigned int rngSeed = 0;
+        Optional<unsigned int> rngSeed;
 
         bool benchmarkNoAnalysis = false;
         unsigned int benchmarkSamples = 100;

src/catch2/catch_session.cpp

@@ -24,6 +24,7 @@
 #include <catch2/internal/catch_move_and_forward.hpp>
 
 #include <algorithm>
+#include <ctime>
 #include <iomanip>
 #include <set>
 
@@ -170,6 +171,10 @@ namespace Catch {
         if( m_startupExceptions )
             return 1;
 
+        if (!m_configData.rngSeed) {
+            m_configData.rngSeed = static_cast<unsigned int>(std::time(nullptr));
+        }
+
         auto result = m_cli.parse( Clara::Args( argc, argv ) );
         if( !result ) {
             config();
@@ -187,6 +192,7 @@ namespace Catch {
             showHelp();
         if( m_configData.libIdentify )
             libIdentify();
+
         m_config.reset();
         return 0;
     }
@@ -256,6 +262,7 @@ namespace Catch {
             return 0;
         }
 
+
         CATCH_TRY {
             config(); // Force config to be constructed
 

src/catch2/internal/catch_commandline.cpp

@@ -7,6 +7,7 @@
 // SPDX-License-Identifier: BSL-1.0
 #include <catch2/internal/catch_commandline.hpp>
 
+#include <catch2/internal/catch_compiler_capabilities.hpp>
 #include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 #include <catch2/interfaces/catch_interfaces_registry_hub.hpp>
@@ -15,6 +16,7 @@
 
 #include <fstream>
 #include <ctime>
+#include <string>
 
 namespace Catch {
 
@@ -71,10 +73,28 @@ namespace Catch {
                 return ParserResult::ok( ParseResultType::Matched );
             };
         auto const setRngSeed = [&]( std::string const& seed ) {
-                if( seed != "time" )
-                    return Clara::Detail::convertInto( seed, config.rngSeed );
-                config.rngSeed = static_cast<unsigned int>( std::time(nullptr) );
-                return ParserResult::ok( ParseResultType::Matched );
+                if( seed == "time" ) {
+                    config.rngSeed = static_cast<unsigned int>(std::time(nullptr));
+                    return ParserResult::ok(ParseResultType::Matched);
+                }
+
+                CATCH_TRY {
+                    std::size_t parsedTo = 0;
+                    unsigned long parsedSeed = std::stoul(seed, &parsedTo, 0);
+                    if (parsedTo != seed.size()) {
+                        return ParserResult::runtimeError("Could not parse '" + seed + "' as seed");
+                    }
+
+                    // TODO: Ideally we could parse unsigned int directly,
+                    //       but the stdlib doesn't provide helper for that
+                    //       type. After this is refactored to use fixed size
+                    //       type, we should check the parsed value is in range
+                    //       of the underlying type.
+                    config.rngSeed = static_cast<unsigned int>(parsedSeed);
+                    return ParserResult::ok(ParseResultType::Matched);
+                } CATCH_CATCH_ANON(std::exception const&) {
+                    return ParserResult::runtimeError("Could not parse '" + seed + "' as seed");
+                }
             };
         auto const setColourUsage = [&]( std::string const& useColour ) {
                     auto mode = toLower( useColour );