fix edge case with config files pointer (#1199)

generating a seg fault if no default and no config file provided.

Fixes #1197 

This was likely introduced by the combination of fixes for some issues
with the config parsing and some updates to the as<T> method a while
back. This edge case on the handling of the config pointer with as was
not overlooked in the earlier testing.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: phlptp

include/CLI/Option.hpp

@@ -681,7 +681,15 @@ class Option : public OptionBase<Option> {
         bool retval = false;
         if(current_option_state_ >= option_state::reduced || (results_.size() == 1 && validators_.empty())) {
             const results_t &res = (proc_results_.empty()) ? results_ : proc_results_;
-            retval = detail::lexical_conversion<T, T>(res, output);
+            if(!res.empty()) {
+                retval = detail::lexical_conversion<T, T>(res, output);
+            } else {
+                results_t res2;
+                res2.emplace_back();
+                proc_results_ = std::move(res2);
+                retval = detail::lexical_conversion<T, T>(proc_results_, output);
+            }
+
         } else {
             results_t res;
             if(results_.empty()) {

tests/ConfigFileTest.cpp

@@ -4208,3 +4208,24 @@ TEST_CASE_METHOD(TApp, "RoundTripArrayFloat", "[config]") {
     CHECK(cv[0] == -1.0F);
     CHECK(cv[1] == 1.0F);
 }
+
+// Code from https://github.com/CLIUtils/CLI11/issues/1197
+TEST_CASE_METHOD(TApp, "CrashTest", "[config]") {
+    args = {"spdlog", "--level=off"};
+
+    app.configurable()->allow_config_extras(false);
+    app.set_config("--conf")->check(CLI::ExistingFile);
+
+    std::string level;
+
+    auto *command = app.add_subcommand("spdlog");
+    command->add_option("--level", level, "Log level")->default_val("info");
+
+    run();
+
+    auto *ptr = app.get_config_ptr();
+    std::string conf_filename;
+    CHECK_NOTHROW(conf_filename = ptr->as<std::string>());
+    CHECK(conf_filename.empty());
+    CHECK(level == "off");
+}

tests/HelpersTest.cpp

@@ -1516,6 +1516,14 @@ TEST_CASE("Types: LexicalConversionVectorDouble", "[helpers]") {
     CHECK(-3.54 == Approx(x[2]));
 }
 
+TEST_CASE("Types: LexicalConversionEmptyVectorDouble", "[helpers]") {
+    CLI::results_t input = {};
+    std::vector<double> x;
+    bool res = CLI::detail::lexical_conversion<std::vector<double>, std::vector<double>>(input, x);
+    CHECK(res);
+    CHECK(x.empty());
+}
+
 static_assert(!CLI::detail::is_tuple_like<std::vector<double>>::value, "vector should not be like a tuple");
 static_assert(CLI::detail::is_tuple_like<std::pair<double, double>>::value, "pair of double should be like a tuple");
 static_assert(CLI::detail::is_tuple_like<std::array<double, 4>>::value, "std::array<double,4> should be like a tuple");