2 files changed, 13 insertions, 12 deletions
diff --git a/src/buildtool/common/cli.hpp b/src/buildtool/common/cli.hpp
index 865ef500..da67d8e7 100644
--- a/src/buildtool/common/cli.hpp
+++ b/src/buildtool/common/cli.hpp
@@ -51,7 +51,7 @@ struct LogArguments {
 /// \brief Arguments required for analysing targets.
 struct AnalysisArguments {
     std::optional<std::size_t> expression_log_limit{};
-    std::string defines{};
+    std::vector<std::string> defines{};
     std::filesystem::path config_file{};
     std::optional<nlohmann::json> target{};
     std::optional<std::string> request_action_input{};
@@ -218,12 +218,14 @@ static inline auto SetupAnalysisArguments(
                                 "in error messages (Default {})",
                                 Evaluator::kDefaultExpressionLogLimit))
         ->type_name("NUM");
-    app->add_option(
+    app->add_option_function<std::string>(
            "-D,--defines",
-           clargs->defines,
+           [clargs](auto const& d) { clargs->defines.emplace_back(d); },
            "Define an overlay configuration via an in-line JSON object."
-           "Latest wins.")
-        ->type_name("JSON");
+           " Multiple options overlay.")
+        ->type_name("JSON")
+        ->trigger_on_parse();  // run callback on all instances while parsing,
+                               // not after all parsing is done
     app->add_option(
            "-c,--config", clargs->config_file, "Path to configuration file.")
         ->type_name("PATH");
diff --git a/src/buildtool/main/main.cpp b/src/buildtool/main/main.cpp
index 98812abd..869ed146 100644
--- a/src/buildtool/main/main.cpp
+++ b/src/buildtool/main/main.cpp
@@ -487,21 +487,20 @@ void SetupHashFunction() {
         }
     }
 
-    if (not clargs.defines.empty()) {
+    for (auto const& s : clargs.defines) {
         try {
-            auto map =
-                Expression::FromJson(nlohmann::json::parse(clargs.defines));
+            auto map = Expression::FromJson(nlohmann::json::parse(s));
             if (not map->IsMap()) {
                 Logger::Log(LogLevel::Error,
-                            "Defines {} does not contain a map.",
-                            clargs.defines);
+                            "Defines entry {} does not contain a map.",
+                            nlohmann::json(s).dump());
                 std::exit(kExitFailure);
             }
             config = config.Update(map);
         } catch (std::exception const& e) {
             Logger::Log(LogLevel::Error,
-                        "Parsing defines {} failed with error:\n{}",
-                        clargs.defines,
+                        "Parsing defines entry {} failed with error:\n{}",
+                        nlohmann::json(s).dump(),
                         e.what());
             std::exit(kExitFailure);
         }