1 files changed, 23 insertions, 0 deletions
diff --git a/tests/unit/libutil/fmt.cc b/tests/unit/libutil/fmt.cc
new file mode 100644
index 000000000..383a688d3
--- /dev/null
+++ b/tests/unit/libutil/fmt.cc
@@ -0,0 +1,23 @@
+#include "fmt.hh"
+#include "ansicolor.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+TEST(HintFmt, arg_count)
+{
+    // Single arg is treated as a literal string.
+    ASSERT_EQ(HintFmt("%s").str(), "%s");
+
+    // Other strings format as expected:
+    ASSERT_EQ(HintFmt("%s", 1).str(), ANSI_MAGENTA "1" ANSI_NORMAL);
+    ASSERT_EQ(HintFmt("%1%", "hello").str(), ANSI_MAGENTA "hello" ANSI_NORMAL);
+
+    // Argument counts are detected at construction.
+    ASSERT_THROW(HintFmt("%s %s", 1), boost::io::too_few_args);
+
+    ASSERT_THROW(HintFmt("%s", 1, 2), boost::io::too_many_args);
+}
+
+}