diff --git a/src/libstore/tests/derivation.cc b/src/libstore/tests/derivation.cc
index 29d5693db..7becfa5ab 100644
--- a/src/libstore/tests/derivation.cc
+++ b/src/libstore/tests/derivation.cc
@@ -87,7 +87,7 @@ TEST_F(DynDerivationTest, BadATerm_oldVersionDynDeps) {
     }                                                                     \
                                                                           \
     TEST_F(FIXTURE, DerivationOutput_ ## NAME ## _to_json) {              \
-        writeTest<json>("output-" #NAME ".json", [&]() -> json {          \
+        writeTest("output-" #NAME ".json", [&]() -> json {                \
             return DerivationOutput { (VAL) }.toJSON(                     \
                 *store,                                                   \
                 (DRV_NAME),                                               \
@@ -165,7 +165,7 @@ TEST_JSON(ImpureDerivationTest, impure,
     }                                                                     \
                                                                           \
     TEST_F(FIXTURE, Derivation_ ## NAME ## _to_json) {                    \
-        writeTest<json>(#NAME ".json", [&]() -> json {                    \
+        writeTest(#NAME ".json", [&]() -> json {                          \
             return Derivation { VAL }.toJSON(*store);                     \
         }, [](const auto & file) {                                        \
             return json::parse(readFile(file));                           \
diff --git a/src/libutil/tests/characterization.hh b/src/libutil/tests/characterization.hh
index 10c8b4f7e..6698c5239 100644
--- a/src/libutil/tests/characterization.hh
+++ b/src/libutil/tests/characterization.hh
@@ -66,16 +66,12 @@ public:
      * @param test hook that produces contents of the file and does the
      * actual work
      */
-    template<typename T>
     void writeTest(
-        PathView testStem,
-        std::invocable<> auto && test,
-        std::invocable<const Path &> auto && readFile2,
-        std::invocable<const Path &, const T &> auto && writeFile2)
+        PathView testStem, auto && test, auto && readFile2, auto && writeFile2)
     {
         auto file = goldenMaster(testStem);
 
-        T got = test();
+        auto got = test();
 
         if (testAccept())
         {
@@ -87,7 +83,7 @@ public:
         }
         else
         {
-            T expected = readFile2(file);
+            decltype(got) expected = readFile2(file);
             ASSERT_EQ(got, expected);
         }
     }
@@ -97,7 +93,7 @@ public:
      */
     void writeTest(PathView testStem, auto && test)
     {
-        writeTest<std::string>(
+        writeTest(
             testStem, test,
             [](const Path & f) -> std::string {
                 return readFile(f);