diff --git a/nix-meson-build-support/common/meson.build b/nix-meson-build-support/common/meson.build
index 1405974d2..5fcf557e7 100644
--- a/nix-meson-build-support/common/meson.build
+++ b/nix-meson-build-support/common/meson.build
@@ -42,6 +42,26 @@ if cxx.get_id() == 'clang'
   add_project_arguments('-fpch-instantiate-templates', language : 'cpp')
 endif
 
+# Detect if we're using libstdc++ (GCC's standard library)
+# libstdc++ uses Intel TBB as backend for C++17 parallel algorithms when <execution> is included.
+# boost::concurrent_flat_map includes <execution>, which would require linking against TBB.
+# Since we don't actually use parallel algorithms, disable the TBB backend to avoid the dependency.
+# TBB is a dependency of blake3 and leaking into our build environment.
+is_using_libstdcxx = cxx.compiles(
+  '''
+  #include <ciso646>
+  #ifndef __GLIBCXX__
+  #error "not libstdc++"
+  #endif
+  int main() { return 0; }
+''',
+  name : 'using libstdc++',
+)
+
+if is_using_libstdcxx
+  add_project_arguments('-D_GLIBCXX_USE_TBB_PAR_BACKEND=0', language : 'cpp')
+endif
+
 # Darwin ld doesn't like "X.Y.ZpreABCD+W"
 nix_soversion = meson.project_version().split('+')[0].split('pre')[0]
 
diff --git a/src/libexpr-tests/error_traces.cc b/src/libexpr-tests/error_traces.cc
index 7e7b5eb84..e722cc484 100644
--- a/src/libexpr-tests/error_traces.cc
+++ b/src/libexpr-tests/error_traces.cc
@@ -139,63 +139,6 @@ TEST_F(ErrorTraceTest, NestedThrows)
 #define ASSERT_DERIVATION_TRACE3(args, type, message, context1, context2) \
     ASSERT_TRACE4(args, type, message, context1, context2, DERIVATION_TRACE_HINTFMT("foo"))
 
-TEST_F(ErrorTraceTest, genericClosure)
-{
-    ASSERT_TRACE2(
-        "genericClosure 1",
-        TypeError,
-        HintFmt("expected a set but found %s: %s", "an integer", Uncolored(ANSI_CYAN "1" ANSI_NORMAL)),
-        HintFmt("while evaluating the first argument passed to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure {}",
-        TypeError,
-        HintFmt("attribute '%s' missing", "startSet"),
-        HintFmt("in the attrset passed as argument to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = 1; }",
-        TypeError,
-        HintFmt("expected a list but found %s: %s", "an integer", Uncolored(ANSI_CYAN "1" ANSI_NORMAL)),
-        HintFmt("while evaluating the 'startSet' attribute passed as argument to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [{ key = 1;}]; operator = true; }",
-        TypeError,
-        HintFmt("expected a function but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt("while evaluating the 'operator' attribute passed as argument to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: true; }",
-        TypeError,
-        HintFmt("expected a list but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt("while evaluating the return value of the `operator` passed to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: [ true ]; }",
-        TypeError,
-        HintFmt("expected a set but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt("while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: [ {} ]; }",
-        TypeError,
-        HintFmt("attribute '%s' missing", "key"),
-        HintFmt("in one of the attrsets generated by (or initially passed to) builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: [{ key = ''a''; }]; }",
-        EvalError,
-        HintFmt("cannot compare %s with %s", "a string", "an integer"),
-        HintFmt("while comparing the `key` attributes of two genericClosure elements"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [ true ]; operator = item: [{ key = ''a''; }]; }",
-        TypeError,
-        HintFmt("expected a set but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt("while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure"));
-}
-
 TEST_F(ErrorTraceTest, replaceStrings)
 {
     ASSERT_TRACE2(
@@ -1050,17 +993,35 @@ TEST_F(ErrorTraceTest, bitXor)
 
 TEST_F(ErrorTraceTest, lessThan)
 {
-    ASSERT_TRACE1("lessThan 1 \"foo\"", EvalError, HintFmt("cannot compare %s with %s", "an integer", "a string"));
+    ASSERT_TRACE1(
+        "lessThan 1 \"foo\"",
+        EvalError,
+        HintFmt(
+            "cannot compare %s with %s; values are %s and %s",
+            "an integer",
+            "a string",
+            Uncolored(ANSI_CYAN "1" ANSI_NORMAL),
+            Uncolored(ANSI_MAGENTA "\"foo\"" ANSI_NORMAL)));
 
     ASSERT_TRACE1(
         "lessThan {} {}",
         EvalError,
-        HintFmt("cannot compare %s with %s; values of that type are incomparable", "a set", "a set"));
+        HintFmt(
+            "cannot compare %s with %s; values of that type are incomparable (values are %s and %s)",
+            "a set",
+            "a set",
+            Uncolored("{ }"),
+            Uncolored("{ }")));
 
     ASSERT_TRACE2(
         "lessThan [ 1 2 ] [ \"foo\" ]",
         EvalError,
-        HintFmt("cannot compare %s with %s", "an integer", "a string"),
+        HintFmt(
+            "cannot compare %s with %s; values are %s and %s",
+            "an integer",
+            "a string",
+            Uncolored(ANSI_CYAN "1" ANSI_NORMAL),
+            Uncolored(ANSI_MAGENTA "\"foo\"" ANSI_NORMAL)),
         HintFmt("while comparing two list elements"));
 }
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index d1aae64fa..0bd03b232 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -681,7 +681,14 @@ struct CompareValues
             if (v1->type() == nInt && v2->type() == nFloat)
                 return v1->integer().value < v2->fpoint();
             if (v1->type() != v2->type())
-                state.error<EvalError>("cannot compare %s with %s", showType(*v1), showType(*v2)).debugThrow();
+                state
+                    .error<EvalError>(
+                        "cannot compare %s with %s; values are %s and %s",
+                        showType(*v1),
+                        showType(*v2),
+                        ValuePrinter(state, *v1, errorPrintOptions),
+                        ValuePrinter(state, *v2, errorPrintOptions))
+                    .debugThrow();
 // Allow selecting a subset of enum values
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wswitch-enum"
@@ -711,7 +718,11 @@ struct CompareValues
             default:
                 state
                     .error<EvalError>(
-                        "cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2))
+                        "cannot compare %s with %s; values of that type are incomparable (values are %s and %s)",
+                        showType(*v1),
+                        showType(*v2),
+                        ValuePrinter(state, *v1, errorPrintOptions),
+                        ValuePrinter(state, *v2, errorPrintOptions))
                     .debugThrow();
 #pragma GCC diagnostic pop
             }
@@ -757,42 +768,79 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value ** ar
        `workSet', adding the result to `workSet', continuing until
        no new elements are found. */
     ValueList res;
-    // `doneKeys' doesn't need to be a GC root, because its values are
-    // reachable from res.
-    auto cmp = CompareValues(state, noPos, "while comparing the `key` attributes of two genericClosure elements");
-    std::set<Value *, decltype(cmp)> doneKeys(cmp);
+    // Track which element each key came from
+    auto cmp = CompareValues(state, noPos, "");
+    std::map<Value *, Value *, decltype(cmp)> keyToElem(cmp);
     while (!workSet.empty()) {
         Value * e = *(workSet.begin());
         workSet.pop_front();
 
-        state.forceAttrs(
-            *e,
-            noPos,
-            "while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure");
+        try {
+            state.forceAttrs(*e, noPos, "");
+        } catch (Error & err) {
+            err.addTrace(nullptr, "in genericClosure element %s", ValuePrinter(state, *e, errorPrintOptions));
+            throw;
+        }
 
-        auto key = state.getAttr(
-            state.s.key,
-            e->attrs(),
-            "in one of the attrsets generated by (or initially passed to) builtins.genericClosure");
+        const Attr * key;
+        try {
+            key = state.getAttr(state.s.key, e->attrs(), "");
+        } catch (Error & err) {
+            err.addTrace(nullptr, "in genericClosure element %s", ValuePrinter(state, *e, errorPrintOptions));
+            throw;
+        }
         state.forceValue(*key->value, noPos);
 
-        if (!doneKeys.insert(key->value).second)
-            continue;
+        try {
+            auto [it, inserted] = keyToElem.insert({key->value, e});
+            if (!inserted)
+                continue;
+        } catch (Error & err) {
+            // Try to find which element we're comparing against
+            Value * otherElem = nullptr;
+            for (auto & [otherKey, elem] : keyToElem) {
+                try {
+                    cmp(key->value, otherKey);
+                } catch (Error &) {
+                    // Found the element we're comparing against
+                    otherElem = elem;
+                    break;
+                }
+            }
+            if (otherElem) {
+                // Traces are printed in reverse order; pre-swap them.
+                err.addTrace(nullptr, "with element %s", ValuePrinter(state, *otherElem, errorPrintOptions));
+                err.addTrace(nullptr, "while comparing element %s", ValuePrinter(state, *e, errorPrintOptions));
+            } else {
+                // Couldn't find the specific element, just show current
+                err.addTrace(nullptr, "while checking key of element %s", ValuePrinter(state, *e, errorPrintOptions));
+            }
+            throw;
+        }
         res.push_back(e);
 
         /* Call the `operator' function with `e' as argument. */
         Value newElements;
-        state.callFunction(*op->value, {&e, 1}, newElements, noPos);
-        state.forceList(
-            newElements,
-            noPos,
-            "while evaluating the return value of the `operator` passed to builtins.genericClosure");
+        try {
+            state.callFunction(*op->value, {&e, 1}, newElements, noPos);
+            state.forceList(
+                newElements,
+                noPos,
+                "while evaluating the return value of the `operator` passed to builtins.genericClosure");
 
-        /* Add the values returned by the operator to the work set. */
-        for (auto elem : newElements.listView()) {
-            state.forceValue(*elem, noPos); // "while evaluating one one of the elements returned by the `operator`
-                                            // passed to builtins.genericClosure");
-            workSet.push_back(elem);
+            /* Add the values returned by the operator to the work set. */
+            for (auto elem : newElements.listView()) {
+                state.forceValue(*elem, noPos); // "while evaluating one one of the elements returned by the `operator`
+                                                // passed to builtins.genericClosure");
+                workSet.push_back(elem);
+            }
+        } catch (Error & err) {
+            err.addTrace(
+                nullptr,
+                "while calling %s on genericClosure element %s",
+                state.symbols[state.s.operator_],
+                ValuePrinter(state, *e, errorPrintOptions));
+            throw;
         }
     }
 
diff --git a/src/libstore/http-binary-cache-store.cc b/src/libstore/http-binary-cache-store.cc
index fdb7c74fc..555da12ea 100644
--- a/src/libstore/http-binary-cache-store.cc
+++ b/src/libstore/http-binary-cache-store.cc
@@ -4,7 +4,6 @@
 #include "nix/store/nar-info-disk-cache.hh"
 #include "nix/util/callback.hh"
 #include "nix/store/store-registration.hh"
-#include "nix/util/compression.hh"
 
 namespace nix {
 
@@ -139,13 +138,14 @@ void HttpBinaryCacheStore::upload(
     RestartableSource & source,
     uint64_t sizeHint,
     std::string_view mimeType,
-    std::optional<std::string_view> contentEncoding)
+    std::optional<Headers> headers)
 {
     auto req = makeRequest(path);
     req.method = HttpMethod::PUT;
 
-    if (contentEncoding) {
-        req.headers.emplace_back("Content-Encoding", *contentEncoding);
+    if (headers) {
+        req.headers.reserve(req.headers.size() + headers->size());
+        std::ranges::move(std::move(*headers), std::back_inserter(req.headers));
     }
 
     req.data = {sizeHint, source};
@@ -154,18 +154,14 @@ void HttpBinaryCacheStore::upload(
     getFileTransfer()->upload(req);
 }
 
-void HttpBinaryCacheStore::upload(std::string_view path, CompressedSource & source, std::string_view mimeType)
-{
-    upload(path, static_cast<RestartableSource &>(source), source.size(), mimeType, source.getCompressionMethod());
-}
-
 void HttpBinaryCacheStore::upsertFile(
     const std::string & path, RestartableSource & source, const std::string & mimeType, uint64_t sizeHint)
 {
     try {
         if (auto compressionMethod = getCompressionMethod(path)) {
             CompressedSource compressed(source, *compressionMethod);
-            upload(path, compressed, mimeType);
+            Headers headers = {{"Content-Encoding", *compressionMethod}};
+            upload(path, compressed, compressed.size(), mimeType, std::move(headers));
         } else {
             upload(path, source, sizeHint, mimeType, std::nullopt);
         }
diff --git a/src/libstore/include/nix/store/http-binary-cache-store.hh b/src/libstore/include/nix/store/http-binary-cache-store.hh
index b092b5b5e..ea3d77b79 100644
--- a/src/libstore/include/nix/store/http-binary-cache-store.hh
+++ b/src/libstore/include/nix/store/http-binary-cache-store.hh
@@ -103,18 +103,7 @@ protected:
         RestartableSource & source,
         uint64_t sizeHint,
         std::string_view mimeType,
-        std::optional<std::string_view> contentEncoding);
-
-    /**
-     * Uploads data to the binary cache (CompressedSource overload).
-     *
-     * This overload infers both the size and compression method from the CompressedSource.
-     *
-     * @param path The path in the binary cache to upload to
-     * @param source The compressed source (knows size and compression method)
-     * @param mimeType The MIME type of the content
-     */
-    void upload(std::string_view path, CompressedSource & source, std::string_view mimeType);
+        std::optional<Headers> headers);
 
     void getFile(const std::string & path, Sink & sink) override;
 
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 37264dfae..51b7a05fc 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -50,7 +50,7 @@ private:
         RestartableSource & source,
         uint64_t sizeHint,
         std::string_view mimeType,
-        std::optional<std::string_view> contentEncoding);
+        std::optional<Headers> headers);
 
     /**
      * Uploads a file to S3 using multipart upload.
@@ -67,7 +67,7 @@ private:
         RestartableSource & source,
         uint64_t sizeHint,
         std::string_view mimeType,
-        std::optional<std::string_view> contentEncoding);
+        std::optional<Headers> headers);
 
     /**
      * A Sink that manages a complete S3 multipart upload lifecycle.
@@ -89,7 +89,7 @@ private:
             std::string_view path,
             uint64_t sizeHint,
             std::string_view mimeType,
-            std::optional<std::string_view> contentEncoding);
+            std::optional<Headers> headers);
 
         void operator()(std::string_view data) override;
         void finish();
@@ -102,8 +102,7 @@ private:
      * @see
      * https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateMultipartUpload.html#API_CreateMultipartUpload_RequestSyntax
      */
-    std::string createMultipartUpload(
-        std::string_view key, std::string_view mimeType, std::optional<std::string_view> contentEncoding);
+    std::string createMultipartUpload(std::string_view key, std::string_view mimeType, std::optional<Headers> headers);
 
     /**
      * Uploads a single part of a multipart upload
@@ -134,18 +133,19 @@ private:
 void S3BinaryCacheStore::upsertFile(
     const std::string & path, RestartableSource & source, const std::string & mimeType, uint64_t sizeHint)
 {
-    auto doUpload = [&](RestartableSource & src, uint64_t size, std::optional<std::string_view> encoding) {
+    auto doUpload = [&](RestartableSource & src, uint64_t size, std::optional<Headers> headers) {
         if (s3Config->multipartUpload && size > s3Config->multipartThreshold) {
-            uploadMultipart(path, src, size, mimeType, encoding);
+            uploadMultipart(path, src, size, mimeType, std::move(headers));
         } else {
-            upload(path, src, size, mimeType, encoding);
+            upload(path, src, size, mimeType, std::move(headers));
         }
     };
 
     try {
         if (auto compressionMethod = getCompressionMethod(path)) {
             CompressedSource compressed(source, *compressionMethod);
-            doUpload(compressed, compressed.size(), compressed.getCompressionMethod());
+            Headers headers = {{"Content-Encoding", *compressionMethod}};
+            doUpload(compressed, compressed.size(), std::move(headers));
         } else {
             doUpload(source, sizeHint, std::nullopt);
         }
@@ -161,7 +161,7 @@ void S3BinaryCacheStore::upload(
     RestartableSource & source,
     uint64_t sizeHint,
     std::string_view mimeType,
-    std::optional<std::string_view> contentEncoding)
+    std::optional<Headers> headers)
 {
     debug("using S3 regular upload for '%s' (%d bytes)", path, sizeHint);
     if (sizeHint > AWS_MAX_PART_SIZE)
@@ -170,7 +170,7 @@ void S3BinaryCacheStore::upload(
             renderSize(sizeHint),
             renderSize(AWS_MAX_PART_SIZE));
 
-    HttpBinaryCacheStore::upload(path, source, sizeHint, mimeType, contentEncoding);
+    HttpBinaryCacheStore::upload(path, source, sizeHint, mimeType, std::move(headers));
 }
 
 void S3BinaryCacheStore::uploadMultipart(
@@ -178,10 +178,10 @@ void S3BinaryCacheStore::uploadMultipart(
     RestartableSource & source,
     uint64_t sizeHint,
     std::string_view mimeType,
-    std::optional<std::string_view> contentEncoding)
+    std::optional<Headers> headers)
 {
     debug("using S3 multipart upload for '%s' (%d bytes)", path, sizeHint);
-    MultipartSink sink(*this, path, sizeHint, mimeType, contentEncoding);
+    MultipartSink sink(*this, path, sizeHint, mimeType, std::move(headers));
     source.drainInto(sink);
     sink.finish();
 }
@@ -191,7 +191,7 @@ S3BinaryCacheStore::MultipartSink::MultipartSink(
     std::string_view path,
     uint64_t sizeHint,
     std::string_view mimeType,
-    std::optional<std::string_view> contentEncoding)
+    std::optional<Headers> headers)
     : store(store)
     , path(path)
 {
@@ -227,7 +227,7 @@ S3BinaryCacheStore::MultipartSink::MultipartSink(
 
     buffer.reserve(chunkSize);
     partEtags.reserve(estimatedParts);
-    uploadId = store.createMultipartUpload(path, mimeType, contentEncoding);
+    uploadId = store.createMultipartUpload(path, mimeType, std::move(headers));
 }
 
 void S3BinaryCacheStore::MultipartSink::operator()(std::string_view data)
@@ -279,7 +279,7 @@ void S3BinaryCacheStore::MultipartSink::uploadChunk(std::string chunk)
 }
 
 std::string S3BinaryCacheStore::createMultipartUpload(
-    std::string_view key, std::string_view mimeType, std::optional<std::string_view> contentEncoding)
+    std::string_view key, std::string_view mimeType, std::optional<Headers> headers)
 {
     auto req = makeRequest(key);
 
@@ -296,8 +296,9 @@ std::string S3BinaryCacheStore::createMultipartUpload(
     req.data = {payload};
     req.mimeType = mimeType;
 
-    if (contentEncoding) {
-        req.headers.emplace_back("Content-Encoding", *contentEncoding);
+    if (headers) {
+        req.headers.reserve(req.headers.size() + headers->size());
+        std::move(headers->begin(), headers->end(), std::back_inserter(req.headers));
     }
 
     auto result = getFileTransfer()->enqueueFileTransfer(req).get();
diff --git a/tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.err.exp b/tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.err.exp
new file mode 100644
index 000000000..a5567cbfc
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.err.exp
@@ -0,0 +1,18 @@
+error:
+       … while calling the 'seq' builtin
+         at /pwd/lang/eval-fail-genericClosure-deeply-nested-element.nix:25:1:
+           24| in
+           25| builtins.seq finiteVal (
+             | ^
+           26|   builtins.genericClosure {
+
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-deeply-nested-element.nix:26:3:
+           25| builtins.seq finiteVal (
+           26|   builtins.genericClosure {
+             |   ^
+           27|     startSet = [
+
+       … in genericClosure element { finite = { a0 = { a1 = { a2 = { a3 = { a4 = { a5 = { a6 = { a7 = { a8 = { ... }; }; }; }; }; }; }; }; }; }; «1 attribute elided» }
+
+       error: attribute 'key' missing
diff --git a/tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.nix b/tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.nix
new file mode 100644
index 000000000..abc0591bb
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.nix
@@ -0,0 +1,35 @@
+let
+  finite = {
+    a0 = {
+      a1 = {
+        a2 = {
+          a3 = {
+            a4 = {
+              a5 = {
+                a6 = {
+                  a7 = {
+                    a8 = {
+                      a9 = "deep";
+                    };
+                  };
+                };
+              };
+            };
+          };
+        };
+      };
+    };
+  };
+  finiteVal = builtins.deepSeq finite finite;
+in
+builtins.seq finiteVal (
+  builtins.genericClosure {
+    startSet = [
+      {
+        infinite = import ./infinite-nesting.nix;
+        finite = finiteVal;
+      }
+    ];
+    operator = x: [ (import ./infinite-nesting.nix) ];
+  }
+)
diff --git a/tests/functional/lang/eval-fail-genericClosure-element-missing-key.err.exp b/tests/functional/lang/eval-fail-genericClosure-element-missing-key.err.exp
new file mode 100644
index 000000000..3ba2a7ea8
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-element-missing-key.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-element-missing-key.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [ { nokey = 1; } ];
+
+       … in genericClosure element { nokey = 1; }
+
+       error: attribute 'key' missing
diff --git a/tests/functional/lang/eval-fail-genericClosure-element-missing-key.nix b/tests/functional/lang/eval-fail-genericClosure-element-missing-key.nix
new file mode 100644
index 000000000..e39e4043b
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-element-missing-key.nix
@@ -0,0 +1,4 @@
+builtins.genericClosure {
+  startSet = [ { nokey = 1; } ];
+  operator = x: [ ];
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-element-not-attrset.err.exp b/tests/functional/lang/eval-fail-genericClosure-element-not-attrset.err.exp
new file mode 100644
index 000000000..b469f6043
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-element-not-attrset.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-element-not-attrset.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [ "not an attrset" ];
+
+       … in genericClosure element "not an attrset"
+
+       error: expected a set but found a string: "not an attrset"
diff --git a/tests/functional/lang/eval-fail-genericClosure-element-not-attrset.nix b/tests/functional/lang/eval-fail-genericClosure-element-not-attrset.nix
new file mode 100644
index 000000000..6850be1c2
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-element-not-attrset.nix
@@ -0,0 +1,4 @@
+builtins.genericClosure {
+  startSet = [ "not an attrset" ];
+  operator = x: [ ];
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.err.exp b/tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.err.exp
new file mode 100644
index 000000000..04b458a48
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.err.exp
@@ -0,0 +1,12 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-keys-incompatible-types.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [
+
+       … while comparing element { key = "string"; }
+
+       … with element { key = 1; }
+
+       error: cannot compare a string with an integer; values are "string" and 1
diff --git a/tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.nix b/tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.nix
new file mode 100644
index 000000000..3335416fd
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.nix
@@ -0,0 +1,7 @@
+builtins.genericClosure {
+  startSet = [
+    { key = 1; }
+    { key = "string"; }
+  ];
+  operator = x: [ ];
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.err.exp b/tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.err.exp
new file mode 100644
index 000000000..97e2bed02
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.err.exp
@@ -0,0 +1,12 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-keys-uncomparable.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [
+
+       … while comparing element { key = { }; }
+
+       … with element { key = { }; }
+
+       error: cannot compare a set with a set; values of that type are incomparable (values are { } and { })
diff --git a/tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.nix b/tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.nix
new file mode 100644
index 000000000..6a1915b6a
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.nix
@@ -0,0 +1,7 @@
+builtins.genericClosure {
+  startSet = [
+    { key = { }; }
+    { key = { }; }
+  ];
+  operator = x: [ ];
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-missing-operator.err.exp b/tests/functional/lang/eval-fail-genericClosure-missing-operator.err.exp
new file mode 100644
index 000000000..0dce0ffd9
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-missing-operator.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-missing-operator.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [ { key = 1; } ];
+
+       … in the attrset passed as argument to builtins.genericClosure
+
+       error: attribute 'operator' missing
diff --git a/tests/functional/lang/eval-fail-genericClosure-missing-operator.nix b/tests/functional/lang/eval-fail-genericClosure-missing-operator.nix
new file mode 100644
index 000000000..0b7c63f6d
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-missing-operator.nix
@@ -0,0 +1,3 @@
+builtins.genericClosure {
+  startSet = [ { key = 1; } ];
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-missing-startSet.err.exp b/tests/functional/lang/eval-fail-genericClosure-missing-startSet.err.exp
new file mode 100644
index 000000000..b68c6542a
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-missing-startSet.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-missing-startSet.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   operator = x: [ ];
+
+       … in the attrset passed as argument to builtins.genericClosure
+
+       error: attribute 'startSet' missing
diff --git a/tests/functional/lang/eval-fail-genericClosure-missing-startSet.nix b/tests/functional/lang/eval-fail-genericClosure-missing-startSet.nix
new file mode 100644
index 000000000..b62802986
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-missing-startSet.nix
@@ -0,0 +1,3 @@
+builtins.genericClosure {
+  operator = x: [ ];
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-not-attrset.err.exp b/tests/functional/lang/eval-fail-genericClosure-not-attrset.err.exp
new file mode 100644
index 000000000..fd3360310
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-not-attrset.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-not-attrset.nix:1:1:
+            1| builtins.genericClosure "not an attrset"
+             | ^
+            2|
+
+       … while evaluating the first argument passed to builtins.genericClosure
+
+       error: expected a set but found a string: "not an attrset"
diff --git a/tests/functional/lang/eval-fail-genericClosure-not-attrset.nix b/tests/functional/lang/eval-fail-genericClosure-not-attrset.nix
new file mode 100644
index 000000000..3998c3432
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-not-attrset.nix
@@ -0,0 +1 @@
+builtins.genericClosure "not an attrset"
diff --git a/tests/functional/lang/eval-fail-genericClosure-operator-not-function.err.exp b/tests/functional/lang/eval-fail-genericClosure-operator-not-function.err.exp
new file mode 100644
index 000000000..d3c5a627a
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-operator-not-function.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-operator-not-function.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [ { key = 1; } ];
+
+       … while evaluating the 'operator' attribute passed as argument to builtins.genericClosure
+
+       error: expected a function but found a string: "not a function"
diff --git a/tests/functional/lang/eval-fail-genericClosure-operator-not-function.nix b/tests/functional/lang/eval-fail-genericClosure-operator-not-function.nix
new file mode 100644
index 000000000..425cd427d
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-operator-not-function.nix
@@ -0,0 +1,4 @@
+builtins.genericClosure {
+  startSet = [ { key = 1; } ];
+  operator = "not a function";
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-operator-not-list.err.exp b/tests/functional/lang/eval-fail-genericClosure-operator-not-list.err.exp
new file mode 100644
index 000000000..49d478033
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-operator-not-list.err.exp
@@ -0,0 +1,12 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-operator-not-list.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [ { key = 1; } ];
+
+       … while calling operator on genericClosure element { key = 1; }
+
+       … while evaluating the return value of the `operator` passed to builtins.genericClosure
+
+       error: expected a list but found a string: "not a list"
diff --git a/tests/functional/lang/eval-fail-genericClosure-operator-not-list.nix b/tests/functional/lang/eval-fail-genericClosure-operator-not-list.nix
new file mode 100644
index 000000000..26f97c51c
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-operator-not-list.nix
@@ -0,0 +1,4 @@
+builtins.genericClosure {
+  startSet = [ { key = 1; } ];
+  operator = x: "not a list";
+}
diff --git a/tests/functional/lang/eval-fail-genericClosure-startSet-not-list.err.exp b/tests/functional/lang/eval-fail-genericClosure-startSet-not-list.err.exp
new file mode 100644
index 000000000..e711a23f5
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-startSet-not-list.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-startSet-not-list.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = "not a list";
+
+       … while evaluating the 'startSet' attribute passed as argument to builtins.genericClosure
+
+       error: expected a list but found a string: "not a list"
diff --git a/tests/functional/lang/eval-fail-genericClosure-startSet-not-list.nix b/tests/functional/lang/eval-fail-genericClosure-startSet-not-list.nix
new file mode 100644
index 000000000..834c82f65
--- /dev/null
+++ b/tests/functional/lang/eval-fail-genericClosure-startSet-not-list.nix
@@ -0,0 +1,4 @@
+builtins.genericClosure {
+  startSet = "not a list";
+  operator = x: [ ];
+}
diff --git a/tests/functional/lang/infinite-nesting.nix b/tests/functional/lang/infinite-nesting.nix
new file mode 100644
index 000000000..1f937e63d
--- /dev/null
+++ b/tests/functional/lang/infinite-nesting.nix
@@ -0,0 +1,4 @@
+let
+  mkInfinite = i: { "a${toString i}" = mkInfinite (i + 1); };
+in
+mkInfinite 0