From b08cee18c424fdb9c365d37b820d349d872bbccd Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Thu, 8 Aug 2024 16:09:10 -0600
Subject: [PATCH] :children_crossing: Improve error message for cyclic flows

---
 .github/workflows/unit_tests.yml |  6 +++---
 include/flow/graph_builder.hpp   |  3 ++-
 test/CMakeLists.txt              |  1 +
 test/flow/fail/cyclic_flow.cpp   | 23 +++++++++++++++++++++++
 4 files changed, 29 insertions(+), 4 deletions(-)
 create mode 100644 test/flow/fail/cyclic_flow.cpp

diff --git a/.github/workflows/unit_tests.yml b/.github/workflows/unit_tests.yml
index 2c0f6e73..3bbb31aa 100644
--- a/.github/workflows/unit_tests.yml
+++ b/.github/workflows/unit_tests.yml
@@ -86,7 +86,7 @@ jobs:
             cxx_flags: ""
           - version: 13
             compiler: gcc
-            install: sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test && sudo apt update && sudo apt-get install -y gcc-13 g++-13
+            install: sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test && sudo apt update && sudo apt install -y gcc-13 g++-13
             cc: "gcc-13"
             cxx: "g++-13"
           - version: 12
@@ -232,7 +232,7 @@ jobs:
           - compiler: gcc
             cc: "gcc-13"
             cxx: "g++-13"
-            install: sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test && sudo apt update && sudo apt-get install -y gcc-13 g++-13
+            install: sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test && sudo apt update && sudo apt install -y gcc-13 g++-13
             toolchain_root: "/usr"
 
     steps:
@@ -288,7 +288,7 @@ jobs:
       - name: Install build tools
         run: |
           sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
-          sudo apt update && sudo apt-get install -y gcc-${{env.DEFAULT_GCC_VERSION}} g++-${{env.DEFAULT_GCC_VERSION}} ninja-build valgrind
+          sudo apt update && sudo apt install -y gcc-${{env.DEFAULT_GCC_VERSION}} g++-${{env.DEFAULT_GCC_VERSION}} ninja-build valgrind
 
       - name: Restore CPM cache
         env:
diff --git a/include/flow/graph_builder.hpp b/include/flow/graph_builder.hpp
index d124c80b..83604675 100644
--- a/include/flow/graph_builder.hpp
+++ b/include/flow/graph_builder.hpp
@@ -147,7 +147,8 @@ struct graph_builder {
         constexpr static auto built() {
             constexpr auto v = Initialized::value;
             constexpr auto built = build(v);
-            static_assert(built.has_value());
+            static_assert(built.has_value(),
+                          "Topological sort failed: cycle in flow");
             return *built;
         }
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 8263d957..54a93eab 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -119,6 +119,7 @@ add_unit_test(
     catalog2_lib
     catalog_strings)
 
+add_compile_fail_test(flow/fail/cyclic_flow.cpp LIBRARIES warnings cib)
 add_compile_fail_test(msg/fail/callback_bad_field_name.cpp LIBRARIES warnings
                       cib)
 add_compile_fail_test(msg/fail/field_location.cpp LIBRARIES warnings cib)
diff --git a/test/flow/fail/cyclic_flow.cpp b/test/flow/fail/cyclic_flow.cpp
new file mode 100644
index 00000000..c0810291
--- /dev/null
+++ b/test/flow/fail/cyclic_flow.cpp
@@ -0,0 +1,23 @@
+#include <cib/cib.hpp>
+#include <flow/flow.hpp>
+
+// EXPECT: Topological sort failed: cycle in flow
+
+namespace {
+using namespace flow::literals;
+
+constexpr auto a = flow::milestone<"a">();
+constexpr auto b = flow::milestone<"b">();
+
+struct TestFlowAlpha : public flow::service<> {};
+
+struct CyclicFlowConfig {
+    constexpr static auto config = cib::config(
+        cib::exports<TestFlowAlpha>, cib::extend<TestFlowAlpha>(a >> b >> a));
+};
+} // namespace
+
+auto main() -> int {
+    cib::nexus<CyclicFlowConfig> nexus{};
+    nexus.service<TestFlowAlpha>();
+}