flake.nix

@@ -99,9 +99,10 @@
       ];
 
       stdenvs = [
-        "gccStdenv"
+        # see assertion in package.nix why these two are disabled
+        # "stdenv"
+        # "gccStdenv"
         "clangStdenv"
-        "stdenv"
         "libcxxStdenv"
         "ccacheStdenv"
       ];
@@ -121,7 +122,11 @@
             name = "${stdenvName}Packages";
             value = f stdenvName;
           }) stdenvs
-        );
+        )
+        // {
+          # TODO delete this and reënable gcc stdenvs once gcc compiles kj coros correctly
+          stdenvPackages = f "clangStdenv";
+        };
 
       # Memoize nixpkgs for different platforms for efficiency.
       nixpkgsFor = forAllSystems (

meson.build

@@ -167,10 +167,18 @@ endif
 # frees one would expect when the objects are unique_ptrs. these problems
 # often show up as memory corruption when nesting generators (since we do
 # treat generators like owned memory) and will cause inexplicable crashs.
+#
+# gcc 13 does not compile capnp coroutine code correctly. a newer version
+# may fix this. (cf. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102051)
+# we allow gcc 13 here anyway because CI uses it for clang-tidy, and when
+# the compiler crashes outright if won't produce any bad binaries either.
 assert(
   cxx.get_id() != 'gcc' or cxx.version().version_compare('>=13'),
-  'GCC 12 and earlier are known to miscompile lix coroutines, use GCC 13 or clang.'
+  'GCC is known to miscompile coroutines, use clang.'
 )
+if cxx.get_id() == 'gcc'
+  warning('GCC is known to crash while building coroutines, use clang.')
+endif
 
 
 # Translate some historical and Mesony CPU names to Lixy CPU names.

package.nix

@@ -90,6 +90,10 @@
     capnproto-lix = capnproto.overrideAttrs { CXXFLAGS = "-std=c++20"; };
   },
 }:
+
+# gcc miscompiles coroutines at least until 13.2, possibly longer
+assert stdenv.cc.isClang || lintInsteadOfBuild;
+
 let
   inherit (__forDefaults) canRunInstalled;
   inherit (lib) fileset;