[Wasm GC] Support and fuzz function subtyping (#5420)

Support function subtyping with contravariant parameters and covariant results.
The actual change is a single line in wasm-type.cpp, so most of the patch is
updating the type fuzzer to generate interesting function subtypes. Since
function parameters are covariant, generating a function subtype requires
generating supertypes of its parameter types, which required new functionality
in the fuzzer. Also update the fuzzer to choose to reuse types at a finer grain,
so for example individual function parameters or results might be reused
unmodified while other parameters or results are still modified.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/tools/fuzzing/fuzzing.cpp b/src/tools/fuzzing/fuzzing.cpp
index 98f9ea733..d91266d49 100644
--- a/src/tools/fuzzing/fuzzing.cpp
+++ b/src/tools/fuzzing/fuzzing.cpp
@@ -1967,7 +1967,7 @@ Expression* TranslateToFuzzReader::makeRefFuncConst(Type type) {
   }
   // TODO: randomize the order
   for (auto& func : wasm.functions) {
-    if (Type::isSubType(type, Type(func->type, NonNullable))) {
+    if (Type::isSubType(Type(func->type, NonNullable), type)) {
       return builder.makeRefFunc(func->name, func->type);
     }
   }