[Types] Fix missing inverse check for described field in isValidSupertype

[sumleo]

Summary

• Add the missing else branch in isValidSupertype for the described field check, making it symmetric with the existing descriptor field check.
• Without this fix, a type without a describes clause can illegally subtype a type that has a describes clause (i.e., a descriptor type), as long as all features are enabled.
• The descriptor check already handles both directions (if sub has no descriptor, super must also have no descriptor), but the described check was missing the inverse direction.

Details

In isValidSupertype (src/wasm/wasm-type.cpp), the validation for descriptor is fully symmetric:

if (sub.descriptor) {
    if (super.descriptor && sub.descriptor->supertype != super.descriptor) {
        return false;
    }
} else {
    if (super.descriptor) { return false; }
}

But the described check was missing the else branch:

if (sub.described) {
    if (!super.described || sub.described->supertype != super.described) {
        return false;
    }
}
// Missing: else { if (super.described) { return false; } }

The spec test at test/spec/descriptors.wast (lines 152-161) already covers this case with an assert_invalid for "supertype of non-descriptor type cannot be a descriptor". However, the test currently passes for the wrong reason: wasm-shell re-parses assert_invalid modules as quoted text modules without features enabled, so TypeBuilder::build() rejects the module with RequiresCustomDescriptors before ever reaching isValidSupertype. When the module is loaded through the normal inline parse path (with all features enabled), the invalid subtyping relationship is incorrectly accepted.

Test plan

• Verified that wasm-shell test/spec/descriptors.wast passes all 14 checks (including the one at line 152)
• Verified that all 309 GTest unit tests pass

[kripken]

I believe this is not correct, but the spec changed and I'm not sure - @tlively ?

[tlively]

This is correct! We do not allow this:

(type $foo (descriptor $super) (struct))
(type $super (sub (describes $foo) (struct)))
(type $sub (sub $super) (struct))

[tlively]

However, the test currently passes for the wrong reason: wasm-shell re-parses assert_invalid modules as quoted text modules without features enabled, so TypeBuilder::build() rejects the module with RequiresCustomDescriptors before ever reaching isValidSupertype.

That seems pretty bad. Would you be able to upload a fix for that as well?