heh, JSC allows it

$ eshost -h JavaScriptCore -sx '
  const iter = (function*(){ return (class { static foo = yield }).foo; })(); 
  iter.next();
  print(JSON.stringify(iter.next(42)));
'
#### JavaScriptCore
{"done":true}

yes:

eshost -h JavaScriptCore -sx '
  const iter = (function*(){ return (new class { foo = yield }).foo; })();
  iter.next();
  print(JSON.stringify(iter.next(42)));
'
#### JavaScriptCore
{"done":true}

I do think the spec may be messed up here, because |PrimaryExpression| propagates [?Yield, ?Await] to |ClassExpression|, which propagates them to |FieldDefinition|, which propagates them to |Initializer|, which propagates them to |AssignmentExpression|, which can expand to |YieldExpression| with [+Yield] and to |AwaitExpression| with [+Await] (and likewise |StatementList| to |ClassDeclaration|). The engines that reject source like function* f(){ class C { static foo = yield }; } are treating the initializer as containing an identifier that is reserved in strict mode code, rather than as containing a yield expression. It seems that only JSC parses per spec, although in evaluation it acts like the above (and engine262 dies with an assertion failure):

$ eshost -h JavaScriptCore,engine262 -sx '
  print("START");
  const iter = (function* f(){ return (class C { static foo = yield }).foo; })();
  try {
    print(JSON.stringify(iter.next("A")));
    print(JSON.stringify(iter.next("B")));
  } catch (err) { print(err); }
'
#### engine262

AssertError: genContext.Generator !== undefined

#### JavaScriptCore
START
{"done":true}
{"done":true}

yeah https://tc39.es/ecma262/multipage/ecmascript-language-functions-and-classes.html#sec-class-definitions-static-semantics-early-errors

It is a Syntax Error if |ClassStaticBlockStatementList| Contains await is true.