A yield inside an expression is an error, and a yield's value is undefined

[ASDAlexander77]

mlirGen(YieldExpression) returned success with no value, so any expression around a yield was silently dropped. const t = ("a" + i) + (yield 1) never wrote t, and the program crashed reading it (every memory model, gc included). A value computed before the yield in the same expression would also be an SSA value of the turn that returned, gone when the generator resumes at the state label.

• A yield now evaluates to undefined, made after the label (next() takes no argument yet). This applies where nothing is computed before the yield: yield x;, const v = yield x; and v = yield x; (with v an identifier).
• The enclosing statement marks those yields (SupportedYieldScope), because the AST has no parent links. Compiler-made yields sit in expression statements, so they count as statement-level.
• Any other yield is now a compile error: a yield inside an expression is not supported ... assign the yield to a variable first. Using the value of yield* is also an error, since it still gives nothing back.

Tests:

• 00generator_yield_value.ts (compile + jit, in the corpus, so it runs under rc/none too).
• yield-in-expression/{binary,call_argument,yield_star_value}.ts, checked for the error under gc/rc/none.

Full ctest -C Release -j 16: 100% passed, 0 failed out of 4782.

Not covered here: a variable declaration whose initializer fails with no diagnostic still declares an uninitialized variable. That silent-failure gap is general and has nothing to do with yield.

🤖 Generated with Claude Code