False positive `local ambiguity` in macro

MCVE:

macro_rules! mcve {
    ($($my_lft:lifetime)? $($my_ty:ty)?) => {};
}

mcve!('a);

error: local ambiguity when calling macro `mcve`: multiple parsing options: built-in NTs lifetime ('my_lft') or ty ('my_ty').
 --> src/lib.rs:5:7
  |
5 | mcve!('a);
  |       ^^

This piece of code should only be able to match $my_lft, but it complains about being able to reach $my_ty.
To prove that $my_ty cannot be reached, and that this problem happens with lifetime but not with other types such as block, i created the following playground link:

• https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=837c3d88bb5f00ad8936e3ede274f8f2
• (gist) https://gist.github.com/rust-play/837c3d88bb5f00ad8936e3ede274f8f2

Meta

Tried it in editions 2024, 2021 and 2018, using nightly, 1.94.1, 1.80.1, 1.70.0, 1.60.0, 1.50.0, 1.40.0 and 1.31.0 (i.e. the first version with edition 2018, where the $(...)? operator wasn't even stable).

Similar issues (some probably too old to be relevant, but still included just in case):

• https://github.com/rust-lang/rust/issues/128915
• https://github.com/rust-lang/rust/issues/59881
• PR https://github.com/rust-lang/rust/pull/70768
  • Closed https://github.com/rust-lang/rust/issues/70446
• PR https://github.com/rust-lang/rust/pull/51480
  • Closed https://github.com/rust-lang/rust/issues/50903
• PR https://github.com/rust-lang/rust/pull/42913
  • Closed https://github.com/rust-lang/rust/issues/24189
  • Closed https://github.com/rust-lang/rust/issues/34030
  • Closed https://github.com/rust-lang/rust/issues/26444
• Commit https://github.com/rust-lang/rust/commit/fc60797f7f3079288bdd1717dedbb885eaf1dbfa
  • Closed https://github.com/rust-lang/rust/issues/5902

Start by reproducing the MCVE in the linked Rust Playground and compare the lifetime case with the block case. Trace macro parsing behavior for the optional lifetime and type metavariables, then verify that the reported local ambiguity disappears without changing valid ambiguity diagnostics.