`Decidable` synthesis failure when `if`-conditioning on a tactic-containing `match`

Prerequisites

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Description

An if expression that cases on a local binding of a Bool-valued result of matching on x[n] notation leads to a Decidable synthesis failure.

Context

This issue arose when trying to condition on a local let-binding that used the matches syntax sugar—something of the form

...
let cond := stx[1].getHeadInfo matches .synthetic
let val := if cond then ...
...

Steps to Reproduce

Refer to the following code:

def f [GetElem α Nat Bool P] (x : α) (h : P x 0) : String :=
  let cond :=
    match x[0] with
    | true => false
    | false => true
  if cond then "foo" else "bar" -- typeclass instance problem is stuck, it is often due to metavariables\n Decidable ?m.210

Expected behavior: The declaration elaborates successfully.

Actual behavior: The indicated type-class synthesis failure occurs.

Versions

• Lean 4.23.0-nightly-2025-07-05
• Lean 4.21.0

Additional Information

Note that the error disappears if (a) one inlines the match (e.g., if x[0] matches true ...) or (b) one writes getElem x 0 h directly, eliminating the get_elem_tactic-containing macro expansion. More generally, other (but not all) tactic invocations in the match discriminant also induce similar errors, such as this one:

#check
  let cond := match not (by exact false) with | true => false | false => true
  if cond then "foo" else "bar"  -- Application type mismatch: ... cond has type Bool but is expected to have type Prop

(But delete the not and it elaborates successfully.)

Impact

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Start by reproducing the provided GetElem example on Lean 4.23.0-nightly-2025-07-05, then compare it with the inline-match and direct getElem variants. Trace elaboration around the tactic-containing match discriminant and local let; done means the declaration elaborates successfully without the Decidable synthesis failure.