Resolving meta variables in function applications through TC synthesis

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

Consider:

class A (a : Nat) (b : Nat) (c : outParam Nat) where 
  wit : Nat × Nat × Nat := ⟨ a , b , c ⟩
instance instAZero : A (.zero) (n) (n) where 
instance instASucc [A m n o]  : A (.succ m) (n) (.succ o) where

#eval A.wit 1 2
#eval @A.wit 1 2 _

Lean is able to execute the first version perfectly fine and in particular automatically resolves the c argument through TC synthesis assigning it with the outParam mechanism. However when passing an _ explicitly this mechanism does not kick in.

Context

Brought up to me by @MangoIV

Expected behavior: Both versions should type check and execute the same way

Actual behavior: Only the first version type checks, the second fails to resolve the mvar

Versions

Lean 4.25.0-nightly-2025-10-21
Target: x86_64-unknown-linux-gnu

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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Start with the minimal Lean example in the issue and reproduce it against the specified nightly release, comparing implicit and explicit _ applications of A.wit. Investigate typeclass synthesis and outParam handling; done means both versions type check and execute equivalently without Mathlib or Batteries.