`opaque` fails to synthesize `Inhabited`

Prerequisites

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• Reduce the issue to a minimal, self-contained, reproducible test case.
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Description

opaque can fail to synthesize an Inhabited instance even when one is available. Manually writing := default is a workaround.

Context

Originally reported as #9689

Steps to Reproduce

class C1 (b : outParam Bool) (m : Type u → Type) where
  val : m α

class C2 (m : Type → Type) where
  val : m α

instance [C2 m] : C1 false m where
  val := C2.val

instance [C1 b m] : Inhabited (m α) where
  default := C1.val

def T (_ : Type u) := Unit

instance : C1 true T where
  val := ()

variable {α}
#synth Inhabited (T α)     -- succeeds
opaque t : T α             -- failed to synthesize  Inhabited (T α)
opaque t' : T α := default -- succeeds

Expected behavior: t should succeed, like t'

Actual behavior: t gives the error failed to synthesize Inhabited (T α)

Versions

Lean 4.23.0-nightly-2025-08-10

Impact

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Start by running the minimal reproduction in the Lean nightly environment and compare opaque t : T α with opaque t' : T α := default and #synth Inhabited (T α). Trace how opaque handles typeclass synthesis, using the reported C1, C2, and Inhabited instances; done means the first declaration succeeds like the second.