Compiler fails to optimize out irrelevant comparison

Prerequisites

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Description

set_option trace.compiler.ir.result true in
@[inline]
def foo (b : UInt8) : Option UInt8 :=
  if 0xd8 ≤ b ∧ b ≤ 0xdf then
    none
  else
    some b

set_option trace.compiler.ir.result true in
def foo' (b) (h : (foo b).isSome) : UInt8 :=
  (foo b).get h

Here we get the IR

    def foo'._redArg (x_1 : u8) : u8 :=
      let x_2 : u8 := 216;
      let x_3 : u8 := UInt8.decLe x_2 x_1;
      case x_3 : u8 of
      Bool.false →
        ret x_1
      Bool.true →
        ret x_1

which is clearly suboptimal.

Expected behavior: Compiler should generate IR ret x_1 for foo'.redArg

Actual behavior: See above

Versions

Lean 4.23.0-nightly-2025-08-12 on live.lean-lang.org

Impact

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