Optimizer: treat EI_ilzero (Unchecked.defaultof) as effect-free to enable dead binding elimination

[Copilot]

Unchecked.defaultof<T> unused bindings were not eliminated even with optimizations enabled, because the optimizer's effect analysis incorrectly classified the underlying EI_ilzero IL instruction as having side effects.

Root cause

Unchecked.defaultof<T> inlines to (# "ilzero !0" type ('T) : 'T #), which the optimizer represents as TOp.ILAsm([EI_ilzero ty], ...). The IlAssemblyCodeInstrHasEffect function in Optimizer.fs did not recognize EI_ilzero as effect-free, so it fell through to the conservative _ -> true default, preventing dead binding elimination.

Change

src/Compiler/Optimize/Optimizer.fs — add EI_ilzero _ to the no-effect branch of IlAssemblyCodeInstrHasEffect. The instruction only produces a zero/default value (mapping to ldnull, ldc.i4.0, or initobj depending on type), with no observable side effects.

// Before: initobj instructions emitted for all three unused bindings
let f (n: float32) =
    Console.WriteLine n
    let _ = Unchecked.defaultof<decimal>   // initobj Decimal
    let _ = Unchecked.defaultof<decimal>   // initobj Decimal
    let _ = Unchecked.defaultof<decimal>   // initobj Decimal
    let n' = n * 2.f
    Console.WriteLine n'

// After: all three bindings eliminated; IL only contains the two Console.WriteLine calls

This is particularly relevant for SRTP-based patterns (common in libraries like FSharpPlus) where Unchecked.defaultof<'T> is used as a witness/dummy argument to drive overload resolution and ends up as unused bindings at call sites.

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md