ICE: assert failure on witnessTableVal when dispatching through derived interface existential with autodiff

[jvepsalainen-nv]

Summary

Dispatching through a derived interface existential (e.g., IDerived that extends IBase) in a differentiable context triggers an internal compiler error: assert failure: witnessTableVal.

Repro

Define an interface hierarchy and attempt backward differentiation through the derived interface:

[anyValueSize(16)]
interface IBase : IDifferentiable
{
    [Differentiable]
    float baseCalc(float x);
}

[anyValueSize(16)]
interface IDerived : IBase
{
    [Differentiable]
    float derivedCalc(float x);
}

struct A : IDerived
{
    float a;
    __init(float a) { this.a = a; }

    [Differentiable]
    [NoDiffThis]
    float baseCalc(float x) { return a * x * x; }

    [Differentiable]
    [NoDiffThis]
    float derivedCalc(float x) { return a * x * x * x; }
}

[Differentiable]
float useDerived(IDerived obj, float x)
{
    return obj.derivedCalc(x);
}

[Differentiable]
float dispatchDerived(uint id, float x, no_diff float param)
{
    IDerived obj;
    if (id == 0)
        obj = no_diff(A(param));
    else
        obj = no_diff(A(param));  // simplified
    return useDerived(obj, x);
}

Calling bwd_diff(dispatchDerived)(...) triggers the ICE.

Expected Behavior

Differentiation through derived interface existentials should work the same as through base interface existentials.

Discovered In

PR #10734 — test autodiff-dynamic-dispatch-interface-inherit.slang documents this as a known limitation.

[nv-slang-bot]

Internal Triage Report

Reproduced: Yes (debug assert only, c01388b)
Crash site: IRBuilder::emitLookupInterfaceMethodInst() in slang-ir.cpp:3777
Pipeline stage: Autodiff backward differentiation synthesis
Severity: medium

Analysis

bwd_diff through a derived interface existential (IDerived extends IBase : IDifferentiable) triggers assertion witnessTableVal && witnessTableVal->getOp() != kIROp_StructKey. The autodiff dadd synthesis at slang-ir-autodiff.cpp:393 calls emitExtractExistentialWitnessTable() on the derived interface existential. For derived interfaces where IDifferentiable conformance is inherited (not direct), extraction returns a StructKey instead of a witness table. The existing TODO at slang-ir.cpp:3771-3775 acknowledges this: "if somebody tries to declare a struct that inherits an interface conformance from a base type, then we hit this assert."

Reporter's PR #10734 documents this as a known limitation with a test case.

Relevant Code

• slang-ir.cpp:3771-3777 — assertion and TODO comment in emitLookupInterfaceMethodInst()
• slang-ir-autodiff.cpp:388-414 — dadd synthesis, witness table extraction and method lookup
• slang-ir-autodiff.cpp:393 — emitExtractExistentialWitnessTable(aObj) returns StructKey for derived interface

Potential Fix Approaches

1. Walk interface hierarchy for witness table (cross-ref PR Refactor auto-diff implementation. #9808 autodiff refactor by saipraveenb25)
   When extracting IDifferentiable witness table from a derived interface existential, resolve through the inheritance chain rather than direct extraction.

2. Handle StructKey in emitLookupInterfaceMethodInst
   Resolve StructKey-typed witness tables to actual witness tables through the type system.

Relevant Engineers

• saipraveenb25 — authored autodiff refactor (Refactor auto-diff implementation. #9808) and dadd synthesis code
• jvepsalainen-nv — reporter, actively testing autodiff+dispatch intersection

Test Gap

No tests for autodiff through derived interface existentials. PR #10734 (open) would add coverage.