From 7349dc5cff49cf22c82eb912813e47f30cd7a757 Mon Sep 17 00:00:00 2001
From: Julius Ikkala <julius.ikkala@gmail.com>
Date: Sat, 28 Jun 2025 05:39:24 +0300
Subject: Minimal optional constraints (#7422)

* Parse optional witness syntax

* Allow failing optional constraint

* Make `is` work with optional constraint

* Allow using optional constraint in checked if statements

* Fix tests

* Make it work with structs

* Fix MSVC build error

* Disallow using `as` with optional constraints

* Update test to match split is/as errors

* Add tests

* Fix uninitialized variables in tests

* Add tests of incorrect uses & fix related bugs

* Mention optional constraints in docs

* format code

* Fix type unification with NoneWitness

* Fix formatting

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Nathan V. Morrical <natemorrical@gmail.com>
---
 source/slang/slang-ir-lower-generic-function.cpp | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

(limited to 'source/slang/slang-ir-lower-generic-function.cpp')

diff --git a/source/slang/slang-ir-lower-generic-function.cpp b/source/slang/slang-ir-lower-generic-function.cpp
index f2b1d1a6a..c003d6125 100644
--- a/source/slang/slang-ir-lower-generic-function.cpp
+++ b/source/slang/slang-ir-lower-generic-function.cpp
@@ -296,8 +296,11 @@ struct GenericFunctionLoweringContext
     // and emission of wrapper functions.
     void lowerWitnessTable(IRWitnessTable* witnessTable)
     {
-        auto interfaceType =
-            maybeLowerInterfaceType(cast<IRInterfaceType>(witnessTable->getConformanceType()));
+        IRInterfaceType* conformanceType = as<IRInterfaceType>(witnessTable->getConformanceType());
+        if (!conformanceType)
+            return;
+
+        auto interfaceType = maybeLowerInterfaceType(conformanceType);
         IRBuilder builderStorage(sharedContext->module);
         auto builder = &builderStorage;
         builder->setInsertBefore(witnessTable);
@@ -353,8 +356,17 @@ struct GenericFunctionLoweringContext
             return;
         if (witnessTableType->getConformanceType()->findDecoration<IRComInterfaceDecoration>())
             return;
-        auto interfaceType =
-            maybeLowerInterfaceType(cast<IRInterfaceType>(witnessTableType->getConformanceType()));
+
+        IRInterfaceType* conformanceType =
+            as<IRInterfaceType>(witnessTableType->getConformanceType());
+
+        // NoneWitness generates conformance types which aren't interfaces. In
+        // that case, the method can just be skipped entirely, since there's no
+        // real witness for it and it should be in unreachable code at this
+        // point.
+        if (!conformanceType)
+            return;
+        auto interfaceType = maybeLowerInterfaceType(conformanceType);
         interfaceRequirementVal = sharedContext->findInterfaceRequirementVal(
             interfaceType,
             lookupInst->getRequirementKey());
-- 
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