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>

1 files changed, 21 insertions, 13 deletions

diff --git a/source/slang/slang-ir-specialize-dispatch.cpp b/source/slang/slang-ir-specialize-dispatch.cpp
index e4d1af93a..0c8b248b0 100644
--- a/source/slang/slang-ir-specialize-dispatch.cpp
+++ b/source/slang/slang-ir-specialize-dispatch.cpp
@@ -232,23 +232,31 @@ void ensureWitnessTableSequentialIDs(SharedGenericsLoweringContext* sharedContex
             {
                 auto interfaceType =
                     cast<IRWitnessTableType>(inst->getDataType())->getConformanceType();
-                auto interfaceLinkage = interfaceType->findDecoration<IRLinkageDecoration>();
-                SLANG_ASSERT(
-                    interfaceLinkage && "An interface type does not have a linkage,"
-                                        "but a witness table associated with it has one.");
-                auto interfaceName = interfaceLinkage->getMangledName();
-                auto idAllocator =
-                    linkage->mapInterfaceMangledNameToSequentialIDCounters.tryGetValue(
-                        interfaceName);
-                if (!idAllocator)
+                if (as<IRInterfaceType>(interfaceType))
                 {
-                    linkage->mapInterfaceMangledNameToSequentialIDCounters[interfaceName] = 0;
-                    idAllocator =
+                    auto interfaceLinkage = interfaceType->findDecoration<IRLinkageDecoration>();
+                    SLANG_ASSERT(
+                        interfaceLinkage && "An interface type does not have a linkage,"
+                                            "but a witness table associated with it has one.");
+                    auto interfaceName = interfaceLinkage->getMangledName();
+                    auto idAllocator =
                         linkage->mapInterfaceMangledNameToSequentialIDCounters.tryGetValue(
                             interfaceName);
+                    if (!idAllocator)
+                    {
+                        linkage->mapInterfaceMangledNameToSequentialIDCounters[interfaceName] = 0;
+                        idAllocator =
+                            linkage->mapInterfaceMangledNameToSequentialIDCounters.tryGetValue(
+                                interfaceName);
+                    }
+                    seqID = *idAllocator;
+                    ++(*idAllocator);
+                }
+                else
+                {
+                    // NoneWitness, has special ID of -1.
+                    seqID = uint32_t(-1);
                 }
-                seqID = *idAllocator;
-                ++(*idAllocator);
                 linkage->mapMangledNameToRTTIObjectIndex[witnessTableMangledName] = seqID;
             }