fix a crash when using type equality constaint (#8515)

Close #8193.

When constructing `TransitiveTypeWitness` node, we should check if there
is operand that represents two equal times. Currently, we only check
whether the operand is `TypeEqualityWitness`, which is not good enough,
because a `DeclaredSubtypeWitness` could also be representing two same
types, in that case, we should also const fold this kind of witness.

Fails to do so, we could finally ends up with a generating a lookup
witness IR on a generic parameter that is not supposed to be looked up.

3 files changed, 19 insertions, 6 deletions

diff --git a/source/slang/slang-ast-base.h b/source/slang/slang-ast-base.h
index 828bbdb5c..fe3554f84 100644
--- a/source/slang/slang-ast-base.h
+++ b/source/slang/slang-ast-base.h
@@ -673,12 +673,7 @@ class DeclRefBase : public Val
     SourceLoc getNameLoc() const;
     SourceLoc getLoc() const;
     DeclRefBase* getParent();
-    String toString() const
-    {
-        StringBuilder sb;
-        const_cast<DeclRefBase*>(this)->toText(sb);
-        return sb.produceString();
-    }
+    String toString() const;
     DeclRefBase* getBase();
     void toText(StringBuilder& out);
 };
diff --git a/source/slang/slang-ast-builder.cpp b/source/slang/slang-ast-builder.cpp
index f7304f308..be71fc334 100644
--- a/source/slang/slang-ast-builder.cpp
+++ b/source/slang/slang-ast-builder.cpp
@@ -936,6 +936,11 @@ top:
     {
         return bIsSubtypeOfCWitness;
     }
+    else if (auto declAIsSubtypeOfBWitness = as<DeclaredSubtypeWitness>(aIsSubtypeOfBWitness))
+    {
+        if (declAIsSubtypeOfBWitness->isEquality())
+            return bIsSubtypeOfCWitness;
+    }
 
     // Similarly, if `b == c`, then the `a <: b` witness is a witness for `a <: c`
     //
@@ -943,6 +948,11 @@ top:
     {
         return aIsSubtypeOfBWitness;
     }
+    else if (auto declBIsSubtypeOfCWitness = as<DeclaredSubtypeWitness>(bIsSubtypeOfCWitness))
+    {
+        if (declBIsSubtypeOfCWitness->isEquality())
+            return declBIsSubtypeOfCWitness;
+    }
 
     // HACK: There is downstream code generation logic that assumes that
     // a `TransitiveSubtypeWitness` will never have a transitive witness
diff --git a/source/slang/slang-ast-decl-ref.cpp b/source/slang/slang-ast-decl-ref.cpp
index 7eec32a3a..c054ef994 100644
--- a/source/slang/slang-ast-decl-ref.cpp
+++ b/source/slang/slang-ast-decl-ref.cpp
@@ -426,6 +426,14 @@ SourceLoc DeclRefBase::getLoc() const
     return getDecl()->loc;
 }
 
+// Keep this function here for better debuggin purpose
+String DeclRefBase::toString() const
+{
+    StringBuilder sb;
+    const_cast<DeclRefBase*>(this)->toText(sb);
+    return sb.produceString();
+}
+
 DeclRefBase* DeclRefBase::getParent()
 {
     auto astBuilder = getCurrentASTBuilder();