1 files changed, 113 insertions, 11 deletions

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index 4c6bf98d2..306687bd8 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -1023,6 +1023,100 @@ LookupResult SemanticsVisitor::filterLookupResultByVisibilityAndDiagnose(
     return result;
 }
 
+bool SemanticsVisitor::isWitnessUncheckedOptional(SubtypeWitness* witness)
+{
+    auto declaredWitness = as<DeclaredSubtypeWitness>(witness);
+    if (!declaredWitness)
+        return false;
+
+    auto decl = declaredWitness->getDeclRef().getDecl();
+    if (!decl || !decl->hasModifier<OptionalConstraintModifier>())
+        return false;
+
+    // Okay, we've found an optional subtype witness. This result needs
+    // to be removed if we're not inside a block that directly checks
+    // if (sub is sup)
+    auto sub = witness->getSub();
+    auto sup = witness->getSup();
+
+    for (auto outerStmtInfo = m_outerStmts; outerStmtInfo; outerStmtInfo = outerStmtInfo->next)
+    {
+        auto outerStmt = outerStmtInfo->stmt;
+        auto ifStmt = as<IfStmt>(outerStmt);
+
+        if (!ifStmt)
+            continue;
+
+        IsTypeExpr* isType = as<IsTypeExpr>(ifStmt->predicate);
+        if (!isType)
+            continue;
+        VarExpr* var = as<VarExpr>(isType->value);
+        if (!var)
+            continue;
+        TypeType* typeType = as<TypeType>(var->type);
+
+        // var->type works for `variable is Interface`, while
+        // typeType->getType() is for `T is Interface`.
+        auto type = typeType ? typeType->getType() : var->type.type;
+        if (type == sub && isType->typeExpr.type == sup)
+        {
+            return false;
+        }
+    }
+
+    // If we got this far, it's both an optional witness and there's no
+    // statement checking its validity.
+    return true;
+}
+
+LookupResult SemanticsVisitor::filterLookupResultByCheckedOptional(const LookupResult& lookupResult)
+{
+    LookupResult filteredResult;
+    for (auto item : lookupResult)
+    {
+        bool optionalConstraintsChecked = true;
+
+        for (auto bb = item.breadcrumbs; bb; bb = bb->next)
+        {
+            auto witness = as<SubtypeWitness>(bb->val);
+            if (!witness)
+                continue;
+
+            if (isWitnessUncheckedOptional(witness))
+            {
+                optionalConstraintsChecked = false;
+                break;
+            }
+        }
+
+        if (optionalConstraintsChecked)
+            AddToLookupResult(filteredResult, item);
+    }
+    return filteredResult;
+}
+
+LookupResult SemanticsVisitor::filterLookupResultByCheckedOptionalAndDiagnose(
+    const LookupResult& lookupResult,
+    SourceLoc loc,
+    bool& outDiagnosed)
+{
+    auto result = filterLookupResultByCheckedOptional(lookupResult);
+    if (lookupResult.isValid() && !result.isValid())
+    {
+        getSink()->diagnose(
+            loc,
+            Diagnostics::requiredConstraintIsNotChecked,
+            lookupResult.item.declRef);
+        outDiagnosed = true;
+
+        if (getShared()->isInLanguageServer())
+        {
+            return lookupResult;
+        }
+    }
+    return result;
+}
+
 LookupResult SemanticsVisitor::resolveOverloadedLookup(LookupResult const& inResult)
 {
     // If the result isn't actually overloaded, it is fine as-is
@@ -4068,19 +4162,16 @@ Expr* SemanticsExprVisitor::visitIsTypeExpr(IsTypeExpr* expr)
     expr->type = m_astBuilder->getBoolType();
     expr->value = originalVal;
 
-    // Check if the right-hand side type is an interface type
-    if (isInterfaceType(expr->typeExpr.type))
-    {
-        getSink()->diagnose(expr, Diagnostics::isAsOperatorCannotUseInterfaceAsRHS);
-        return expr;
-    }
-
     auto valueType = expr->value->type.type;
     if (auto typeType = as<TypeType>(valueType))
         valueType = typeType->getType();
 
     // If value is a subtype of `type`, then this expr is always true.
-    if (isSubtype(valueType, expr->typeExpr.type, IsSubTypeOptions::None))
+    auto witness = isSubtype(valueType, expr->typeExpr.type, IsSubTypeOptions::None);
+    auto declWitness = as<DeclaredSubtypeWitness>(witness);
+    bool optionalWitness = declWitness && declWitness->isOptional();
+
+    if (witness && !optionalWitness)
     {
         // Instead of returning a BoolLiteralExpr, we use a field to indicate this scenario,
         // so that the language server can still see the original syntax tree.
@@ -4091,15 +4182,24 @@ Expr* SemanticsExprVisitor::visitIsTypeExpr(IsTypeExpr* expr)
         return expr;
     }
 
+    // Check if the right-hand side type is an interface type. For 'is'
+    // statements, that's only allowed if it's related to an optional
+    // constraint.
+    if (isInterfaceType(expr->typeExpr.type) && !optionalWitness)
+    {
+        getSink()->diagnose(expr, Diagnostics::isOperatorCannotUseInterfaceAsRHS);
+        return expr;
+    }
+
     // Otherwise, if the target type is a subtype of value->type, we need to grab the
     // subtype witness for runtime checks.
 
     expr->value = maybeOpenExistential(originalVal);
-    expr->witnessArg = tryGetSubtypeWitness(expr->typeExpr.type, valueType);
+    expr->witnessArg = witness ? witness : tryGetSubtypeWitness(expr->typeExpr.type, valueType);
     if (expr->witnessArg)
     {
         // For now we can only support the scenario where `expr->value` is an interface type.
-        if (!isInterfaceType(originalVal->type))
+        if (!optionalWitness && !isInterfaceType(originalVal->type))
         {
             getSink()->diagnose(expr, Diagnostics::isOperatorValueMustBeInterfaceType);
         }
@@ -4117,7 +4217,7 @@ Expr* SemanticsExprVisitor::visitAsTypeExpr(AsTypeExpr* expr)
     // Check if the right-hand side type is an interface type
     if (isInterfaceType(typeExpr.type))
     {
-        getSink()->diagnose(expr, Diagnostics::isAsOperatorCannotUseInterfaceAsRHS);
+        getSink()->diagnose(expr, Diagnostics::asOperatorCannotUseInterfaceAsRHS);
         expr->type = m_astBuilder->getErrorType();
         return expr;
     }
@@ -5162,6 +5262,8 @@ Expr* SemanticsVisitor::checkGeneralMemberLookupExpr(MemberExpr* expr, Type* bas
         lookUpMember(m_astBuilder, this, expr->name, baseType, m_outerScope);
     bool diagnosed = false;
     lookupResult = filterLookupResultByVisibilityAndDiagnose(lookupResult, expr->loc, diagnosed);
+    lookupResult =
+        filterLookupResultByCheckedOptionalAndDiagnose(lookupResult, expr->loc, diagnosed);
     if (!lookupResult.isValid())
     {
         return lookupMemberResultFailure(expr, baseType, diagnosed);