From 786f48d32340c36a06865a333ff9066033b5b2bc Mon Sep 17 00:00:00 2001
From: Yong He <yonghe@outlook.com>
Date: Fri, 12 Aug 2022 07:57:41 -0700
Subject: Fix logic of `is` operator. (#2359)

---
 source/slang/slang-check-expr.cpp | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

(limited to 'source/slang/slang-check-expr.cpp')

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index cb75e3078..2ac02b978 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -1771,30 +1771,35 @@ namespace Slang
         {
             // 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.
-            expr->isAlwaysTrue = true;
+            expr->constantVal = m_astBuilder->create<BoolLiteralExpr>();
+            expr->constantVal->type = m_astBuilder->getBoolType();
+            expr->constantVal->value = true;
+            expr->constantVal->loc = expr->loc;
             return expr;
         }
 
         // Otherwise, we need to ensure the target type is a subtype of value->type.
-        // For now we can only support the scenario where `expr->value` is an interface type.
-        if (!isInterfaceType(originalVal->type))
-        {
-            getSink()->diagnose(expr, Diagnostics::isOperatorValueMustBeInterfaceType);
-        }
 
         expr->value = maybeOpenExistential(originalVal);
         expr->witnessArg = tryGetSubtypeWitness(expr->typeExpr.type, originalVal->type.type);
         if (expr->witnessArg)
         {
+            // For now we can only support the scenario where `expr->value` is an interface type.
+            if (!isInterfaceType(originalVal->type))
+            {
+                getSink()->diagnose(expr, Diagnostics::isOperatorValueMustBeInterfaceType);
+            }
             return expr;
         }
 
         if (!as<ErrorType>(expr->typeExpr.type) && !as<ErrorType>(expr->value->type.type))
         {
-            getSink()->diagnose(expr, Diagnostics::typeNotInTheSameHierarchy, expr->value->type.type, expr->typeExpr.type);
+            // The type is not in the same hierarchy, so we evaluate to false.
+            expr->constantVal = m_astBuilder->create<BoolLiteralExpr>();
+            expr->constantVal->type = m_astBuilder->getBoolType();
+            expr->constantVal->value = false;
+            expr->constantVal->loc = expr->loc;
         }
-
-        expr->type = m_astBuilder->getErrorType();
         return expr;
     }
 
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