3 files changed, 84 insertions, 22 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index e5e8e8acc..1dc81e861 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -4226,7 +4226,7 @@ namespace Slang
         //
         DiagnosticSink tempSink(getSourceManager(), nullptr);
         ExprLocalScope localScope;
-        SemanticsVisitor subVisitor(withSink(&tempSink).withExprLocalScope(&localScope));
+        SemanticsVisitor subVisitor(withSink(&tempSink).withParentFunc(synFuncDecl).withExprLocalScope(&localScope));
 
         // With our temporary diagnostic sink soaking up any messages
         // from overload resolution, we can now try to resolve
diff --git a/source/slang/slang-check-overload.cpp b/source/slang/slang-check-overload.cpp
index 0e01eeed2..7318fa390 100644
--- a/source/slang/slang-check-overload.cpp
+++ b/source/slang/slang-check-overload.cpp
@@ -2266,7 +2266,6 @@ namespace Slang
 
         // Look at the base expression for the call, and figure out how to invoke it.
         auto funcExpr = expr->functionExpr;
-        auto funcExprType = funcExpr->type;
 
         // If we are trying to apply an erroneous expression, then just bail out now.
         if(IsErrorExpr(funcExpr))
@@ -2295,25 +2294,6 @@ namespace Slang
         for (auto& arg : expr->arguments)
         {
             arg = maybeOpenRef(arg);
-        }
-
-        auto funcType = as<FuncType>(funcExprType);
-        for (Index i = 0; i < expr->arguments.getCount(); i++)
-        {
-            auto& arg = expr->arguments[i];
-            if (funcType && i < funcType->getParamCount())
-            {
-                switch (funcType->getParamDirection(i))
-                {
-                case kParameterDirection_Out:
-                case kParameterDirection_InOut:
-                case kParameterDirection_Ref:
-                case kParameterDirection_ConstRef:
-                    continue;
-                default:
-                    break;
-                }
-            }
             arg = maybeOpenExistential(arg);
         }
 
@@ -2443,6 +2423,45 @@ namespace Slang
             // the user the most help we can.
             if (shouldAddToCache)
                 typeCheckingCache->resolvedOperatorOverloadCache[key] = *context.bestCandidate;
+
+            // Now that we have resolved the overload candidate, we need to undo an `openExistential`
+            // operation that was applied to `out` arguments.
+            //
+            auto funcType = context.bestCandidate->funcType;
+            ShortList<ParameterDirection> paramDirections;
+            if (funcType)
+            {
+                for (Index i = 0; i < funcType->getParamCount(); i++)
+                {
+                    paramDirections.add(funcType->getParamDirection(i));
+                }
+            }
+            else if (auto callableDeclRef = context.bestCandidate->item.declRef.as<CallableDecl>())
+            {
+                for (auto param : callableDeclRef.getDecl()->getParameters())
+                {
+                    paramDirections.add(getParameterDirection(param));
+                }
+            }
+            for (Index i = 0; i < expr->arguments.getCount(); i++)
+            {
+                auto& arg = expr->arguments[i];
+                if (i < paramDirections.getCount())
+                {
+                    switch (paramDirections[i])
+                    {
+                    case kParameterDirection_Out:
+                    case kParameterDirection_InOut:
+                    case kParameterDirection_Ref:
+                    case kParameterDirection_ConstRef:
+                        break;
+                    default:
+                        continue;
+                    }
+                }
+                if (auto extractExistentialExpr = as<ExtractExistentialValueExpr>(arg))
+                    arg = extractExistentialExpr->originalExpr;
+            }
             return CompleteOverloadCandidate(context, *context.bestCandidate);
         }
 
@@ -2475,7 +2494,7 @@ namespace Slang
 
         // Nothing at all was found that we could even consider invoking.
         // In all other cases, this is an error.
-        getSink()->diagnose(expr->functionExpr, Diagnostics::expectedFunction, funcExprType);
+        getSink()->diagnose(expr->functionExpr, Diagnostics::expectedFunction, funcExpr->type);
         expr->type = QualType(m_astBuilder->getErrorType());
         return expr;
     }
diff --git a/tests/bugs/gh-4467.slang b/tests/bugs/gh-4467.slang
new file mode 100644
index 000000000..dc97b9890
--- /dev/null
+++ b/tests/bugs/gh-4467.slang
@@ -0,0 +1,43 @@
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHK): -d3d12 -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=CHK): -vk -compute -shaderobj -output-using-type
+
+
+// Test that we can synthesize a [mutating] interface requirement from a nonmutating implementation,
+// and the interface requirement signature contains an output interface-typed parameter.
+
+//TEST_INPUT: ubuffer(data=[0 0], stride=4):out,name outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+interface IFoo
+{
+    int getVal();
+};
+
+interface IBar
+{
+    [mutating] void method(out IFoo o);
+};
+
+struct FooImpl : IFoo
+{
+    int x;
+    int getVal() { return x; }
+}
+
+struct BarImpl : IBar
+{
+    void method(out IFoo o)
+    {
+        o = FooImpl(1);
+    }
+};
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    BarImpl bar;
+    IFoo foo;
+    bar.method(foo);
+    // CHK: 1
+    outputBuffer[0] = foo.getVal();
+}
\ No newline at end of file