2 files changed, 70 insertions, 4 deletions
diff --git a/source/slang/slang-ir-remove-unused-generic-param.cpp b/source/slang/slang-ir-remove-unused-generic-param.cpp
index dd4efeddd..c79e236b8 100644
--- a/source/slang/slang-ir-remove-unused-generic-param.cpp
+++ b/source/slang/slang-ir-remove-unused-generic-param.cpp
@@ -72,17 +72,38 @@ struct RemoveUnusedGenericParamContext : InstPassBase
                         child = next;
                     }
                     SLANG_ASSERT(returnVal);
-                    List<IRUse*> uses;
+
+                    // Collect all specialize uses that we might optimize
+                    List<IRUse*> specializeUses;
                     for (auto use = genInst->firstUse; use; use = use->nextUse)
-                        uses.add(use);
-                    for (auto use : uses)
                     {
                         if (use->getUser()->getOp() == kIROp_Specialize &&
                             use == use->getUser()->getOperands())
                         {
-                            use->getUser()->replaceUsesWith(returnVal);
+                            specializeUses.add(use);
+                        }
+                    }
+
+                    // Check if any of these specialize uses are used by witness tables
+                    // If so, we cannot apply the optimization because witness tables are immutable
+                    for (auto use : specializeUses)
+                    {
+                        for (auto specializeUse = use->getUser()->firstUse; specializeUse;
+                             specializeUse = specializeUse->nextUse)
+                        {
+                            auto userOp = specializeUse->getUser()->getOp();
+                            if (userOp == kIROp_WitnessTable)
+                            {
+                                goto skipOptimization;
+                            }
                         }
                     }
+
+                    // Apply the optimization: replace all specialize uses with the return value
+                    for (auto use : specializeUses)
+                    {
+                        use->getUser()->replaceUsesWith(returnVal);
+                    }
                     genInst->replaceUsesWith(returnVal);
                     genInst->removeAndDeallocate();
                 }
@@ -118,6 +139,7 @@ struct RemoveUnusedGenericParamContext : InstPassBase
                     for (auto param : paramsToRemove)
                         param->removeAndDeallocate();
                 }
+            skipOptimization:;
             }
         }
         return changed;
diff --git a/tests/language-feature/interfaces/generic-value-parameter-via-interface.slang b/tests/language-feature/interfaces/generic-value-parameter-via-interface.slang
new file mode 100644
index 000000000..abc1131e8
--- /dev/null
+++ b/tests/language-feature/interfaces/generic-value-parameter-via-interface.slang
@@ -0,0 +1,44 @@
+//TEST:INTERPRET(filecheck=CHECK):
+
+interface IFoo
+{
+    void test();
+};
+
+struct Foo<let GenericValue : uint> : IFoo
+{
+    void test()
+    {
+        printf("GenericValue=%d, value=%d\n", GenericValue, value);
+    }
+
+    uint value;
+};
+
+void testInterfaceAsParameter(IFoo foo)
+{
+    foo.test();
+}
+
+void testInterfaceAsGeneric<T:IFoo>(T foo)
+{
+    foo.test();
+}
+
+void main()
+{
+    // CHECK-COUNT-2:GenericValue=0, value=0
+    Foo<0> foo0 = {0};
+    testInterfaceAsParameter(foo0);
+    testInterfaceAsGeneric(foo0);
+
+    // CHECK-COUNT-2:GenericValue=1, value=1
+    Foo<1> foo1 = {1};
+    testInterfaceAsParameter(foo1);
+    testInterfaceAsGeneric(foo1);
+
+    // CHECK-COUNT-2:GenericValue=2, value=2
+    Foo<2> foo2 = {2};
+    testInterfaceAsParameter(foo2);
+    testInterfaceAsGeneric(foo2);
+}