Avoid early specialization for witness tables in SimplifyIR (#7636)

* Avoid early specialization for witness tables in SimplifyIR

Prevents SimplifyIR from prematurely specializing witness tables before
the main specialization pass. Witness tables are hoistable immutable
objects that must maintain consistent signatures to avoid incorrect
deduplication. SimplifyIR was incorrectly transforming expressions like
"witness_table_t(%IFoo)(specialize(%7, %GenericValue4))" into
"witness_table_t(%IFoo)(%Foo)" even when %GenericValue4 was unused.

Fixes #7233

* Add a missing test file

1 files changed, 44 insertions, 0 deletions

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);
+}