Fix issue of double lowering issue a differentiable function (#8182)

Close #8054.
For detailed root cause is at:
https://github.com/shader-slang/slang/issues/8054#issuecomment-3189579508

2 files changed, 31 insertions, 5 deletions

diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index 52d24ff7d..8e1f85f8e 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -10756,6 +10756,11 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
                 });
         }
 
+        // Register the value now, to avoid any possible infinite recursion when lowering the body
+        // or attributes.
+        IRFunc* irFunc = subBuilder->createFunc();
+        context->setGlobalValue(decl, LoweredValInfo::simple(findOuterMostGeneric(irFunc)));
+
         FuncDeclBaseTypeInfo info;
         _lowerFuncDeclBaseTypeInfo(
             subContext,
@@ -10763,15 +10768,10 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
             info);
 
         // need to create an IR function here
-
-        IRFunc* irFunc = subBuilder->createFunc();
         addNameHint(subContext, irFunc, decl);
         addLinkageDecoration(subContext, irFunc, decl);
         maybeAddDebugLocationDecoration(subContext, irFunc);
 
-        // Register the value now, to avoid any possible infinite recursion when lowering the body
-        // or attributes.
-        context->setGlobalValue(decl, LoweredValInfo::simple(findOuterMostGeneric(irFunc)));
 
         // Always force inline diff setter accessor to prevent downstream compiler from complaining
         // fields are not fully initialized for the first `inout` parameter.
diff --git a/tests/autodiff/avoid-double-lowering.slang b/tests/autodiff/avoid-double-lowering.slang
new file mode 100644
index 000000000..354d1d27b
--- /dev/null
+++ b/tests/autodiff/avoid-double-lowering.slang
@@ -0,0 +1,26 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+// CHECK-NOT: an internal error threw
+// CHECK-NOT: error:
+
+interface IFoo<T, int N>: IDifferentiable
+    where T : __BuiltinFloatingPointType
+{
+    This myFunc(This other);
+}
+
+struct Foo<T, int N>
+    where T : __BuiltinFloatingPointType
+{
+    [BackwardDifferentiable]
+    This myFunc(This other)
+    {
+        [MaxIters(N)]
+        for (int i = 0; i < N; i++) {}
+
+        return this;
+    }
+}
+
+extension <T, int N> Foo<T, N> : IFoo<T, N>
+    where T : __BuiltinFloatingPointType {}
+