From a670bafc121c20168624f70a388dbe8556402c7f Mon Sep 17 00:00:00 2001
From: kaizhangNV <149626564+kaizhangNV@users.noreply.github.com>
Date: Wed, 9 Jul 2025 11:25:29 -0500
Subject: no_diff diagnostics improvement (#7655)

close #6286.

This PR is to improve the diagnostics for no_diff usage.

In a differentiable function, any calls to a non-diff function with constant arguments should not require no_diff attribute.
This PR adds this extra check at `checkAutoDiffUsages` where it checks the differentiability on IR.

In a differentiable method, we will force to use `[NoDiffThis]` attribute if there is access to non-differentiable `This` type. Once this access is detected we will report a warning to bring users attention that this access won't generate any derivative, they have to use `[NoDiffThis]` to suppress that warning.
This PR adds this check at type checking stage, because it's the easiest way to find out all the `This` accesses.
---
 source/slang/slang-check-expr.cpp | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

(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 b28b458da..c7e58a888 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -1632,6 +1632,31 @@ void SemanticsVisitor::maybeRegisterDifferentiableTypeImplRecursive(ASTBuilder*
     }
 }
 
+// This checks that if a differentiable function access a non-diff type "This", in such case we
+// want to provide a non-error diagnostic to the user to notify that there could be an unexpected
+// behavior because every member access will not have derivative computed for it. User can use
+// [NoDiffThis] to clarify that this is intended.
+void SemanticsVisitor::maybeCheckMissingNoDiffThis(Expr* expr)
+{
+    if (auto memberExpr = as<MemberExpr>(expr))
+    {
+        auto thisExpr = as<ThisExpr>(memberExpr->baseExpression);
+        if (thisExpr && isTypeDifferentiable(memberExpr->type.type))
+        {
+            if (isTypeDifferentiable(calcThisType(thisExpr->type.type)) ||
+                this->m_parentFunc->findModifier<NoDiffThisAttribute>())
+            {
+                return;
+            }
+
+            getSink()->diagnose(
+                memberExpr->loc,
+                Diagnostics::noDerivativeOnNonDifferentiableThisType,
+                memberExpr->declRef.getDecl(),
+                this->m_parentFunc);
+        }
+    }
+}
 
 Expr* SemanticsVisitor::CheckTerm(Expr* term)
 {
@@ -1649,7 +1674,13 @@ Expr* SemanticsVisitor::CheckTerm(Expr* term)
     if (this->m_parentFunc && this->m_parentFunc->findModifier<DifferentiableAttribute>())
     {
         maybeRegisterDifferentiableType(getASTBuilder(), checkedTerm->type.type);
+
+        if (!this->m_parentFunc->findModifier<TreatAsDifferentiableAttribute>())
+        {
+            maybeCheckMissingNoDiffThis(checkedTerm);
+        }
     }
+
     return checkedTerm;
 }
 
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