[SYCL] Do not allow template instantiation to create null attributes. (#3575)

Consumers of the attributes to be added on the attributed statement do
not typically expect to get a null attribute, so make that impossible.

Author: AaronBallman

clang/lib/Sema/SemaStmtAttr.cpp

@@ -166,14 +166,14 @@ template <typename T>
 static void FilterAttributeList(ArrayRef<const Attr *> Attrs,
                     SmallVectorImpl<const T *> &FilteredAttrs) {
 
-  llvm::transform(Attrs, std::back_inserter(FilteredAttrs), [](const Attr *A) {
-    if (const auto *Cast = dyn_cast_or_null<const T>(A))
-      return Cast->isDependent() ? nullptr : Cast;
-    return static_cast<const T*>(nullptr);
-  });
+  llvm::transform(Attrs, std::back_inserter(FilteredAttrs),
+                  [](const Attr *A) -> const T * {
+                    if (const auto *Cast = dyn_cast<T>(A))
+                      return Cast->isDependent() ? nullptr : Cast;
+                    return nullptr;
+                  });
   FilteredAttrs.erase(
-      std::remove(FilteredAttrs.begin(), FilteredAttrs.end(),
-                  static_cast<const T*>(nullptr)),
+      std::remove(FilteredAttrs.begin(), FilteredAttrs.end(), nullptr),
       FilteredAttrs.end());
 }
 

clang/lib/Sema/TreeTransform.h

@@ -7321,7 +7321,8 @@ TreeTransform<Derived>::TransformAttributedStmt(AttributedStmt *S,
   for (const auto *I : S->getAttrs()) {
     const Attr *R = getDerived().TransformAttr(I);
     AttrsChanged |= (I != R);
-    Attrs.push_back(R);
+    if (R)
+      Attrs.push_back(R);
   }
 
   StmtResult SubStmt = getDerived().TransformStmt(S->getSubStmt(), SDK);
@@ -7331,6 +7332,11 @@ TreeTransform<Derived>::TransformAttributedStmt(AttributedStmt *S,
   if (SubStmt.get() == S->getSubStmt() && !AttrsChanged)
     return S;
 
+  // If transforming the attributes failed for all of the attributes in the
+  // statement, don't make an AttributedStmt without attributes.
+  if (Attrs.empty())
+    return SubStmt;
+
   return getDerived().RebuildAttributedStmt(S->getAttrLoc(), Attrs,
                                             SubStmt.get());
 }

clang/test/SemaSYCL/attr-instantiate.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsycl-is-device -verify -fsyntax-only %s
+
+// Test to ensure that template instantiation of an invalid attribute argument
+// does not result in a null pointer crash when rebuilding the attributed
+// statement.
+template <int A> void bar() {
+  // expected-error@+1 {{'loop_unroll' attribute requires a positive integral compile time constant expression}}
+  [[clang::loop_unroll(A)]] for (int i = 0; i < 10; ++i);
+}
+
+void foo() {
+  bar<-1>(); // expected-note {{in instantiation of function template specialization 'bar<-1>' requested here}}
+}