[Clang] Do not try to transform invalid bindings

In the presence of an invalid structured binding decomposition,
some binding packs may be invalid and trying to transform them
would produce a recovery expression that does not contains a pack,
leading to assertions in places where we would expect a pack
at that stage.

Fixes #125165

Author: cor3ntin

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -2481,7 +2481,7 @@ TemplateInstantiator::TransformDeclRefExpr(DeclRefExpr *E) {
 
   if (BindingDecl *BD = dyn_cast<BindingDecl>(D); BD && BD->isParameterPack()) {
     BD = cast_or_null<BindingDecl>(TransformDecl(BD->getLocation(), BD));
-    if (!BD)
+    if (!BD || BD->isInvalidDecl())
       return ExprError();
     if (auto *RP =
             dyn_cast_if_present<ResolvedUnexpandedPackExpr>(BD->getBinding()))

clang/test/SemaCXX/cxx2c-binding-pack.cpp

@@ -188,3 +188,19 @@ void other_main() {
   static_assert(f<int>() == 2);
 }
 }  // namespace
+
+
+namespace GH125165 {
+
+template <typename = void>
+auto f(auto t) {
+    const auto& [...pack] = t;
+    // expected-error@-1 {{cannot decompose non-class, non-array type 'char const'}}
+    (pack, ...);
+};
+
+void g() {
+    f('x'); // expected-note {{in instantiation}}
+}
+
+}