Do not normalize empty FoldExprs?

Author: zyn0217

clang/lib/Sema/SemaConcept.cpp

@@ -1818,6 +1818,18 @@ substituteParameterMappings(Sema &S, NormalizedConstraint &N,
     return substituteParameterMappings(S, static_cast<AtomicConstraint &>(N),
                                        MLTAL, ArgsAsWritten);
   case NormalizedConstraint::ConstraintKind::FoldExpanded: {
+    auto &FE = static_cast<FoldExpandedConstraint &>(N);
+    // FIXME: This is not used.
+    ConstraintSatisfaction CS;
+    UnsignedOrNone PackSize = EvaluateFoldExpandedConstraintSize(
+        S, FE, /*Template=*/nullptr, /*TemplateNameLoc=*/SourceLocation(),
+        MLTAL, CS);
+    if (!PackSize)
+      return true;
+
+    // Do we need to do something else when pack is empty?
+    if (!*PackSize)
+      return false;
     Sema::ArgPackSubstIndexRAII _(S, std::nullopt);
     return substituteParameterMappings(
         S, static_cast<FoldExpandedConstraint &>(N).getNormalizedPattern(),

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1896,15 +1896,15 @@ class TemplateInstantiator : public TreeTransform<TemplateInstantiator> {
       return false;
     }
 
-  TemplateParameterList *
-  TransformTemplateParameterList(TemplateParameterList *OrigTPL) {
-    if (!OrigTPL || !OrigTPL->size())
-      return OrigTPL;
+    TemplateParameterList *
+    TransformTemplateParameterList(TemplateParameterList *OrigTPL) {
+      if (!OrigTPL || !OrigTPL->size())
+        return OrigTPL;
 
       DeclContext *Owner = OrigTPL->getParam(0)->getDeclContext();
-      TemplateDeclInstantiator  DeclInstantiator(getSema(),
-                                              /* DeclContext *Owner */ Owner,
-                                              TemplateArgs);
+      TemplateDeclInstantiator DeclInstantiator(getSema(),
+                                                /* DeclContext *Owner */ Owner,
+                                                TemplateArgs);
       DeclInstantiator.setEvaluateConstraints(EvaluateConstraints);
       return DeclInstantiator.SubstTemplateParams(OrigTPL);
     }