[Clang] Give empty template parameter mapping an empty MLTAL

clang/lib/Sema/SemaConcept.cpp

@@ -570,6 +570,11 @@ ConstraintSatisfactionChecker::SubstitutionInTemplateArguments(
   if (!Constraint.hasParameterMapping())
     return std::move(MLTAL);
 
+  // The mapping is empty, meaning no template arguments are needed for
+  // evaluation.
+  if (Constraint.getParameterMapping().empty())
+    return MultiLevelTemplateArgumentList();
+
   TemplateDeductionInfo Info(Constraint.getBeginLoc());
   Sema::InstantiatingTemplate Inst(
       S, Constraint.getBeginLoc(),
@@ -2017,8 +2022,13 @@ void SubstituteParameterMappings::buildParameterMapping(
       SemaRef.MarkUsedTemplateParameters(Args->arguments(),
                                          /*Depth=*/0, OccurringIndices);
   }
+  unsigned Size = OccurringIndices.count();
+  // When the constraint is independent of any template parameters,
+  // we build an empty mapping so that we can distinguish these cases
+  // from cases where no mapping exists at all, e.g. when there are only atomic
+  // constraints.
   TemplateArgumentLoc *TempArgs =
-      new (SemaRef.Context) TemplateArgumentLoc[OccurringIndices.count()];
+      new (SemaRef.Context) TemplateArgumentLoc[Size];
   llvm::SmallVector<NamedDecl *> UsedParams;
   for (unsigned I = 0, J = 0, C = TemplateParams->size(); I != C; ++I) {
     SourceLocation Loc = ArgsAsWritten->NumTemplateArgs > I
@@ -2039,7 +2049,6 @@ void SubstituteParameterMappings::buildParameterMapping(
       TemplateParams->getLAngleLoc(), UsedParams,
       /*RAngleLoc=*/SourceLocation(),
       /*RequiresClause=*/nullptr);
-  unsigned Size = OccurringIndices.count();
   N.updateParameterMapping(
       std::move(OccurringIndices), std::move(OccurringIndicesForSubsumption),
       MutableArrayRef<TemplateArgumentLoc>{TempArgs, Size}, UsedList);
@@ -2050,6 +2059,10 @@ bool SubstituteParameterMappings::substitute(
   if (!N.hasParameterMapping())
     buildParameterMapping(N);
 
+  // If the parameter mapping is empty, there is nothing to substitute.
+  if (N.getParameterMapping().empty())
+    return false;
+
   SourceLocation InstLocBegin, InstLocEnd;
   llvm::ArrayRef Arguments = ArgsAsWritten->arguments();
   if (Arguments.empty()) {

clang/test/SemaTemplate/concepts.cpp

@@ -1441,6 +1441,24 @@ void main() { Feeder<int>{}.feed<Cat<int>>(); }
 
 }
 
+namespace case9 {
+
+template <typename>
+concept a = requires { requires true; };
+template <typename T>
+concept b = a<typename T::EntitySpec>;
+template <typename T>
+concept c = requires { b<T>; };
+template <typename T>
+  requires c<T>
+struct s;
+template <typename> constexpr bool f() { return true; }
+template <typename T> constexpr bool d = f<T>();
+struct s2;
+static_assert(d<s<s2>>);
+
+}
+
 }
 
 namespace GH162125 {