llvm · mizvekov · Sep 28, 2025 · Sep 24, 2025
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -299,7 +299,8 @@ Improvements to Clang's diagnostics
   "format specifies type 'unsigned int' but the argument has type 'int', which differs in signedness [-Wformat-signedness]"
   "signedness of format specifier 'u' is incompatible with 'c' [-Wformat-signedness]"
   and the API-visible diagnostic id will be appropriate.
-
+- Clang now produces better diagnostics for template template parameter matching
+  involving 'auto' template parameters.
 - Fixed false positives in ``-Waddress-of-packed-member`` diagnostics when
   potential misaligned members get processed before they can get discarded.
   (#GH144729)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
@@ -7068,22 +7068,8 @@ ExprResult Sema::CheckTemplateArgument(NamedDecl *Param, QualType ParamType,
 
   // If the parameter type somehow involves auto, deduce the type now.
   DeducedType *DeducedT = ParamType->getContainedDeducedType();
-  if (getLangOpts().CPlusPlus17 && DeducedT && !DeducedT->isDeduced()) {
-    // During template argument deduction, we allow 'decltype(auto)' to
-    // match an arbitrary dependent argument.
-    // FIXME: The language rules don't say what happens in this case.
-    // FIXME: We get an opaque dependent type out of decltype(auto) if the
-    // expression is merely instantiation-dependent; is this enough?
-    if (DeductionArg->isTypeDependent()) {
-      auto *AT = dyn_cast<AutoType>(DeducedT);
-      if (AT && AT->isDecltypeAuto()) {
-        SugaredConverted = TemplateArgument(Arg, /*IsCanonical=*/false);
-        CanonicalConverted = TemplateArgument(
-            Context.getCanonicalTemplateArgument(SugaredConverted));
-        return Arg;
-      }
-    }
-
+  bool IsDeduced = DeducedT && !DeducedT->isDeduced();
+  if (IsDeduced) {
     // When checking a deduced template argument, deduce from its type even if
     // the type is dependent, in order to check the types of non-type template
     // arguments line up properly in partial ordering.
@@ -7112,17 +7098,21 @@ ExprResult Sema::CheckTemplateArgument(NamedDecl *Param, QualType ParamType,
                          // along with the other associated constraints after
                          // checking the template argument list.
                          /*IgnoreConstraints=*/true);
-      if (Result == TemplateDeductionResult::AlreadyDiagnosed) {
-        return ExprError();
-      } else if (Result != TemplateDeductionResult::Success) {
-        if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
-          Diag(Arg->getExprLoc(),
-               diag::err_non_type_template_parm_type_deduction_failure)
-              << Param->getDeclName() << NTTP->getType() << Arg->getType()
-              << Arg->getSourceRange();
+      if (Result != TemplateDeductionResult::Success) {
+        ParamType = TSI->getType();
+        if (StrictCheck || !DeductionArg->isTypeDependent()) {
+          if (Result == TemplateDeductionResult::AlreadyDiagnosed)
+            return ExprError();
+          if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param))
+            Diag(Arg->getExprLoc(),
+                 diag::err_non_type_template_parm_type_deduction_failure)
+                << Param->getDeclName() << NTTP->getType() << Arg->getType()
+                << Arg->getSourceRange();
+          NoteTemplateParameterLocation(*Param);
+          return ExprError();
         }
-        NoteTemplateParameterLocation(*Param);
-        return ExprError();
+        ParamType = SubstAutoTypeDependent(ParamType);
+        assert(!ParamType.isNull() && "substituting DependentTy can't fail");
       }
     }
     // CheckNonTypeTemplateParameterType will produce a diagnostic if there's
@@ -7144,14 +7134,16 @@ ExprResult Sema::CheckTemplateArgument(NamedDecl *Param, QualType ParamType,
   // type-dependent, there's nothing we can check now.
   if (ParamType->isDependentType() || DeductionArg->isTypeDependent()) {
     // Force the argument to the type of the parameter to maintain invariants.
-    ExprResult E = ImpCastExprToType(
-        DeductionArg, ParamType.getNonLValueExprType(Context), CK_Dependent,
-        ParamType->isLValueReferenceType()   ? VK_LValue
-        : ParamType->isRValueReferenceType() ? VK_XValue
-                                             : VK_PRValue);
-    if (E.isInvalid())
-      return ExprError();
-    setDeductionArg(E.get());
+    if (!IsDeduced) {
+      ExprResult E = ImpCastExprToType(
+          DeductionArg, ParamType.getNonLValueExprType(Context), CK_Dependent,
+          ParamType->isLValueReferenceType()   ? VK_LValue
+          : ParamType->isRValueReferenceType() ? VK_XValue
+                                               : VK_PRValue);
+      if (E.isInvalid())
+        return ExprError();
+      setDeductionArg(E.get());
+    }
     SugaredConverted = TemplateArgument(Arg, /*IsCanonical=*/false);
     CanonicalConverted = TemplateArgument(
         Context.getCanonicalTemplateArgument(SugaredConverted));
@@ -8555,6 +8547,7 @@ static SourceRange findTemplateParameter(unsigned Depth, TypeLoc TL) {
 static bool CheckNonTypeTemplatePartialSpecializationArgs(
     Sema &S, SourceLocation TemplateNameLoc, NonTypeTemplateParmDecl *Param,
     const TemplateArgument *Args, unsigned NumArgs, bool IsDefaultArgument) {
+  bool HasError = false;
   for (unsigned I = 0; I != NumArgs; ++I) {
     if (Args[I].getKind() == TemplateArgument::Pack) {
       if (CheckNonTypeTemplatePartialSpecializationArgs(
@@ -8569,6 +8562,10 @@ static bool CheckNonTypeTemplatePartialSpecializationArgs(
       continue;
 
     Expr *ArgExpr = Args[I].getAsExpr();
+    if (ArgExpr->containsErrors()) {
+      HasError = true;
+      continue;
+    }
 
     // We can have a pack expansion of any of the bullets below.
     if (PackExpansionExpr *Expansion = dyn_cast<PackExpansionExpr>(ArgExpr))
@@ -8638,7 +8635,7 @@ static bool CheckNonTypeTemplatePartialSpecializationArgs(
     }
   }
 
-  return false;
+  return HasError;
 }
 
 bool Sema::CheckTemplatePartialSpecializationArgs(

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -5262,18 +5262,6 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *Init, QualType &Result,
   SmallVector<DeducedTemplateArgument, 1> Deduced;
   Deduced.resize(1);
 
-  // If deduction failed, don't diagnose if the initializer is dependent; it
-  // might acquire a matching type in the instantiation.
-  auto DeductionFailed = [&](TemplateDeductionResult TDK) {
-    if (Init->isTypeDependent()) {
-      Result =
-          SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
-      assert(!Result.isNull() && "substituting DependentTy can't fail");
-      return TemplateDeductionResult::Success;
-    }
-    return TDK;
-  };
-
   SmallVector<OriginalCallArg, 4> OriginalCallArgs;
 
   QualType DeducedType;
@@ -5323,9 +5311,9 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *Init, QualType &Result,
             Diag(Info.getLocation(), diag::err_auto_inconsistent_deduction)
                 << Info.FirstArg << Info.SecondArg << DeducedFromInitRange
                 << Init->getSourceRange();
-            return DeductionFailed(TemplateDeductionResult::AlreadyDiagnosed);
+            return TemplateDeductionResult::AlreadyDiagnosed;
           }
-          return DeductionFailed(TDK);
+          return TDK;
         }
 
         if (DeducedFromInitRange.isInvalid() &&
@@ -5347,12 +5335,12 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *Init, QualType &Result,
               OriginalCallArgs,
               /*Decomposed=*/false, /*ArgIdx=*/0, /*TDF=*/0, FailedTSC);
           TDK != TemplateDeductionResult::Success)
-        return DeductionFailed(TDK);
+        return TDK;
     }
 
     // Could be null if somehow 'auto' appears in a non-deduced context.
     if (Deduced[0].getKind() != TemplateArgument::Type)
-      return DeductionFailed(TemplateDeductionResult::Incomplete);
+      return TemplateDeductionResult::Incomplete;
     DeducedType = Deduced[0].getAsType();
 
     if (InitList) {
@@ -5366,7 +5354,7 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *Init, QualType &Result,
     if (!Context.hasSameType(DeducedType, Result)) {
       Info.FirstArg = Result;
       Info.SecondArg = DeducedType;
-      return DeductionFailed(TemplateDeductionResult::Inconsistent);
+      return TemplateDeductionResult::Inconsistent;
     }
     DeducedType = Context.getCommonSugaredType(Result, DeducedType);
   }
@@ -5390,7 +5378,7 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *Init, QualType &Result,
             CheckOriginalCallArgDeduction(*this, Info, OriginalArg, DeducedA);
         TDK != TemplateDeductionResult::Success) {
       Result = QualType();
-      return DeductionFailed(TDK);
+      return TDK;
     }
   }
 
@@ -5412,13 +5400,17 @@ TypeSourceInfo *Sema::SubstAutoTypeSourceInfo(TypeSourceInfo *TypeWithAuto,
 }
 
 QualType Sema::SubstAutoTypeDependent(QualType TypeWithAuto) {
-  return SubstituteDeducedTypeTransform(*this, DependentAuto{false})
+  return SubstituteDeducedTypeTransform(
+             *this,
+             DependentAuto{/*IsPack=*/isa<PackExpansionType>(TypeWithAuto)})
       .TransformType(TypeWithAuto);
 }
 
 TypeSourceInfo *
 Sema::SubstAutoTypeSourceInfoDependent(TypeSourceInfo *TypeWithAuto) {
-  return SubstituteDeducedTypeTransform(*this, DependentAuto{false})
+  return SubstituteDeducedTypeTransform(
+             *this, DependentAuto{/*IsPack=*/isa<PackExpansionType>(
+                        TypeWithAuto->getType())})
       .TransformType(TypeWithAuto);
 }
 

diff --git a/clang/test/SemaTemplate/temp_arg_template_p0522.cpp b/clang/test/SemaTemplate/temp_arg_template_p0522.cpp
@@ -83,11 +83,11 @@ namespace DependentType {
 namespace Auto {
   template<template<int> typename T> struct TInt {}; // #TInt
   template<template<int*> typename T> struct TIntPtr {}; // #TIntPtr
-  template<template<auto> typename T> struct TAuto {};
+  template<template<auto> typename T> struct TAuto {}; // #TAuto
   template<template<auto*> typename T> struct TAutoPtr {};
-  template<template<decltype(auto)> typename T> struct TDecltypeAuto {};
+  template<template<decltype(auto)> typename T> struct TDecltypeAuto {}; // #TDecltypeAuto
   template<auto> struct Auto;
-  template<auto*> struct AutoPtr; // #AutoPtr
+  template<auto*> struct AutoPtr;
   template<decltype(auto)> struct DecltypeAuto;
   template<int> struct Int;
   template<int*> struct IntPtr;
@@ -108,7 +108,7 @@ namespace Auto {
   TIntPtr<IntPtr> ipip;
 
   TAuto<Auto> aa;
-  TAuto<AutoPtr> aap; // expected-error@#AutoPtr {{could not match 'auto *' against 'auto'}}
+  TAuto<AutoPtr> aap; // expected-error@#TAuto {{non-type template parameter '' with type 'auto *' has incompatible initializer of type 'auto'}}
                       // expected-note@-1 {{different template parameters}}
   TAuto<Int> ai; // FIXME: ill-formed (?)
   TAuto<IntPtr> aip; // FIXME: ill-formed (?)
@@ -130,7 +130,7 @@ namespace Auto {
   // parameters (such as 'user-defined-type &') that are not valid 'auto'
   // parameters.
   TDecltypeAuto<Auto> daa;
-  TDecltypeAuto<AutoPtr> daap; // expected-error@#AutoPtr {{could not match 'auto *' against 'decltype(auto)'}}
+  TDecltypeAuto<AutoPtr> daap; // expected-error@#TDecltypeAuto {{non-type template parameter '' with type 'auto *' has incompatible initializer of type 'decltype(auto)'}}
                                // expected-note@-1 {{different template parameters}}
 
   int n;