[Clang][Sema] Always use latest redeclaration of primary template (#114258)

This patch fixes a couple of regressions introduced in #111852.

Consider:

```
template<typename T>
struct A
{
    template<bool U>
    static constexpr bool f() requires U
    {
        return true;
    }
};

template<>
template<bool U>
constexpr bool A<short>::f() requires U
{
    return A<long>::f<U>();
}

template<>
template<bool U>
constexpr bool A<long>::f() requires U
{
    return true;
}

static_assert(A<short>::f<true>()); // crash here
```

This crashes because when collecting template arguments from the _first_
declaration of `A<long>::f<true>` for constraint checking, we don't add
the template arguments from the enclosing class template specialization
because there exists another redeclaration that is a member
specialization.

This also fixes the following example, which happens for a similar
reason:
```
// input.cppm

export module input;

export template<int N>
constexpr int f();

template<int N>
struct A {
  template<int J>
  friend constexpr int f();
};

template struct A<0>;

template<int N>
constexpr int f() {
  return N;
}
```

```
// input.cpp

import input;

static_assert(f<1>() == 1); // error: static assertion failed
```

Author: sdkrystian

clang/include/clang/AST/DeclTemplate.h

@@ -857,16 +857,6 @@ class RedeclarableTemplateDecl : public TemplateDecl,
   /// \endcode
   bool isMemberSpecialization() const { return Common.getInt(); }
 
-  /// Determines whether any redeclaration of this template was
-  /// a specialization of a member template.
-  bool hasMemberSpecialization() const {
-    for (const auto *D : redecls()) {
-      if (D->isMemberSpecialization())
-        return true;
-    }
-    return false;
-  }
-
   /// Note that this member template is a specialization.
   void setMemberSpecialization() {
     assert(!isMemberSpecialization() && "already a member specialization");
@@ -1965,13 +1955,7 @@ class ClassTemplateSpecializationDecl : public CXXRecordDecl,
   /// specialization which was specialized by this.
   llvm::PointerUnion<ClassTemplateDecl *,
                      ClassTemplatePartialSpecializationDecl *>
-  getSpecializedTemplateOrPartial() const {
-    if (const auto *PartialSpec =
-            SpecializedTemplate.dyn_cast<SpecializedPartialSpecialization *>())
-      return PartialSpec->PartialSpecialization;
-
-    return SpecializedTemplate.get<ClassTemplateDecl*>();
-  }
+  getSpecializedTemplateOrPartial() const;
 
   /// Retrieve the set of template arguments that should be used
   /// to instantiate members of the class template or class template partial
@@ -2208,17 +2192,6 @@ class ClassTemplatePartialSpecializationDecl
     return InstantiatedFromMember.getInt();
   }
 
-  /// Determines whether any redeclaration of this this class template partial
-  /// specialization was a specialization of a member partial specialization.
-  bool hasMemberSpecialization() const {
-    for (const auto *D : redecls()) {
-      if (cast<ClassTemplatePartialSpecializationDecl>(D)
-              ->isMemberSpecialization())
-        return true;
-    }
-    return false;
-  }
-
   /// Note that this member template is a specialization.
   void setMemberSpecialization() { return InstantiatedFromMember.setInt(true); }
 
@@ -2740,13 +2713,7 @@ class VarTemplateSpecializationDecl : public VarDecl,
   /// Retrieve the variable template or variable template partial
   /// specialization which was specialized by this.
   llvm::PointerUnion<VarTemplateDecl *, VarTemplatePartialSpecializationDecl *>
-  getSpecializedTemplateOrPartial() const {
-    if (const auto *PartialSpec =
-            SpecializedTemplate.dyn_cast<SpecializedPartialSpecialization *>())
-      return PartialSpec->PartialSpecialization;
-
-    return SpecializedTemplate.get<VarTemplateDecl *>();
-  }
+  getSpecializedTemplateOrPartial() const;
 
   /// Retrieve the set of template arguments that should be used
   /// to instantiate the initializer of the variable template or variable
@@ -2980,18 +2947,6 @@ class VarTemplatePartialSpecializationDecl
     return InstantiatedFromMember.getInt();
   }
 
-  /// Determines whether any redeclaration of this this variable template
-  /// partial specialization was a specialization of a member partial
-  /// specialization.
-  bool hasMemberSpecialization() const {
-    for (const auto *D : redecls()) {
-      if (cast<VarTemplatePartialSpecializationDecl>(D)
-              ->isMemberSpecialization())
-        return true;
-    }
-    return false;
-  }
-
   /// Note that this member template is a specialization.
   void setMemberSpecialization() { return InstantiatedFromMember.setInt(true); }
 
@@ -3164,6 +3119,9 @@ class VarTemplateDecl : public RedeclarableTemplateDecl {
     return makeSpecIterator(getSpecializations(), true);
   }
 
+  /// Merge \p Prev with our RedeclarableTemplateDecl::Common.
+  void mergePrevDecl(VarTemplateDecl *Prev);
+
   // Implement isa/cast/dyncast support
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == VarTemplate; }

clang/lib/AST/Decl.cpp

@@ -2708,7 +2708,7 @@ VarDecl *VarDecl::getTemplateInstantiationPattern() const {
     if (isTemplateInstantiation(VDTemplSpec->getTemplateSpecializationKind())) {
       auto From = VDTemplSpec->getInstantiatedFrom();
       if (auto *VTD = From.dyn_cast<VarTemplateDecl *>()) {
-        while (!VTD->hasMemberSpecialization()) {
+        while (!VTD->isMemberSpecialization()) {
           if (auto *NewVTD = VTD->getInstantiatedFromMemberTemplate())
             VTD = NewVTD;
           else
@@ -2718,7 +2718,7 @@ VarDecl *VarDecl::getTemplateInstantiationPattern() const {
       }
       if (auto *VTPSD =
               From.dyn_cast<VarTemplatePartialSpecializationDecl *>()) {
-        while (!VTPSD->hasMemberSpecialization()) {
+        while (!VTPSD->isMemberSpecialization()) {
           if (auto *NewVTPSD = VTPSD->getInstantiatedFromMember())
             VTPSD = NewVTPSD;
           else
@@ -2732,7 +2732,7 @@ VarDecl *VarDecl::getTemplateInstantiationPattern() const {
   // If this is the pattern of a variable template, find where it was
   // instantiated from. FIXME: Is this necessary?
   if (VarTemplateDecl *VTD = VD->getDescribedVarTemplate()) {
-    while (!VTD->hasMemberSpecialization()) {
+    while (!VTD->isMemberSpecialization()) {
       if (auto *NewVTD = VTD->getInstantiatedFromMemberTemplate())
         VTD = NewVTD;
       else
@@ -4153,7 +4153,7 @@ FunctionDecl::getTemplateInstantiationPattern(bool ForDefinition) const {
   if (FunctionTemplateDecl *Primary = getPrimaryTemplate()) {
     // If we hit a point where the user provided a specialization of this
     // template, we're done looking.
-    while (!ForDefinition || !Primary->hasMemberSpecialization()) {
+    while (!ForDefinition || !Primary->isMemberSpecialization()) {
       if (auto *NewPrimary = Primary->getInstantiatedFromMemberTemplate())
         Primary = NewPrimary;
       else
@@ -4170,7 +4170,7 @@ FunctionTemplateDecl *FunctionDecl::getPrimaryTemplate() const {
   if (FunctionTemplateSpecializationInfo *Info
         = TemplateOrSpecialization
             .dyn_cast<FunctionTemplateSpecializationInfo*>()) {
-    return Info->getTemplate();
+    return Info->getTemplate()->getMostRecentDecl();
   }
   return nullptr;
 }

clang/lib/AST/DeclCXX.cpp

@@ -2030,7 +2030,7 @@ const CXXRecordDecl *CXXRecordDecl::getTemplateInstantiationPattern() const {
   if (auto *TD = dyn_cast<ClassTemplateSpecializationDecl>(this)) {
     auto From = TD->getInstantiatedFrom();
     if (auto *CTD = From.dyn_cast<ClassTemplateDecl *>()) {
-      while (!CTD->hasMemberSpecialization()) {
+      while (!CTD->isMemberSpecialization()) {
         if (auto *NewCTD = CTD->getInstantiatedFromMemberTemplate())
           CTD = NewCTD;
         else
@@ -2040,7 +2040,7 @@ const CXXRecordDecl *CXXRecordDecl::getTemplateInstantiationPattern() const {
     }
     if (auto *CTPSD =
             From.dyn_cast<ClassTemplatePartialSpecializationDecl *>()) {
-      while (!CTPSD->hasMemberSpecialization()) {
+      while (!CTPSD->isMemberSpecialization()) {
         if (auto *NewCTPSD = CTPSD->getInstantiatedFromMemberTemplate())
           CTPSD = NewCTPSD;
         else

clang/lib/AST/DeclTemplate.cpp

@@ -993,7 +993,17 @@ ClassTemplateSpecializationDecl::getSpecializedTemplate() const {
   if (const auto *PartialSpec =
           SpecializedTemplate.dyn_cast<SpecializedPartialSpecialization*>())
     return PartialSpec->PartialSpecialization->getSpecializedTemplate();
-  return SpecializedTemplate.get<ClassTemplateDecl*>();
+  return SpecializedTemplate.get<ClassTemplateDecl *>()->getMostRecentDecl();
+}
+
+llvm::PointerUnion<ClassTemplateDecl *,
+                   ClassTemplatePartialSpecializationDecl *>
+ClassTemplateSpecializationDecl::getSpecializedTemplateOrPartial() const {
+  if (const auto *PartialSpec =
+          SpecializedTemplate.dyn_cast<SpecializedPartialSpecialization *>())
+    return PartialSpec->PartialSpecialization->getMostRecentDecl();
+
+  return SpecializedTemplate.get<ClassTemplateDecl *>()->getMostRecentDecl();
 }
 
 SourceRange
@@ -1283,6 +1293,39 @@ VarTemplateDecl::newCommon(ASTContext &C) const {
   return CommonPtr;
 }
 
+void VarTemplateDecl::mergePrevDecl(VarTemplateDecl *Prev) {
+  // If we haven't created a common pointer yet, then it can just be created
+  // with the usual method.
+  if (!getCommonPtrInternal())
+    return;
+
+  Common *ThisCommon = static_cast<Common *>(getCommonPtrInternal());
+  Common *PrevCommon = nullptr;
+  SmallVector<VarTemplateDecl *, 8> PreviousDecls;
+  for (; Prev; Prev = Prev->getPreviousDecl()) {
+    if (CommonBase *C = Prev->getCommonPtrInternal()) {
+      PrevCommon = static_cast<Common *>(C);
+      break;
+    }
+    PreviousDecls.push_back(Prev);
+  }
+
+  // If the previous redecl chain hasn't created a common pointer yet, then just
+  // use this common pointer.
+  if (!PrevCommon) {
+    for (auto *D : PreviousDecls)
+      D->setCommonPtr(ThisCommon);
+    return;
+  }
+
+  // Ensure we don't leak any important state.
+  assert(ThisCommon->Specializations.empty() &&
+         ThisCommon->PartialSpecializations.empty() &&
+         "Can't merge incompatible declarations!");
+
+  setCommonPtr(PrevCommon);
+}
+
 VarTemplateSpecializationDecl *
 VarTemplateDecl::findSpecialization(ArrayRef<TemplateArgument> Args,
                                     void *&InsertPos) {
@@ -1405,7 +1448,16 @@ VarTemplateDecl *VarTemplateSpecializationDecl::getSpecializedTemplate() const {
   if (const auto *PartialSpec =
           SpecializedTemplate.dyn_cast<SpecializedPartialSpecialization *>())
     return PartialSpec->PartialSpecialization->getSpecializedTemplate();
-  return SpecializedTemplate.get<VarTemplateDecl *>();
+  return SpecializedTemplate.get<VarTemplateDecl *>()->getMostRecentDecl();
+}
+
+llvm::PointerUnion<VarTemplateDecl *, VarTemplatePartialSpecializationDecl *>
+VarTemplateSpecializationDecl::getSpecializedTemplateOrPartial() const {
+  if (const auto *PartialSpec =
+          SpecializedTemplate.dyn_cast<SpecializedPartialSpecialization *>())
+    return PartialSpec->PartialSpecialization->getMostRecentDecl();
+
+  return SpecializedTemplate.get<VarTemplateDecl *>()->getMostRecentDecl();
 }
 
 SourceRange VarTemplateSpecializationDecl::getSourceRange() const {

clang/lib/Sema/SemaDecl.cpp

@@ -4696,8 +4696,10 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
 
   // Keep a chain of previous declarations.
   New->setPreviousDecl(Old);
-  if (NewTemplate)
+  if (NewTemplate) {
+    NewTemplate->mergePrevDecl(OldTemplate);
     NewTemplate->setPreviousDecl(OldTemplate);
+  }
 
   // Inherit access appropriately.
   New->setAccess(Old->getAccess());

clang/lib/Sema/SemaInit.cpp

@@ -9954,7 +9954,7 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
     auto SynthesizeAggrGuide = [&](InitListExpr *ListInit) {
       auto *Pattern = Template;
       while (Pattern->getInstantiatedFromMemberTemplate()) {
-        if (Pattern->hasMemberSpecialization())
+        if (Pattern->isMemberSpecialization())
           break;
         Pattern = Pattern->getInstantiatedFromMemberTemplate();
       }

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -343,7 +343,7 @@ struct TemplateInstantiationArgumentCollecter
       // If this function was instantiated from a specialized member that is
       // a function template, we're done.
       assert(FD->getPrimaryTemplate() && "No function template?");
-      if (FD->getPrimaryTemplate()->hasMemberSpecialization())
+      if (FD->getPrimaryTemplate()->isMemberSpecialization())
         return Done();
 
       // If this function is a generic lambda specialization, we are done.
@@ -442,11 +442,11 @@ struct TemplateInstantiationArgumentCollecter
         Specialized = CTSD->getSpecializedTemplateOrPartial();
     if (auto *CTPSD =
             Specialized.dyn_cast<ClassTemplatePartialSpecializationDecl *>()) {
-      if (CTPSD->hasMemberSpecialization())
+      if (CTPSD->isMemberSpecialization())
         return Done();
     } else {
       auto *CTD = Specialized.get<ClassTemplateDecl *>();
-      if (CTD->hasMemberSpecialization())
+      if (CTD->isMemberSpecialization())
         return Done();
     }
     return UseNextDecl(CTSD);
@@ -478,11 +478,11 @@ struct TemplateInstantiationArgumentCollecter
         Specialized = VTSD->getSpecializedTemplateOrPartial();
     if (auto *VTPSD =
             Specialized.dyn_cast<VarTemplatePartialSpecializationDecl *>()) {
-      if (VTPSD->hasMemberSpecialization())
+      if (VTPSD->isMemberSpecialization())
         return Done();
     } else {
       auto *VTD = Specialized.get<VarTemplateDecl *>();
-      if (VTD->hasMemberSpecialization())
+      if (VTD->isMemberSpecialization())
         return Done();
     }
     return UseNextDecl(VTSD);
@@ -4141,7 +4141,7 @@ getPatternForClassTemplateSpecialization(
   CXXRecordDecl *Pattern = nullptr;
   Specialized = ClassTemplateSpec->getSpecializedTemplateOrPartial();
   if (auto *CTD = Specialized.dyn_cast<ClassTemplateDecl *>()) {
-    while (!CTD->hasMemberSpecialization()) {
+    while (!CTD->isMemberSpecialization()) {
       if (auto *NewCTD = CTD->getInstantiatedFromMemberTemplate())
         CTD = NewCTD;
       else
@@ -4151,7 +4151,7 @@ getPatternForClassTemplateSpecialization(
   } else if (auto *CTPSD =
                  Specialized
                      .dyn_cast<ClassTemplatePartialSpecializationDecl *>()) {
-    while (!CTPSD->hasMemberSpecialization()) {
+    while (!CTPSD->isMemberSpecialization()) {
       if (auto *NewCTPSD = CTPSD->getInstantiatedFromMemberTemplate())
         CTPSD = NewCTPSD;
       else

clang/test/AST/ast-dump-decl.cpp

@@ -530,7 +530,7 @@ namespace testCanonicalTemplate {
   // CHECK-NEXT: |   `-ClassTemplateDecl 0x{{.+}} parent 0x{{.+}} <col:5, col:40> col:40 friend_undeclared TestClassTemplate{{$}}
   // CHECK-NEXT: |     |-TemplateTypeParmDecl 0x{{.+}} <col:14, col:23> col:23 typename depth 1 index 0 T2{{$}}
   // CHECK-NEXT: |     `-CXXRecordDecl 0x{{.+}} parent 0x{{.+}} <col:34, col:40> col:40 class TestClassTemplate{{$}}
-  // CHECK-NEXT: `-ClassTemplateSpecializationDecl 0x{{.+}} <line:[[@LINE-19]]:3, line:[[@LINE-17]]:3> line:[[@LINE-19]]:31 class TestClassTemplate definition implicit_instantiation{{$}}
+  // CHECK-NEXT: `-ClassTemplateSpecializationDecl 0x{{.+}} <col:5, col:40> line:[[@LINE-19]]:31 class TestClassTemplate definition implicit_instantiation{{$}}
   // CHECK-NEXT:   |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init{{$}}
   // CHECK-NEXT:   | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr{{$}}
   // CHECK-NEXT:   | |-CopyConstructor simple trivial has_const_param implicit_has_const_param{{$}}