Reland: [clang] Track function template instantiation from definition #125266

mizvekov · 2025-01-31T18:11:06Z

This fixes instantiation of definition for friend function templates,
when the declaration found and the one containing the definition
have different template contexts.

In these cases, the the function declaration corresponding to the
definition is not available; it may not even be instantiated at all.

So this patch adds a bit which tracks which function template
declaration was instantiated from the member template.
It's used to find which primary template serves as a context
for the purpose of obtainining the template arguments needed
to instantiate the definition.

Fixes #55509

llvmbot · 2025-01-31T18:11:41Z

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

This fixes instantiation of definition for friend function templates,
when the declaration found and the one containing the definition
have different template contexts.

In these cases, the the function declaration corresponding to the
definition is not available; it may not even be instantiated at all.

So this patch adds a bit which tracks which function template
declaration was instantiated from the member template.
It's used to find which primary template serves as a context
for the purpose of obtainining the template arguments needed
to instantiate the definition.

Fixes #55509

Full diff: https://github.com/llvm/llvm-project/pull/125266.diff

11 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+2)
(modified) clang/include/clang/AST/Decl.h (+7)
(modified) clang/include/clang/AST/DeclBase.h (+6-4)
(modified) clang/include/clang/AST/DeclTemplate.h (+9)
(modified) clang/lib/AST/Decl.cpp (+1)
(modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+1-16)
(modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+4-5)
(modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+25-2)
(modified) clang/lib/Serialization/ASTReaderDecl.cpp (+1)
(modified) clang/lib/Serialization/ASTWriterDecl.cpp (+2-1)
(added) clang/test/SemaTemplate/GH55509.cpp (+112)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8be1ea2fb01455f..500fce9dcb72745 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -128,6 +128,8 @@ Bug Fixes to Attribute Support
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 499d27a9be5a8a4..f305cbbce4c609a 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -2298,6 +2298,13 @@ class FunctionDecl : public DeclaratorDecl,
     FunctionDeclBits.IsLateTemplateParsed = ILT;
   }
 
+  bool isInstantiatedFromMemberTemplate() const {
+    return FunctionDeclBits.IsInstantiatedFromMemberTemplate;
+  }
+  void setInstantiatedFromMemberTemplate(bool Val = true) {
+    FunctionDeclBits.IsInstantiatedFromMemberTemplate = Val;
+  }
+
   /// Whether this function is "trivial" in some specialized C++ senses.
   /// Can only be true for default constructors, copy constructors,
   /// copy assignment operators, and destructors.  Not meaningful until
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 2c0c3a8dc2f9d5c..3a13309a6100eed 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1777,6 +1777,8 @@ class DeclContext {
     uint64_t HasImplicitReturnZero : 1;
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsLateTemplateParsed : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    uint64_t IsInstantiatedFromMemberTemplate : 1;
 
     /// Kind of contexpr specifier as defined by ConstexprSpecKind.
     LLVM_PREFERRED_TYPE(ConstexprSpecKind)
@@ -1827,7 +1829,7 @@ class DeclContext {
   };
 
   /// Number of inherited and non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = NumDeclContextBits + 31 };
+  enum { NumFunctionDeclBits = NumDeclContextBits + 32 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
@@ -1838,12 +1840,12 @@ class DeclContext {
     LLVM_PREFERRED_TYPE(FunctionDeclBitfields)
     uint64_t : NumFunctionDeclBits;
 
-    /// 20 bits to fit in the remaining available space.
+    /// 19 bits to fit in the remaining available space.
     /// Note that this makes CXXConstructorDeclBitfields take
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 17;
+    uint64_t NumCtorInitializers : 16;
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsInheritingConstructor : 1;
 
@@ -1857,7 +1859,7 @@ class DeclContext {
   };
 
   /// Number of inherited and non-inherited bits in CXXConstructorDeclBitfields.
-  enum { NumCXXConstructorDeclBits = NumFunctionDeclBits + 20 };
+  enum { NumCXXConstructorDeclBits = NumFunctionDeclBits + 19 };
 
   /// Stores the bits used by ObjCMethodDecl.
   /// If modified NumObjCMethodDeclBits and the accessor
diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index 9ecff2c898acd5f..0c706036ff70227 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -1011,6 +1011,15 @@ class FunctionTemplateDecl : public RedeclarableTemplateDecl {
     return getTemplatedDecl()->isThisDeclarationADefinition();
   }
 
+  bool isCompatibleWithDefinition() const {
+    return getTemplatedDecl()->isInstantiatedFromMemberTemplate() ||
+           isThisDeclarationADefinition();
+  }
+  void setInstantiatedFromMemberTemplate(FunctionTemplateDecl *D) {
+    getTemplatedDecl()->setInstantiatedFromMemberTemplate();
+    RedeclarableTemplateDecl::setInstantiatedFromMemberTemplate(D);
+  }
+
   /// Return the specialization with the provided arguments if it exists,
   /// otherwise return the insertion point.
   FunctionDecl *findSpecialization(ArrayRef<TemplateArgument> Args,
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 7e8a172ae4c37fa..610207cf8b9a4a1 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3065,6 +3065,7 @@ FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
   FunctionDeclBits.IsIneligibleOrNotSelected = false;
   FunctionDeclBits.HasImplicitReturnZero = false;
   FunctionDeclBits.IsLateTemplateParsed = false;
+  FunctionDeclBits.IsInstantiatedFromMemberTemplate = false;
   FunctionDeclBits.ConstexprKind = static_cast<uint64_t>(ConstexprKind);
   FunctionDeclBits.BodyContainsImmediateEscalatingExpression = false;
   FunctionDeclBits.InstantiationIsPending = false;
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 137942f0c30bfe1..6aaf86a6a6ff369 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -4074,22 +4074,7 @@ TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
   if (FunctionTemplate->getFriendObjectKind())
     Owner = FunctionTemplate->getLexicalDeclContext();
   FunctionDecl *FD = FunctionTemplate->getTemplatedDecl();
-  // additional check for inline friend,
-  // ```
-  //   template <class F1> int foo(F1 X);
-  //   template <int A1> struct A {
-  //     template <class F1> friend int foo(F1 X) { return A1; }
-  //   };
-  //   template struct A<1>;
-  //   int a = foo(1.0);
-  // ```
-  const FunctionDecl *FDFriend;
-  if (FD->getFriendObjectKind() == Decl::FriendObjectKind::FOK_None &&
-      FD->isDefined(FDFriend, /*CheckForPendingFriendDefinition*/ true) &&
-      FDFriend->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None) {
-    FD = const_cast<FunctionDecl *>(FDFriend);
-    Owner = FD->getLexicalDeclContext();
-  }
+
   MultiLevelTemplateArgumentList SubstArgs(
       FunctionTemplate, CanonicalDeducedArgumentList->asArray(),
       /*Final=*/false);
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index dc3bfa97eff399b..12e98a33d07853e 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -479,9 +479,6 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
   using namespace TemplateInstArgsHelpers;
   const Decl *CurDecl = ND;
 
-  if (!CurDecl)
-    CurDecl = Decl::castFromDeclContext(DC);
-
   if (Innermost) {
     Result.addOuterTemplateArguments(const_cast<NamedDecl *>(ND), *Innermost,
                                      Final);
@@ -495,8 +492,10 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
     // has a depth of 0.
     if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(CurDecl))
       HandleDefaultTempArgIntoTempTempParam(TTP, Result);
-    CurDecl = Response::UseNextDecl(CurDecl).NextDecl;
-  }
+    CurDecl = DC ? Decl::castFromDeclContext(DC)
+                 : Response::UseNextDecl(CurDecl).NextDecl;
+  } else if (!CurDecl)
+    CurDecl = Decl::castFromDeclContext(DC);
 
   while (!CurDecl->isFileContextDecl()) {
     Response R;
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 4855e8a23689cef..5aa8a92032f03d4 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -12,6 +12,7 @@
 #include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTLambda.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DependentDiagnostic.h"
@@ -5276,9 +5277,31 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     RebuildTypeSourceInfoForDefaultSpecialMembers();
     SetDeclDefaulted(Function, PatternDecl->getLocation());
   } else {
+    NamedDecl *ND = Function;
+    DeclContext *DC = ND->getLexicalDeclContext();
+    std::optional<ArrayRef<TemplateArgument>> Innermost;
+    if (auto *Primary = Function->getPrimaryTemplate();
+        Primary &&
+        !isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function) &&
+        Function->getTemplateSpecializationKind() !=
+            TSK_ExplicitSpecialization) {
+      auto It = llvm::find_if(Primary->redecls(),
+                              [](const RedeclarableTemplateDecl *RTD) {
+                                return cast<FunctionTemplateDecl>(RTD)
+                                    ->isCompatibleWithDefinition();
+                              });
+      assert(It != Primary->redecls().end() &&
+             "Should't get here without a definition");
+      if (FunctionDecl *Def = cast<FunctionTemplateDecl>(*It)
+                                  ->getTemplatedDecl()
+                                  ->getDefinition())
+        DC = Def->getLexicalDeclContext();
+      else
+        DC = (*It)->getLexicalDeclContext();
+      Innermost.emplace(Function->getTemplateSpecializationArgs()->asArray());
+    }
     MultiLevelTemplateArgumentList TemplateArgs = getTemplateInstantiationArgs(
-        Function, Function->getLexicalDeclContext(), /*Final=*/false,
-        /*Innermost=*/std::nullopt, false, PatternDecl);
+        Function, DC, /*Final=*/false, Innermost, false, PatternDecl);
 
     // Substitute into the qualifier; we can get a substitution failure here
     // through evil use of alias templates.
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 8210eb2143acf56..7a15e60d87d83a3 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1064,6 +1064,7 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   FD->setHasImplicitReturnZero(FunctionDeclBits.getNextBit());
   FD->setIsMultiVersion(FunctionDeclBits.getNextBit());
   FD->setLateTemplateParsed(FunctionDeclBits.getNextBit());
+  FD->setInstantiatedFromMemberTemplate(FunctionDeclBits.getNextBit());
   FD->setFriendConstraintRefersToEnclosingTemplate(
       FunctionDeclBits.getNextBit());
   FD->setUsesSEHTry(FunctionDeclBits.getNextBit());
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index fa2294da95de8db..6a79444bdb98921 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -679,7 +679,7 @@ void ASTDeclWriter::VisitDeclaratorDecl(DeclaratorDecl *D) {
 }
 
 void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
-  static_assert(DeclContext::NumFunctionDeclBits == 44,
+  static_assert(DeclContext::NumFunctionDeclBits == 45,
                 "You need to update the serializer after you change the "
                 "FunctionDeclBits");
 
@@ -785,6 +785,7 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   FunctionDeclBits.addBit(D->hasImplicitReturnZero());
   FunctionDeclBits.addBit(D->isMultiVersion());
   FunctionDeclBits.addBit(D->isLateTemplateParsed());
+  FunctionDeclBits.addBit(D->isInstantiatedFromMemberTemplate());
   FunctionDeclBits.addBit(D->FriendConstraintRefersToEnclosingTemplate());
   FunctionDeclBits.addBit(D->usesSEHTry());
   Record.push_back(FunctionDeclBits);
diff --git a/clang/test/SemaTemplate/GH55509.cpp b/clang/test/SemaTemplate/GH55509.cpp
new file mode 100644
index 000000000000000..773a84305a0cda8
--- /dev/null
+++ b/clang/test/SemaTemplate/GH55509.cpp
@@ -0,0 +1,112 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++26 %s
+
+namespace t1 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C) {
+      return N;
+    }
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C);
+    // expected-note@-1 {{declared here}}
+  };
+
+  void test() {
+    cica(A<0>{}, 0);
+    // expected-error@-1 {{function 'cica<int>' with deduced return type cannot be used before it is defined}}
+
+    (void)A<1>{};
+    cica(A<0>{}, 0);
+  }
+} // namespace t1
+namespace t2 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C) {
+      return N;
+    }
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C);
+  };
+
+  template <int N, class = decltype(cica(A<N>{}, nullptr))>
+  void MakeCica();
+  // expected-note@-1 {{candidate function}}
+
+  template <int N> void MakeCica(A<N+1> = {});
+  // expected-note@-1 {{candidate function}}
+
+  void test() {
+    MakeCica<0>();
+
+    MakeCica<0>();
+    // expected-error@-1 {{call to 'MakeCica' is ambiguous}}
+  }
+} // namespace t2
+namespace t3 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C) {
+      return N-1;
+    }
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C);
+  };
+
+  template <int N, class AT, class = decltype(cica(AT{}, nullptr))>
+  static constexpr bool MakeCica(int);
+
+  template <int N, class AT>
+  static constexpr bool MakeCica(short, A<N+1> = {});
+
+  template <int N, class AT = A<N>, class Val = decltype(MakeCica<N, AT>(0))>
+  static constexpr bool has_cica = Val{};
+
+  constexpr bool cica2 = has_cica<0> || has_cica<0>;
+} // namespace t3
+namespace t4 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C);
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C) {
+      C a;
+    }
+  };
+
+  template struct A<1>;
+
+  void test() {
+    cica(A<0>{}, 0);
+  }
+} // namespace t4
+namespace regression1 {
+  template <class> class A;
+
+  template <class T> [[gnu::abi_tag("TAG")]] void foo(A<T>);
+
+  template <class> struct A {
+    friend void foo <>(A);
+  };
+
+  template struct A<int>;
+
+  template <class T> [[gnu::abi_tag("TAG")]] void foo(A<T>) {}
+
+  template void foo<int>(A<int>);
+} // namespace regression1
+namespace regression2 {
+  template <class> struct A {
+    template <class T> static void f() {
+      A<int>::f<T>();
+    }
+  };
+  template <> template <class T> void A<int>::f() {
+    static_assert(__is_same(T, long));
+  }
+  template void A<void>::f<long>();
+} // namespace regression2

llvmbot · 2025-01-31T18:11:42Z

@llvm/pr-subscribers-clang-modules

Author: Matheus Izvekov (mizvekov)

Changes

This fixes instantiation of definition for friend function templates,
when the declaration found and the one containing the definition
have different template contexts.

In these cases, the the function declaration corresponding to the
definition is not available; it may not even be instantiated at all.

So this patch adds a bit which tracks which function template
declaration was instantiated from the member template.
It's used to find which primary template serves as a context
for the purpose of obtainining the template arguments needed
to instantiate the definition.

Fixes #55509

Full diff: https://github.com/llvm/llvm-project/pull/125266.diff

11 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+2)
(modified) clang/include/clang/AST/Decl.h (+7)
(modified) clang/include/clang/AST/DeclBase.h (+6-4)
(modified) clang/include/clang/AST/DeclTemplate.h (+9)
(modified) clang/lib/AST/Decl.cpp (+1)
(modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+1-16)
(modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+4-5)
(modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+25-2)
(modified) clang/lib/Serialization/ASTReaderDecl.cpp (+1)
(modified) clang/lib/Serialization/ASTWriterDecl.cpp (+2-1)
(added) clang/test/SemaTemplate/GH55509.cpp (+112)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8be1ea2fb01455f..500fce9dcb72745 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -128,6 +128,8 @@ Bug Fixes to Attribute Support
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 499d27a9be5a8a4..f305cbbce4c609a 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -2298,6 +2298,13 @@ class FunctionDecl : public DeclaratorDecl,
     FunctionDeclBits.IsLateTemplateParsed = ILT;
   }
 
+  bool isInstantiatedFromMemberTemplate() const {
+    return FunctionDeclBits.IsInstantiatedFromMemberTemplate;
+  }
+  void setInstantiatedFromMemberTemplate(bool Val = true) {
+    FunctionDeclBits.IsInstantiatedFromMemberTemplate = Val;
+  }
+
   /// Whether this function is "trivial" in some specialized C++ senses.
   /// Can only be true for default constructors, copy constructors,
   /// copy assignment operators, and destructors.  Not meaningful until
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 2c0c3a8dc2f9d5c..3a13309a6100eed 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1777,6 +1777,8 @@ class DeclContext {
     uint64_t HasImplicitReturnZero : 1;
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsLateTemplateParsed : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    uint64_t IsInstantiatedFromMemberTemplate : 1;
 
     /// Kind of contexpr specifier as defined by ConstexprSpecKind.
     LLVM_PREFERRED_TYPE(ConstexprSpecKind)
@@ -1827,7 +1829,7 @@ class DeclContext {
   };
 
   /// Number of inherited and non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = NumDeclContextBits + 31 };
+  enum { NumFunctionDeclBits = NumDeclContextBits + 32 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
@@ -1838,12 +1840,12 @@ class DeclContext {
     LLVM_PREFERRED_TYPE(FunctionDeclBitfields)
     uint64_t : NumFunctionDeclBits;
 
-    /// 20 bits to fit in the remaining available space.
+    /// 19 bits to fit in the remaining available space.
     /// Note that this makes CXXConstructorDeclBitfields take
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 17;
+    uint64_t NumCtorInitializers : 16;
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsInheritingConstructor : 1;
 
@@ -1857,7 +1859,7 @@ class DeclContext {
   };
 
   /// Number of inherited and non-inherited bits in CXXConstructorDeclBitfields.
-  enum { NumCXXConstructorDeclBits = NumFunctionDeclBits + 20 };
+  enum { NumCXXConstructorDeclBits = NumFunctionDeclBits + 19 };
 
   /// Stores the bits used by ObjCMethodDecl.
   /// If modified NumObjCMethodDeclBits and the accessor
diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index 9ecff2c898acd5f..0c706036ff70227 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -1011,6 +1011,15 @@ class FunctionTemplateDecl : public RedeclarableTemplateDecl {
     return getTemplatedDecl()->isThisDeclarationADefinition();
   }
 
+  bool isCompatibleWithDefinition() const {
+    return getTemplatedDecl()->isInstantiatedFromMemberTemplate() ||
+           isThisDeclarationADefinition();
+  }
+  void setInstantiatedFromMemberTemplate(FunctionTemplateDecl *D) {
+    getTemplatedDecl()->setInstantiatedFromMemberTemplate();
+    RedeclarableTemplateDecl::setInstantiatedFromMemberTemplate(D);
+  }
+
   /// Return the specialization with the provided arguments if it exists,
   /// otherwise return the insertion point.
   FunctionDecl *findSpecialization(ArrayRef<TemplateArgument> Args,
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 7e8a172ae4c37fa..610207cf8b9a4a1 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3065,6 +3065,7 @@ FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
   FunctionDeclBits.IsIneligibleOrNotSelected = false;
   FunctionDeclBits.HasImplicitReturnZero = false;
   FunctionDeclBits.IsLateTemplateParsed = false;
+  FunctionDeclBits.IsInstantiatedFromMemberTemplate = false;
   FunctionDeclBits.ConstexprKind = static_cast<uint64_t>(ConstexprKind);
   FunctionDeclBits.BodyContainsImmediateEscalatingExpression = false;
   FunctionDeclBits.InstantiationIsPending = false;
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 137942f0c30bfe1..6aaf86a6a6ff369 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -4074,22 +4074,7 @@ TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
   if (FunctionTemplate->getFriendObjectKind())
     Owner = FunctionTemplate->getLexicalDeclContext();
   FunctionDecl *FD = FunctionTemplate->getTemplatedDecl();
-  // additional check for inline friend,
-  // ```
-  //   template <class F1> int foo(F1 X);
-  //   template <int A1> struct A {
-  //     template <class F1> friend int foo(F1 X) { return A1; }
-  //   };
-  //   template struct A<1>;
-  //   int a = foo(1.0);
-  // ```
-  const FunctionDecl *FDFriend;
-  if (FD->getFriendObjectKind() == Decl::FriendObjectKind::FOK_None &&
-      FD->isDefined(FDFriend, /*CheckForPendingFriendDefinition*/ true) &&
-      FDFriend->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None) {
-    FD = const_cast<FunctionDecl *>(FDFriend);
-    Owner = FD->getLexicalDeclContext();
-  }
+
   MultiLevelTemplateArgumentList SubstArgs(
       FunctionTemplate, CanonicalDeducedArgumentList->asArray(),
       /*Final=*/false);
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index dc3bfa97eff399b..12e98a33d07853e 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -479,9 +479,6 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
   using namespace TemplateInstArgsHelpers;
   const Decl *CurDecl = ND;
 
-  if (!CurDecl)
-    CurDecl = Decl::castFromDeclContext(DC);
-
   if (Innermost) {
     Result.addOuterTemplateArguments(const_cast<NamedDecl *>(ND), *Innermost,
                                      Final);
@@ -495,8 +492,10 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
     // has a depth of 0.
     if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(CurDecl))
       HandleDefaultTempArgIntoTempTempParam(TTP, Result);
-    CurDecl = Response::UseNextDecl(CurDecl).NextDecl;
-  }
+    CurDecl = DC ? Decl::castFromDeclContext(DC)
+                 : Response::UseNextDecl(CurDecl).NextDecl;
+  } else if (!CurDecl)
+    CurDecl = Decl::castFromDeclContext(DC);
 
   while (!CurDecl->isFileContextDecl()) {
     Response R;
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 4855e8a23689cef..5aa8a92032f03d4 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -12,6 +12,7 @@
 #include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTLambda.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DependentDiagnostic.h"
@@ -5276,9 +5277,31 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     RebuildTypeSourceInfoForDefaultSpecialMembers();
     SetDeclDefaulted(Function, PatternDecl->getLocation());
   } else {
+    NamedDecl *ND = Function;
+    DeclContext *DC = ND->getLexicalDeclContext();
+    std::optional<ArrayRef<TemplateArgument>> Innermost;
+    if (auto *Primary = Function->getPrimaryTemplate();
+        Primary &&
+        !isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function) &&
+        Function->getTemplateSpecializationKind() !=
+            TSK_ExplicitSpecialization) {
+      auto It = llvm::find_if(Primary->redecls(),
+                              [](const RedeclarableTemplateDecl *RTD) {
+                                return cast<FunctionTemplateDecl>(RTD)
+                                    ->isCompatibleWithDefinition();
+                              });
+      assert(It != Primary->redecls().end() &&
+             "Should't get here without a definition");
+      if (FunctionDecl *Def = cast<FunctionTemplateDecl>(*It)
+                                  ->getTemplatedDecl()
+                                  ->getDefinition())
+        DC = Def->getLexicalDeclContext();
+      else
+        DC = (*It)->getLexicalDeclContext();
+      Innermost.emplace(Function->getTemplateSpecializationArgs()->asArray());
+    }
     MultiLevelTemplateArgumentList TemplateArgs = getTemplateInstantiationArgs(
-        Function, Function->getLexicalDeclContext(), /*Final=*/false,
-        /*Innermost=*/std::nullopt, false, PatternDecl);
+        Function, DC, /*Final=*/false, Innermost, false, PatternDecl);
 
     // Substitute into the qualifier; we can get a substitution failure here
     // through evil use of alias templates.
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 8210eb2143acf56..7a15e60d87d83a3 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1064,6 +1064,7 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   FD->setHasImplicitReturnZero(FunctionDeclBits.getNextBit());
   FD->setIsMultiVersion(FunctionDeclBits.getNextBit());
   FD->setLateTemplateParsed(FunctionDeclBits.getNextBit());
+  FD->setInstantiatedFromMemberTemplate(FunctionDeclBits.getNextBit());
   FD->setFriendConstraintRefersToEnclosingTemplate(
       FunctionDeclBits.getNextBit());
   FD->setUsesSEHTry(FunctionDeclBits.getNextBit());
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index fa2294da95de8db..6a79444bdb98921 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -679,7 +679,7 @@ void ASTDeclWriter::VisitDeclaratorDecl(DeclaratorDecl *D) {
 }
 
 void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
-  static_assert(DeclContext::NumFunctionDeclBits == 44,
+  static_assert(DeclContext::NumFunctionDeclBits == 45,
                 "You need to update the serializer after you change the "
                 "FunctionDeclBits");
 
@@ -785,6 +785,7 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   FunctionDeclBits.addBit(D->hasImplicitReturnZero());
   FunctionDeclBits.addBit(D->isMultiVersion());
   FunctionDeclBits.addBit(D->isLateTemplateParsed());
+  FunctionDeclBits.addBit(D->isInstantiatedFromMemberTemplate());
   FunctionDeclBits.addBit(D->FriendConstraintRefersToEnclosingTemplate());
   FunctionDeclBits.addBit(D->usesSEHTry());
   Record.push_back(FunctionDeclBits);
diff --git a/clang/test/SemaTemplate/GH55509.cpp b/clang/test/SemaTemplate/GH55509.cpp
new file mode 100644
index 000000000000000..773a84305a0cda8
--- /dev/null
+++ b/clang/test/SemaTemplate/GH55509.cpp
@@ -0,0 +1,112 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++26 %s
+
+namespace t1 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C) {
+      return N;
+    }
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C);
+    // expected-note@-1 {{declared here}}
+  };
+
+  void test() {
+    cica(A<0>{}, 0);
+    // expected-error@-1 {{function 'cica<int>' with deduced return type cannot be used before it is defined}}
+
+    (void)A<1>{};
+    cica(A<0>{}, 0);
+  }
+} // namespace t1
+namespace t2 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C) {
+      return N;
+    }
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C);
+  };
+
+  template <int N, class = decltype(cica(A<N>{}, nullptr))>
+  void MakeCica();
+  // expected-note@-1 {{candidate function}}
+
+  template <int N> void MakeCica(A<N+1> = {});
+  // expected-note@-1 {{candidate function}}
+
+  void test() {
+    MakeCica<0>();
+
+    MakeCica<0>();
+    // expected-error@-1 {{call to 'MakeCica' is ambiguous}}
+  }
+} // namespace t2
+namespace t3 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C) {
+      return N-1;
+    }
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C);
+  };
+
+  template <int N, class AT, class = decltype(cica(AT{}, nullptr))>
+  static constexpr bool MakeCica(int);
+
+  template <int N, class AT>
+  static constexpr bool MakeCica(short, A<N+1> = {});
+
+  template <int N, class AT = A<N>, class Val = decltype(MakeCica<N, AT>(0))>
+  static constexpr bool has_cica = Val{};
+
+  constexpr bool cica2 = has_cica<0> || has_cica<0>;
+} // namespace t3
+namespace t4 {
+  template<int N> struct A {
+    template<class C> friend auto cica(const A<N-1>&, C);
+  };
+
+  template<> struct A<0> {
+    template<class C> friend auto cica(const A<0>&, C) {
+      C a;
+    }
+  };
+
+  template struct A<1>;
+
+  void test() {
+    cica(A<0>{}, 0);
+  }
+} // namespace t4
+namespace regression1 {
+  template <class> class A;
+
+  template <class T> [[gnu::abi_tag("TAG")]] void foo(A<T>);
+
+  template <class> struct A {
+    friend void foo <>(A);
+  };
+
+  template struct A<int>;
+
+  template <class T> [[gnu::abi_tag("TAG")]] void foo(A<T>) {}
+
+  template void foo<int>(A<int>);
+} // namespace regression1
+namespace regression2 {
+  template <class> struct A {
+    template <class T> static void f() {
+      A<int>::f<T>();
+    }
+  };
+  template <> template <class T> void A<int>::f() {
+    static_assert(__is_same(T, long));
+  }
+  template void A<void>::f<long>();
+} // namespace regression2

mstorsjo · 2025-01-31T18:15:04Z

I’m not in a position where I can test the patch properly right now, but if the whole qtbase can be built (not only the reduced testcase or the specific source file that triggered the issue before), I’m good with it - thanks!

mizvekov · 2025-01-31T18:16:28Z

I’m not in a position where I can test the patch properly right now, but if the whole qtbase can be built (not only the reduced testcase or the specific source file that triggered the issue before), I’m good with it - thanks!

Yes it does, no problem!

cor3ntin · 2025-01-31T19:03:38Z

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

+      if (FunctionDecl *Def = cast<FunctionTemplateDecl>(*It)
+                                  ->getTemplatedDecl()
+                                  ->getDefinition())
+        DC = Def->getLexicalDeclContext();
+      else
+        DC = (*It)->getLexicalDeclContext();


Can we get into a situation like regression2 with friends?
I wonder if we can use DC = (*It)->getDeclContext(); (not lexical!) unconditionally.

We need the lexical context because we are interested in the template context of the declaration, ie what set of parameters it sits under, which is a lexical thing.

I don't follow what you mean this situation with friends, can you provide an example?
regression2 is a situation very specific to member function templates, but friends are not members.

Sorry, nevermind. I understand what you are doing now.

However, I'm wondering if we are not reinventing getTemplateInstantiationPattern(true) here, It looks strangely similar. Have you explored that?

Well, there is commonality in that both are using the same helpers, but getTemplateInstantiationPattern(true) doesn't do what we need here, and this bit doesn't do what the other users of getTemplateInstantiationPattern(true) need. I am not sure it would be worth joining both implementations, adding an extra parameter to getTemplateInstantiationPattern.

clang/include/clang/AST/Decl.h

This fixes instantiation of definition for friend function templates, when the declaration found and the one containing the definition have different template contexts. In these cases, the the function declaration corresponding to the definition is not available; it may not even be instantiated at all. So this patch adds a bit which tracks which function template declaration was instantiated from the member template. It's used to find which primary template serves as a context for the purpose of obtainining the template arguments needed to instantiate the definition. Fixes #55509

mizvekov · 2025-02-07T23:17:44Z

/cherry-pick 346077a

llvmbot · 2025-02-07T23:23:18Z

Failed to cherry-pick: 346077a

https://github.com/llvm/llvm-project/actions/runs/13209645800

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

…llvm#125266)

…#125266) This fixes instantiation of definition for friend function templates, when the declaration found and the one containing the definition have different template contexts. In these cases, the the function declaration corresponding to the definition is not available; it may not even be instantiated at all. So this patch adds a bit which tracks which function template declaration was instantiated from the member template. It's used to find which primary template serves as a context for the purpose of obtainining the template arguments needed to instantiate the definition. Fixes #55509

… (#127777) This fixes instantiation of definition for friend function templates, when the declaration found and the one containing the definition have different template contexts. In these cases, the the function declaration corresponding to the definition is not available; it may not even be instantiated at all. So this patch adds a bit which tracks which function template declaration was instantiated from the member template. It's used to find which primary template serves as a context for the purpose of obtainining the template arguments needed to instantiate the definition. Fixes #55509

mizvekov requested review from mstorsjo and cor3ntin January 31, 2025 18:11

mizvekov self-assigned this Jan 31, 2025

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" clang:modules C++20 modules and Clang Header Modules labels Jan 31, 2025

cor3ntin reviewed Jan 31, 2025

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cor3ntin reviewed Feb 1, 2025

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clang/include/clang/AST/Decl.h Show resolved Hide resolved

erichkeane approved these changes Feb 3, 2025

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mizvekov force-pushed the users/mizvekov/clang-fix-GH55509 branch from ad1af5f to 1282f6a Compare February 5, 2025 16:25

mizvekov merged commit 346077a into main Feb 5, 2025
7 of 8 checks passed

mizvekov deleted the users/mizvekov/clang-fix-GH55509 branch February 5, 2025 17:12

mizvekov added this to the LLVM 20.X Release milestone Feb 7, 2025

llvmbot added the release:cherry-pick-failed label Feb 7, 2025

Icohedron pushed a commit to Icohedron/llvm-project that referenced this pull request Feb 11, 2025

Reland: [clang] Track function template instantiation from definition (…

c2ebd89

…llvm#125266)

mizvekov mentioned this pull request Feb 19, 2025

Backport: [clang] Track function template instantiation from definition (#125266) #127777

Merged

zyn0217 mentioned this pull request Mar 12, 2025

friend function with default parameters causes "redefinition of default argument" error in templates #130917

Closed

efriedma-quic mentioned this pull request Jul 9, 2025

[Clang] [C++20] Crash in special cases #107865

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