[modules] Handle friend function that was a definition but became only a declaration during AST deserialization

clang/include/clang/AST/Decl.h

@@ -2005,6 +2005,10 @@ class FunctionDecl : public DeclaratorDecl,
 
   unsigned ODRHash;
 
+  /// Indicates if given function declaration was a definition but its body
+  /// was removed due to declaration merging.
+  bool ThisDeclarationWasADefinition : 1;
+
   /// End part of this FunctionDecl's source range.
   ///
   /// We could compute the full range in getSourceRange(). However, when we're
@@ -2190,6 +2194,12 @@ class FunctionDecl : public DeclaratorDecl,
     return hasBody(Definition);
   }
 
+  void setThisDeclarationWasADefinition() {
+    ThisDeclarationWasADefinition = true;
+  }
+
+  bool wasThisDeclarationADefinition() { return ThisDeclarationWasADefinition; }
+
   /// Returns whether the function has a trivial body that does not require any
   /// specific codegen.
   bool hasTrivialBody() const;

clang/lib/AST/Decl.cpp

@@ -3083,6 +3083,7 @@ FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
       static_cast<unsigned char>(DeductionCandidate::Normal);
   FunctionDeclBits.HasODRHash = false;
   FunctionDeclBits.FriendConstraintRefersToEnclosingTemplate = false;
+  ThisDeclarationWasADefinition = false;
   if (TrailingRequiresClause)
     setTrailingRequiresClause(TrailingRequiresClause);
 }

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2572,7 +2572,7 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
   // Friend function defined withing class template may stop being function
   // definition during AST merges from different modules, in this case decl
   // with function body should be used for instantiation.
-  if (isFriend) {
+  if (isFriend && D->wasThisDeclarationADefinition()) {
     const FunctionDecl *Defn = nullptr;
     if (D->hasBody(Defn)) {
       D = const_cast<FunctionDecl *>(Defn);

clang/lib/Serialization/ASTReader.cpp

@@ -10432,6 +10432,7 @@ void ASTReader::finishPendingActions() {
       if (!getContext().getLangOpts().Modules || !FD->hasBody(Defn)) {
         FD->setLazyBody(PB->second);
       } else {
+        FD->setThisDeclarationWasADefinition();
         auto *NonConstDefn = const_cast<FunctionDecl*>(Defn);
         mergeDefinitionVisibility(NonConstDefn, FD);
 

clang/test/SemaCXX/friend-default-parameters-modules.cpp

@@ -0,0 +1,39 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -std=c++20 -fmodule-map-file=modules.map -xc++ -emit-module -fmodule-name=foo modules.map -o foo.pcm
+// RUN: %clang_cc1 -std=c++20 -fmodule-map-file=modules.map -O1 -emit-obj main.cc -verify -fmodule-file=foo.pcm
+
+//--- modules.map
+module "foo" {
+  export *
+  module "foo.h" {
+    export *
+    header "foo.h"
+  }
+}
+
+//--- foo.h
+#pragma once
+
+template <int>
+void Create(const void* = nullptr);
+
+template <int>
+struct ObjImpl {
+  template <int>
+  friend void ::Create(const void*);
+};
+
+template <int I>
+void Create(const void*) {
+  (void) ObjImpl<I>{};
+}
+
+//--- main.cc
+// expected-no-diagnostics
+#include "foo.h"
+
+int main() {
+  Create<42>();
+}

clang/test/SemaCXX/friend-default-parameters.cpp

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -std=c++20 -verify -emit-llvm-only %s
+
+template <int>
+void Create(const void* = nullptr);
+
+template <int>
+struct ObjImpl {
+  template <int>
+  friend void ::Create(const void*);
+};
+
+template <int I>
+void Create(const void*) {
+  (void) ObjImpl<I>{};
+}
+
+int main() {
+  Create<42>();
+}
+
+// expected-no-diagnostics