[C++20][Modules] Load function body from the module that gives canonical decl (#111992)

Summary:
Fix crash from reproducer provided in
https://github.com/llvm/llvm-project/pull/109167#issuecomment-2405289565
Also fix issues with merged inline friend functions merged during deserialization.

Test Plan: check-clang

1 files changed, 12 insertions, 1 deletions

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index 68e2d6b..75c1d9a 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -798,6 +798,17 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
     }
   }
 
+  if (D->getFriendObjectKind()) {
+    // For a function defined inline within a class template, we have to force
+    // the canonical definition to be the one inside the canonical definition of
+    // the template. Remember this relation to deserialize them together.
+    if (auto *RD = dyn_cast<CXXRecordDecl>(D->getLexicalParent()))
+      if (RD->isDependentContext() && RD->isThisDeclarationADefinition()) {
+        Writer.RelatedDeclsMap[Writer.GetDeclRef(RD)].push_back(
+            Writer.GetDeclRef(D));
+      }
+  }
+
   Record.push_back(D->param_size());
   for (auto *P : D->parameters())
     Record.AddDeclRef(P);
@@ -1563,7 +1574,7 @@ void ASTDeclWriter::VisitCXXRecordDecl(CXXRecordDecl *D) {
     // For lambdas inside canonical FunctionDecl remember the mapping.
     if (auto FD = llvm::dyn_cast_or_null<FunctionDecl>(D->getDeclContext());
         FD && FD->isCanonicalDecl()) {
-      Writer.FunctionToLambdasMap[Writer.GetDeclRef(FD)].push_back(
+      Writer.RelatedDeclsMap[Writer.GetDeclRef(FD)].push_back(
           Writer.GetDeclRef(D));
     }
   } else {