[clang] [RFC][C++20][Modules] Fix crash when function and lambda inside loaded from different modules (PR #104512)

[Dmitry Polukhin via cfe-commits]

Valentin Clement =?utf-8?b?KOODkOODrOODsw=?=,Kyungwoo Lee <kyulee at meta.com>,Dmitry
 Polukhin <dmitry.polukhin at gmail.com>,Dmitry Polukhin
 <dmitry.polukhin at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/104512 at github.com>


================
@@ -1155,6 +1155,16 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   for (unsigned I = 0; I != NumParams; ++I)
     Params.push_back(readDeclAs<ParmVarDecl>());
   FD->setParams(Reader.getContext(), Params);
+
+  // For the first decl add all lambdas inside for loading them later,
+  // otherwise skip them.
+  unsigned NumLambdas = Record.readInt();
+  if (FD->isFirstDecl()) {
+    for (unsigned I = 0; I != NumLambdas; ++I)
+      Reader.PendingLambdas.push_back(Record.readDeclID());
+  } else {
+    Record.skipInts(NumLambdas);
----------------
dmpolukhin wrote:

@ChuanqiXu9 could you please elaborate why you think it may not be paired? Code in ASTDeclWriter looks like this:
```
  if (D->isCanonicalDecl()) {
    llvm::SmallVector<const Decl *, 2> Lambdas = collectLambdas(D);
    Record.push_back(Lambdas.size());
    for (const auto *L : Lambdas)
      Record.AddDeclRef(L);
  } else {
    Record.push_back(0);
  }
```
So in the first case it writes size + following decls, in the second case it writes 0. I looked the implementation of skipInts for `skipInts(0)` it does nothing.

https://github.com/llvm/llvm-project/pull/104512