Call setMustBuildLookupTable on TagDecls in ExternalASTMerger

[lhames]

Summary

setMustBuildLookupTable should be called on imported TagDecls otherwise we may fail
to import their member decls (if they have any).

Not calling the setMustBuildLookupTable method results in a failure in the attached test
case when lookup for the 'x' member fails on struct S, which hasn't had its decls imported
elsewhere. (By contrast the member-in-struct testcase hasn't run into this issue
because the import of its decls is triggered when the struct instance is defined, and the
member access follows this).

[spyffe]

Thanks for the test. This looks fine.

[rsmith]

If the lookup table is expected to be computed from the lexical decls rather than provided via "external visible storage", this makes sense to me.

[lhames]

In D34253#782476, @rsmith wrote:

If the lookup table is expected to be computed from the lexical decls rather than provided via "external visible storage", this makes sense to me.

This gets in to details that I'm (very) fuzzy on. My understanding is that external lexical storage should be set on DeclContexts whose child decls are enumerable up front (e.g. class member decls), and that external visible storage was for open DeclContexts like namespaces. Based on that, I assumed a TagDecl (which, if I understand correctly, is either a Record or an Enum) would always have external lexical storage, never external visible storage. Is that right?

Anticipating that it may be useful to you in answering this, my favorite Charles Babbage quote is: "I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."

lib/AST/ExternalASTMerger.cpp

 public:
   LazyASTImporter(ASTContext &ToContext, FileManager &ToFileManager,
                   ASTContext &FromContext, FileManager &FromFileManager)
       : ASTImporter(ToContext, ToFileManager, FromContext, FromFileManager,
                     /*MinimalImport=*/true) {}
   Decl *Imported(Decl *From, Decl *To) override {
     if (auto ToTag = dyn_cast<TagDecl>(To)) {
       ToTag->setHasExternalLexicalStorage();
+      ToTag->setMustBuildLookupTable();
     } else if (auto ToNamespace = dyn_cast<NamespaceDecl>(To)) {
       ToNamespace->setHasExternalVisibleStorage();
     }
     return ASTImporter::Imported(From, To);
   }
 };
 
 Source<const DeclContext *>

test/Import/indirect-struct-member-access/Inputs/S.c

+struct S {
+  int a;
+};

test/Import/indirect-struct-member-access/test.c

+// RUN: clang-import-test -import %S/Inputs/S.c -expression %s
+void expr(struct S *MyS) {
+  MyS->a = 3;
+}