[ASTImporter] Extend lookup logic in class templates

During import of a class template, lookup may find a forward
declaration and structural match falsely reports equivalency
between a forward decl and a definition. The result is that
some definitions are not imported if we had imported a forward
decl previously. This patch gives a fix.

Patch by Gabor Marton!

Differential Revision: https://reviews.llvm.org/D46353

llvm-svn: 332338

1 files changed, 8 insertions, 2 deletions

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 57d4585e07d..d8c2259e1c2 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -4108,8 +4108,14 @@ Decl *ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
       if (auto *FoundTemplate = dyn_cast<ClassTemplateDecl>(Found)) {
         if (IsStructuralMatch(D, FoundTemplate)) {
           // The class templates structurally match; call it the same template.
-          // FIXME: We may be filling in a forward declaration here. Handle
-          // this case!
+
+          // We found a forward declaration but the class to be imported has a
+          // definition.
+          // FIXME Add this forward declaration to the redeclaration chain.
+          if (D->isThisDeclarationADefinition() &&
+              !FoundTemplate->isThisDeclarationADefinition())
+            continue;
+
           Importer.Imported(D->getTemplatedDecl(), 
                             FoundTemplate->getTemplatedDecl());
           return Importer.Imported(D, FoundTemplate);