From 0c926e6d245bec176bf2554a9f0bd48ef2276678 Mon Sep 17 00:00:00 2001
From: Raphael Isemann <teemperor@gmail.com>
Date: Tue, 24 Nov 2020 23:08:57 +0100
Subject: [PATCH] [ASTImporter] Make the Import() return value consistent with
 the map of imported decls when merging ClassTemplateSpecializationDecls

When importing a `ClassTemplateSpecializationDecl` definition into a TU with a matching
`ClassTemplateSpecializationDecl` definition and a more recent forward decl, the ASTImporter
currently will call `MapImported()` for the definitions, but will return the forward declaration
from the `ASTImporter::Import()` call.

This is triggering some assertions in LLDB when we try to fully import some DeclContexts
before we delete the 'From' AST. The returned 'To' Decl before this patch is just the most recent
forward decl but that's not the Decl with the definition to which the ASTImporter will import
the child declarations.

This patch just changes that the ASTImporter returns the definition that the imported Decl was
merged with instead of the found forward declaration.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D92016
---
 clang/lib/AST/ASTImporter.cpp           |  7 ++++---
 clang/unittests/AST/ASTImporterTest.cpp | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index de1f13e..0886980 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -5423,8 +5423,9 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateSpecializationDecl(
 
   if (PrevDecl) {
     if (IsStructuralMatch(D, PrevDecl)) {
-      if (D->isThisDeclarationADefinition() && PrevDecl->getDefinition()) {
-        Importer.MapImported(D, PrevDecl->getDefinition());
+      CXXRecordDecl *PrevDefinition = PrevDecl->getDefinition();
+      if (D->isThisDeclarationADefinition() && PrevDefinition) {
+        Importer.MapImported(D, PrevDefinition);
         // Import those default field initializers which have been
         // instantiated in the "From" context, but not in the "To" context.
         for (auto *FromField : D->fields()) {
@@ -5446,7 +5447,7 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateSpecializationDecl(
         //
         // Generally, ASTCommon.h/DeclUpdateKind enum gives a very good hint
         // what else could be fused during an AST merge.
-        return PrevDecl;
+        return PrevDefinition;
       }
     } else { // ODR violation.
       // FIXME HandleNameConflict
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index f869e492..97a18a7 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -3104,6 +3104,25 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   EXPECT_TRUE(ToFun->hasBody());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, MergeTemplateSpecWithForwardDecl) {
+  std::string ClassTemplate =
+      R"(
+      template<typename T>
+      struct X { int m; };
+      template<>
+      struct X<int> { int m; };
+      )";
+  // Append a forward decl for our template specialization.
+  getToTuDecl(ClassTemplate + "template<> struct X<int>;", Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X"), isDefinition()));
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  // Check that our definition got merged with the existing definition.
+  EXPECT_TRUE(FromSpec->isThisDeclarationADefinition());
+  EXPECT_TRUE(ImportedSpec->isThisDeclarationADefinition());
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase,
        ODRViolationOfClassTemplateSpecializationsShouldBeReported) {
   std::string ClassTemplate =
-- 
2.7.4