Fix for import of friend class template with definition. (#691)

If there is a friend class template "prototype" (forward declaration)
and later a definition for it in the existing code, this existing
definition may be not found by ASTImporter because it is not linked
to the prototype (under the friend AST node). The problem is fixed by
looping over all found matching decls instead of break after the first
found one.

Author: balazske

lib/AST/ASTImporter.cpp

@@ -5155,11 +5155,11 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
         if (isStructuralMatch(D, FoundTemplate)) {
           ClassTemplateDecl *TemplateWithDef =
               getTemplateDefinition(FoundTemplate);
-          if (D->isThisDeclarationADefinition() && TemplateWithDef) {
+          if (D->isThisDeclarationADefinition() && TemplateWithDef)
             return Importer.MapImported(D, TemplateWithDef);
-          }
-          FoundByLookup = FoundTemplate;
-          break;
+          if (!FoundByLookup)
+            FoundByLookup = FoundTemplate;
+          continue;
         }
         ConflictingDecls.push_back(FoundDecl);
       }

unittests/AST/ASTImporterTest.cpp

@@ -5264,6 +5264,50 @@ TEST_P(ASTImporterCImplicitFunctionTest,
   EXPECT_EQ(ToImplicitF->getDefinition(), ToImplicitFDef);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ImportExistingFriendClassTemplateDef) {
+  auto Code =
+      R"(
+        template <class T1, class T2>
+        struct Base {
+          template <class U1, class U2>
+          friend struct Class;
+        };
+        template <class T1, class T2>
+        struct Class { };
+        )";
+
+  TranslationUnitDecl *ToTU = getToTuDecl(Code, Lang_CXX);
+  TranslationUnitDecl *FromTU = getTuDecl(Code, Lang_CXX, "input.cc");
+
+  auto *ToClassProto = FirstDeclMatcher<ClassTemplateDecl>().match(
+      ToTU, classTemplateDecl(hasName("Class")));
+  auto *ToClassDef = LastDeclMatcher<ClassTemplateDecl>().match(
+      ToTU, classTemplateDecl(hasName("Class")));
+  ASSERT_FALSE(ToClassProto->isThisDeclarationADefinition());
+  ASSERT_TRUE(ToClassDef->isThisDeclarationADefinition());
+  // Previous friend decl is not linked to it!
+  ASSERT_FALSE(ToClassDef->getPreviousDecl());
+  ASSERT_EQ(ToClassDef->getMostRecentDecl(), ToClassDef);
+  ASSERT_EQ(ToClassProto->getMostRecentDecl(), ToClassProto);
+
+  auto *FromClassProto = FirstDeclMatcher<ClassTemplateDecl>().match(
+      FromTU, classTemplateDecl(hasName("Class")));
+  auto *FromClassDef = LastDeclMatcher<ClassTemplateDecl>().match(
+      FromTU, classTemplateDecl(hasName("Class")));
+  ASSERT_FALSE(FromClassProto->isThisDeclarationADefinition());
+  ASSERT_TRUE(FromClassDef->isThisDeclarationADefinition());
+  ASSERT_FALSE(FromClassDef->getPreviousDecl());
+  ASSERT_EQ(FromClassDef->getMostRecentDecl(), FromClassDef);
+  ASSERT_EQ(FromClassProto->getMostRecentDecl(), FromClassProto);
+
+  auto *ImportedDef = Import(FromClassDef, Lang_CXX);
+  // At import we should find the definition for 'Class' even if the
+  // prototype (inside 'friend') for it comes first in the AST and is not
+  // linked to the definition.
+  EXPECT_EQ(ImportedDef, ToClassDef);
+}
+
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ASTImporterCImplicitFunctionTest,
                         DefaultTestValuesForRunOptions, );