[ASTImporter] Add test RedeclChainShouldBeCorrectAmongstNamespaces

[martong]

Summary

We add a new test to show that redecl chains are not handled properly
amongst namespaces. We cannot pass this test now, so this is disabled.
Subsequent patches will make this test pass.

unittests/AST/ASTImporterTest.cpp

 
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, DeclContextTest,
                         ::testing::Values(ArgVector()), );
 
 INSTANTIATE_TEST_CASE_P(
     ParameterizedTests, CanonicalRedeclChain,
     ::testing::Values(ArgVector()),);
 
+// FIXME This test is disabled currently, upcoming patches will make it
+// possible to enable.
+TEST_P(ASTImporterOptionSpecificTestBase,
+       DISABLED_RedeclChainShouldBeCorrectAmongstNamespaces) {
+  Decl *FromTU = getTuDecl(
+      R"(
+      namespace NS {
+        struct X;
+        struct Y {
+          static const int I = 3;
+        };
+      }
+      namespace NS {
+        struct X {  // <--- To be imported
+          void method(int i = Y::I) {}
+          int f;
+        };
+      }
+      )",
+      Lang_CXX);
+  auto *FromFwd = FirstDeclMatcher<CXXRecordDecl>().match(
+      FromTU, cxxRecordDecl(hasName("X"), unless(isImplicit())));
+  auto *FromDef = LastDeclMatcher<CXXRecordDecl>().match(
+      FromTU,
+      cxxRecordDecl(hasName("X"), isDefinition(), unless(isImplicit())));
+  ASSERT_NE(FromFwd, FromDef);
+  ASSERT_FALSE(FromFwd->isThisDeclarationADefinition());
+  ASSERT_TRUE(FromDef->isThisDeclarationADefinition());
+  ASSERT_EQ(FromFwd->getCanonicalDecl(), FromDef->getCanonicalDecl());
+
+  auto *ToDef = cast_or_null<CXXRecordDecl>(Import(FromDef, Lang_CXX));
+  auto *ToFwd = cast_or_null<CXXRecordDecl>(Import(FromFwd, Lang_CXX));
+  EXPECT_NE(ToFwd, ToDef);
+  EXPECT_FALSE(ToFwd->isThisDeclarationADefinition());
+  EXPECT_TRUE(ToDef->isThisDeclarationADefinition());
+  EXPECT_EQ(ToFwd->getCanonicalDecl(), ToDef->getCanonicalDecl());
+  auto *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  // We expect no (ODR) warning during the import.
+  EXPECT_EQ(0u, ToTU->getASTContext().getDiagnostics().getNumWarnings());
+}
+
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ASTImporterLookupTableTest,
                         DefaultTestValuesForRunOptions, );
 
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ImportExpr,
                         DefaultTestValuesForRunOptions, );
 
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ImportType,
                         DefaultTestValuesForRunOptions, );