Support round-tripping entities through C++ and Carbon. (#7022)

Use the same C++ -> Carbon map for both interop directions, and when
importing an entity from Carbon -> C++, check whether it was originally
a C++ entity and if so return the original.

Assisted-by: Gemini via Google Antigravity

toolchain/check/testdata/interop/cpp/class/roundtrip.carbon

@@ -0,0 +1,43 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/class/roundtrip.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/roundtrip.carbon
+
+// --- carbon_to_cpp_to_carbon.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+class C {}
+
+inline Cpp '''
+using CAlias = Carbon::C;
+''';
+
+fn F(p: C*) {
+  let unused q: Cpp.CAlias* = p;
+}
+
+// --- cpp_to_carbon_to_cpp.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+inline Cpp '''
+class C {};
+''';
+
+alias CAlias = Cpp.C;
+
+inline Cpp '''
+auto F(C *p) -> Carbon::CAlias* {
+  return p;
+}
+''';

toolchain/check/testdata/interop/cpp/namespace/basic.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/namespace.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/namespace/basic.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/namespace.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/namespace/basic.carbon
 
 // ============================================================================
 // Single

toolchain/check/testdata/interop/cpp/namespace/roundtrip.carbon

@@ -0,0 +1,100 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/namespace/roundtrip.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/namespace/roundtrip.carbon
+
+// --- carbon_to_cpp_to_carbon.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+namespace N;
+
+inline Cpp '''
+namespace M = Carbon::N;
+''';
+
+class C {}
+
+fn N.F(c: C);
+
+fn G(c: C) {
+  Cpp.M.F(c);
+}
+
+// --- cpp_to_carbon_to_cpp.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+inline Cpp '''
+namespace N {}
+''';
+
+alias M = Cpp.N;
+
+inline Cpp '''
+namespace N { void F(); }
+
+static_assert(&N::F == &Carbon::M::F);
+''';
+
+// --- cpp_carbon.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+fn F();
+
+fn G() {
+  // TODO: Should we disallow this?
+  Cpp.Carbon.F();
+}
+
+// --- fail_carbon_cpp.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+inline Cpp '''
+void F();
+
+void G() {
+  // This is not found because it looks for the identifier `Cpp`, not the keyword `Cpp`.
+  // CHECK:STDERR: fail_carbon_cpp.carbon:[[@LINE+4]]:11: error: no member named 'Cpp' in namespace 'Carbon' [CppInteropParseError]
+  // CHECK:STDERR:    14 |   Carbon::Cpp::F();
+  // CHECK:STDERR:       |   ~~~~~~~~^
+  // CHECK:STDERR:
+  Carbon::Cpp::F();
+}
+''';
+
+// --- fail_carbon_raw_cpp.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+// CHECK:STDERR: fail_carbon_raw_cpp.carbon:[[@LINE+4]]:7: error: semantics TODO: `interop with non-identifier package name` [SemanticsTodo]
+// CHECK:STDERR: alias r#Cpp = Cpp;
+// CHECK:STDERR:       ^~~
+// CHECK:STDERR:
+alias r#Cpp = Cpp;
+
+inline Cpp '''
+void F();
+
+void G() {
+  // CHECK:STDERR: fail_carbon_raw_cpp.carbon:[[@LINE+4]]:11: error: no member named 'Cpp' in namespace 'Carbon' [CppInteropParseError]
+  // CHECK:STDERR:    19 |   Carbon::Cpp::F();
+  // CHECK:STDERR:       |   ~~~~~~~~^
+  // CHECK:STDERR:
+  Carbon::Cpp::F();
+}
+''';