Add a test for importing a C++ namespace indirectly (#6075)

Part of #6060.

toolchain/check/testdata/interop/cpp/import.carbon

@@ -10,25 +10,65 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/import.carbon
 
-// --- header.h
+// ============================================================================
+// Import C++ namespace indirectly
+// ============================================================================
+
+// --- namespace.h
+
+namespace MyNamespace {
+
+class MyClass {};
+
+}  // namespace MyNamespace
+
+// --- namespace_api.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "namespace.h";
+
+alias MyNamespaceAlias = Cpp.MyNamespace;
+
+// --- fail_todo_import_namespace_api.carbon
+
+library "[[@TEST_NAME]]";
+
+import library "namespace_api";
+
+fn F() {
+  //@dump-sem-ir-begin
+  // CHECK:STDERR: fail_todo_import_namespace_api.carbon:[[@LINE+4]]:10: error: member name `MyClass` not found in `Cpp.MyNamespace` [MemberNameNotFoundInInstScope]
+  // CHECK:STDERR:   var x: MyNamespaceAlias.MyClass;
+  // CHECK:STDERR:          ^~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  var x: MyNamespaceAlias.MyClass;
+  //@dump-sem-ir-end
+}
+
+// ============================================================================
+// Import C++ function indirectly
+// ============================================================================
+
+// --- function.h
 
 auto foo_short(short x) -> void;
 auto foo_int(int x) -> void;
 
-// --- api.carbon
+// --- function_api.carbon
 
 library "[[@TEST_NAME]]";
 
-import Cpp library "header.h";
+import Cpp library "function.h";
 
 alias FooShort = Cpp.foo_short;
 alias FooInt = Cpp.foo_int;
 
-// --- import_api.carbon
+// --- todo_import_function_api.carbon
 
 library "[[@TEST_NAME]]";
 
-import library "api";
+import library "function_api";
 
 // TODO: Fix this test as a follow-up of https://github.com/carbon-language/carbon-lang/pull/5891.
 fn F() {
@@ -38,7 +78,27 @@ fn F() {
   //@dump-sem-ir-end
 }
 
-// CHECK:STDOUT: --- import_api.carbon
+// CHECK:STDOUT: --- fail_todo_import_namespace_api.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %x.patt: <error> = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.var_patt: <error> = var_pattern %x.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x.var: ref <error> = var %x.var_patt [concrete = <error>]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %x: <error> = bind_name x, <error> [concrete = <error>]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_import_function_api.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT: }