Avoid crashing when importing a C++ struct indirectly (#6086)

Return an error constant id instead and output a TODO.

Part of #6060.

toolchain/check/import_ref.cpp

@@ -447,6 +447,9 @@ class ImportRefResolver : public ImportContext {
 
       // Step 1: check for a constant value.
       auto existing = FindResolvedConstId(work.inst_id);
+      if (existing.const_id == SemIR::ErrorInst::ConstantId) {
+        return existing.const_id;
+      }
       if (existing.const_id.has_value() && !work.retry_with_constant_value) {
         work_stack_.pop_back();
         continue;
@@ -583,6 +586,11 @@ class ImportRefResolver : public ImportContext {
         return result;
       }
       auto ir_inst = cursor_ir->import_ir_insts().Get(import_ir_inst_id);
+      if (ir_inst.ir_id() == SemIR::ImportIRId::Cpp) {
+        local_context().TODO(SemIR::LocId::None,
+                             "Unsupported: Importing C++ indirectly");
+        return {.const_id = SemIR::ErrorInst::ConstantId};
+      }
 
       const auto* prev_ir = cursor_ir;
       auto prev_inst_id = cursor_inst_id;

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

@@ -46,6 +46,37 @@ fn F() {
   //@dump-sem-ir-end
 }
 
+// ============================================================================
+// Import C++ struct indirectly
+// ============================================================================
+
+// --- struct.h
+
+struct MyStruct { void Foo(); };
+
+// --- struct_api.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "struct.h";
+
+alias MyStructAlias = Cpp.MyStruct;
+
+// --- fail_todo_import_struct_api.carbon
+// CHECK:STDERR: fail_todo_import_struct_api.carbon: error: semantics TODO: `Unsupported: Importing C++ indirectly` [SemanticsTodo]
+// CHECK:STDERR:
+
+library "[[@TEST_NAME]]";
+
+import library "struct_api";
+
+fn F() {
+  //@dump-sem-ir-begin
+  var x: MyStructAlias;
+  x.Foo();
+  //@dump-sem-ir-end
+}
+
 // ============================================================================
 // Import C++ function indirectly
 // ============================================================================
@@ -101,6 +132,29 @@ fn F() {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_import_struct_api.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Main.MyStructAlias: type = import_ref Main//struct_api, MyStructAlias, loaded [concrete = <error>]
+// 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:   %MyStructAlias.ref: type = name_ref MyStructAlias, imports.%Main.MyStructAlias [concrete = <error>]
+// CHECK:STDOUT:   %x: <error> = bind_name x, <error> [concrete = <error>]
+// CHECK:STDOUT:   %x.ref: <error> = name_ref x, %x [concrete = <error>]
+// CHECK:STDOUT:   %Foo.ref: <error> = name_ref Foo, <error> [concrete = <error>]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_todo_import_function_api.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {