C++ Interop: Make `CppVoidType` always-incomplete (#6302)

Part of https://github.com/carbon-language/carbon-lang/issues/6280.

toolchain/check/testdata/interop/cpp/function/void_pointer.carbon

@@ -84,13 +84,18 @@ fn F() {
   //@dump-sem-ir-end
 }
 
-// --- todo_fail_void.carbon
+// --- fail_void.carbon
 
 library "[[@TEST_NAME]]";
 
 import Cpp inline "";
 
-fn F(x: Cpp.void);
+// CHECK:STDERR: fail_void.carbon:[[@LINE+5]]:6: error: parameter has incomplete type `Cpp.void` in function definition [IncompleteTypeInFunctionParam]
+// CHECK:STDERR: fn F(x: Cpp.void) {}
+// CHECK:STDERR:      ^~~~~~~~~~~
+// CHECK:STDERR: fail_void.carbon: note: `Cpp.void` is always-incomplete [CppVoidIncomplete]
+// CHECK:STDERR:
+fn F(x: Cpp.void) {}
 
 // --- non_nullable_pointer.carbon
 

toolchain/check/type_completion.cpp

@@ -195,8 +195,7 @@ class TypeCompleter {
   auto BuildInfoForInst(SemIR::TypeId /*type_id*/,
                         SemIR::CppVoidType /*inst*/) const
       -> SemIR::CompleteTypeInfo {
-    // TODO: `CppVoidType` should be always-incomplete.
-    return {.value_repr = MakeEmptyValueRepr()};
+    CARBON_FATAL("`CppVoidType` is always-incomplete");
   }
 
   auto BuildInfoForInst(SemIR::TypeId type_id,
@@ -372,6 +371,17 @@ auto TypeCompleter::AddNestedIncompleteTypes(SemIR::Inst type_inst) -> bool {
       Push(context_->types().GetTypeIdForTypeInstId(inst.inner_id));
       break;
     }
+    case SemIR::CppVoidType::Kind: {
+      // TODO: Consider checking `VoidTy` for completeness and checking whether
+      // this extra check triggers an error. For that, reuse the code in
+      // https://github.com/carbon-language/carbon-lang/blob/ca3f95faa610fdb9412c9e58ece524abf30c7a9e/toolchain/check/cpp/import.cpp#L2317-L2325.
+      if (diagnoser_) {
+        CARBON_DIAGNOSTIC(CppVoidIncomplete, Note,
+                          "`Cpp.void` is always-incomplete");
+        diagnoser_().Note(SemIR::LocId::None, CppVoidIncomplete).Emit();
+      }
+      return false;
+    }
     case CARBON_KIND(SemIR::CustomLayoutType inst): {
       for (auto field : context_->struct_type_fields().Get(inst.fields_id)) {
         Push(context_->types().GetTypeIdForTypeInstId(field.type_inst_id));

toolchain/diagnostics/diagnostic_kind.def

@@ -289,6 +289,7 @@ CARBON_DIAGNOSTIC_KIND(ClassForwardDeclaredHere)
 CARBON_DIAGNOSTIC_KIND(ClassSpecificDeclOutsideClass)
 CARBON_DIAGNOSTIC_KIND(ClassSpecificDeclPrevious)
 CARBON_DIAGNOSTIC_KIND(ClassIncompleteWithinDefinition)
+CARBON_DIAGNOSTIC_KIND(CppVoidIncomplete)
 CARBON_DIAGNOSTIC_KIND(GenericVirtual)
 CARBON_DIAGNOSTIC_KIND(OverrideWithoutBase)
 CARBON_DIAGNOSTIC_KIND(OverrideWithoutVirtualInBase)