expected a named type for type argument %d of %s, got %v

Error message

expected a named type for type argument %d of %s, got %v

What it means

renderTypeArgs renders the type arguments of a generic named type. Each type argument must itself be a named type (optionally behind a single pointer) so it can be rendered as <import>.<Name>. This error fires when a type argument is anything else — a basic type, slice, map, or multi-level pointer.

Source

Thrown at go/vt/vtctl/vtctldclient/codegen/main.go:404

	if typeArgs.Len() == 0 {
		return "", nil, nil
	}

	args := make([]string, 0, typeArgs.Len())
	argImports := make([]typeArgImport, 0, typeArgs.Len())

	for i := 0; i < typeArgs.Len(); i++ {
		arg := typeArgs.At(i)

		var prefix string
		if ptr, ok := arg.(*types.Pointer); ok {
			prefix = "*"
			arg = ptr.Elem()
		}

		argNamed, ok := arg.(*types.Named)
		if !ok {
			return "", nil, fmt.Errorf("expected a named type for type argument %d of %s, got %v", i, named.Obj().Name(), arg)
		}

		localImport := rewriteProtoImports(argNamed.Obj().Pkg())
		args = append(args, prefix+localImport+"."+argNamed.Obj().Name())
		argImports = append(argImports, typeArgImport{localImport: localImport, pkgPath: argNamed.Obj().Pkg().Path()})
	}

	return "[" + strings.Join(args, ", ") + "]", argImports, nil
}

func extractLocalPointerType(v *types.Var) (name string, localImport string, pkgPath string, err error) {
	ptr, ok := v.Type().(*types.Pointer)
	if !ok {
		return "", "", "", fmt.Errorf("expected a pointer type for %s, got %v", v.Name(), v.Type())
	}

	typ, ok := ptr.Elem().(*types.Named)
	if !ok {

View on GitHub (pinned to 01a25a7d17)

Solutions

Change the offending type argument (position given in the message) to a named type such as a proto message
Remove generics from the interface method parameter if primitives are required
Extend renderTypeArgs in the codegen tool if support for such args is genuinely needed

Example fix

// before
Get[T any](ctx context.Context, req Wrapper[string]) error
// after
Get[T any](ctx context.Context, req Wrapper[*vtctldatapb.GetRequest]) error

Defensive patterns

Strategy: validation

Validate before calling

// Ensure each generic type argument is a named type
named := t.(*types.Named)
for i := 0; i < named.TypeArgs().Len(); i++ {
    arg := named.TypeArgs().At(i)
    if _, ok := arg.(*types.Named); !ok {
        return fmt.Errorf("type arg %d (%v) is not named", i, arg)
    }
}

Type guard

func hasNamedTypeArgs(t *types.Named) bool {
    for i := 0; i < t.TypeArgs().Len(); i++ {
        if _, ok := t.TypeArgs().At(i).(*types.Named); !ok { return false }
    }
    return true
}

Prevention

Never instantiate codegen-visible generics with primitives, slices, or maps
Prefer proto pointer types as generic arguments

When it happens

Trigger: Declaring a method on the source interface whose parameter instantiates a generic with a non-named type argument, e.g. Wrapper[[]byte], Wrapper[string], Wrapper[**T], or Wrapper[map[K]V].

Common situations: Using generics in vtctldclient service interfaces with primitive type arguments; refactoring a proto-typed generic to a primitive; index i in the message tells which positional argument of the generic is the problem.

Related errors

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/533df5b5864161b9. Report an issue: GitHub.