The following code incorrectly report the "removed_configs" are unused (see  bug 602209  for the reason why I have to use "removed_configs"):

template("wrapped_shared_library") {
  shared_library(target_name) {
    forward_variables_from(invoker, "*", ["configs"])
    if (defined(invoker.configs)) {
      configs += invoker.configs
    }
    if (defined(invoker.removed_configs)) {
      configs -= invoker.removed_configs
    }
  }
}

wrapped_shared_library("foo") {
  sources = ["bar.cc", "bar.h"]
  removed_configs = [
    "//build/config/gcc:symbol_visibility_hidden"
  ]
}

Causes the following error while I expect the variable to be marked as used by the template "configs -= remove_unused":

$ gn gen out/gn
ERROR at //foo/BUILD.gn:16:21: Assignment had no effect.
  removed_configs = [
                    ^
You set the variable "removed_configs" here and it was unused before it went
out of scope.
See //foo/BUILD.gn:14:1: whence it was called.
wrapped_shared_library("foo") {
^------------------------------

Note that the removed value is present (this command is run with "removed_configs = ..." line commented out):

$ gn desc out/gn //foo configs
  //build/config:feature_flags
  //build/config/compiler:compiler
  //build/config/compiler:clang_stackrealign
  //build/config/compiler:compiler_arm_fpu
  //build/config/compiler:chromium_code
  //build/config/compiler:default_include_dirs
  //build/config/compiler:default_optimization
  //build/config/compiler:default_symbols
  //build/config/compiler:no_rtti
  //build/config/compiler:runtime_library
  //build/config/sanitizers:default_sanitizer_flags
  //build/config/sanitizers:default_sanitizer_coverage_flags
  //build/config/gcc:no_exceptions
  //build/config/gcc:symbol_visibility_hidden
  //build/config/clang:find_bad_constructs
  //build/config/clang:extra_warnings
  //build/config:debug
  //build/config:default_libs
  //build/config:shared_library_config