Exported/Virtual resources incorrectly trigger type validation on undef instead of using parameter defaults

[alexjfisher]

When a parameter has a default value (e.g. String $param = 'default'), Puppet’s documentation and language spec both say that passing undef should act as if the parameter was omitted—triggering the default. However:

• With exported resources (@@), if you explicitly set a parameter to undef, Puppet’s type validation fails at compile time. Instead of using the default value, Puppet treats that undef as a type mismatch (if the parameter is typed).
• With virtual resources (@), the same issue only manifests if the resource is never realized—in that case, undef also triggers a type mismatch instead of using the default.

Virtual Resource Example

# @summary virtual.pp: A really simple defined type with 1 typed parameter with a default value
#
# @param param1 A simple `String` parameter with a valid default value
define my_module::example (
  String $param1 = 'test',
) {
  notice($param1)
}

@my_module::example { 'param set with undef (virtual resource)':
  param1 => undef,
}

❯ puppet apply -t virtual.pp
Info: Loading facts
Error: My_module::Example[param set with undef (virtual resource)]: parameter 'param1' expects a String value, got Undef on node example

If I realize the resource (realize(My_module::Example['param set with undef (virtual resource)'])), it works fine.

Notice: Scope(My_module::Example[param set with undef (virtual resource)]): test

Exported Resources Example

# @summary exported.pp: A really simple defined type with 1 typed parameter with a default value
#
# @param param1 A simple `String` parameter with a valid default value
define my_module::example (
  String $param1 = 'test',
) {
  notice($param1)
}

@@my_module::example { 'param set with undef (exported resource)':
  param1 => undef,
}

(When running with puppet apply puppetdb might not be configured, but the bug still triggers in the same way as if doing an agent run)

❯ puppet apply -t exported.pp
Info: Loading facts
Warning: You cannot collect exported resources without storeconfigs being set; the export is ignored (file: /home/halexfis/exported.pp, line: 10, column: 1)
Error: My_module::Example[param set with undef (exported resource)]: parameter 'param1' expects a String value, got Undef on node example

This only appears to be an issue with incorrect parameter validation. In a scenario where I use puppet agent, (with the server connected to PuppetDB), if I export a resource that doesn't have any type validation, (so doesn't trigger the bug), parameters I 'set' to undef do not appear in PuppetDB, so when later collected, default values are correctly used.

This looks like a bug that has existed in Puppet for a long time (since Puppet 4.3.0 AFAICT).