programs listing versions of Go earlier than Go 1.20 are configured to match Go 1.20,
not the older version.

To override these defaults, a main package's source files
can include one or more `//go:debug` directives at the top of the file
(preceding the `package` statement).
Continuing the `panicnil` example, if the module or workspace is updated
to say `go` `1.21`, the program can opt back into the old `panic(nil)`
behavior by including this directive:

	<li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>

	<li>The scope of an identifier denoting a constant, type, variable,
	    or function (but not method) declared at top level (outside any
	    function) is the package block.</li>

	<li>The scope of the package name of an imported package is the file block
	    of the file containing the import declaration.</li>

	<li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>

	<li>The scope of an identifier denoting a constant, type, variable,
	    or function (but not method) declared at top level (outside any
	    function) is the package block.</li>

	<li>The scope of the package name of an imported package is the file block
	    of the file containing the import declaration.</li>