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:

	//go:debug panicnil=1

Starting in Go 1.21, the Go toolchain treats a `//go:debug` directive

// it has sufficient capacity, the destination is resliced to accommodate the
// new elements. If it does not, a new underlying array will be allocated.
// Append returns the updated slice. It is therefore necessary to store the
// result of append, often in the variable holding the slice itself:
//
//	slice = append(slice, elem1, elem2)
//	slice = append(slice, anotherSlice...)
//

<code>Sizeof</code> are compile-time constant expressions of type <code>uintptr</code>.
</p>

<p>
The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.

and RHS match (become identical or assignment-compatible, depending on
context).
To that effect, type inference maintains a map of bound type parameters
to inferred type arguments; this map is consulted and updated during type unification.
Initially, the bound type parameters are known but the map is empty.
During type unification, if a new type argument <code>A</code> is inferred,