Why can't you always use a value of type T where *T is needed for interface satisfaction?

Go's method set rules determine which methods can be called on a value of a given type. The asymmetry is:

type Counter struct{ n int }
func (c Counter)  Get() int  { return c.n }   // value receiver
func (c *Counter) Add(d int) { c.n += d }     // pointer receiver

// Interface requiring pointer receiver method
type Adder interface { Add(int) }

var a1 Adder = &Counter{}   // OK  — *Counter has Add()
// var a2 Adder = Counter{}  // COMPILE ERROR: Counter does not implement Adder
//                            (Add has pointer receiver)

// Why the asymmetry?
// When you assign Counter{} to an interface, Go cannot take its address —
// the interface stores a copy, and pointer receiver methods need the ORIGINAL.
// A *Counter stores the address, so pointer receiver methods can mutate it.

// Addressable variables work fine without interfaces:
c := Counter{}   // addressable
c.Add(5)          // Go auto-takes &c → compiles fine
fmt.Println(c.Get()) // 5

// Non-addressable — method call fails
// Counter{}.Add(5) // COMPILE ERROR: cannot take the address of Counter{}

// Correct pattern: use *Counter when ANY method uses pointer receiver
var a Adder = &Counter{}
a.Add(10)
// Access Get() via type assertion:
fmt.Println(a.(*Counter).Get()) // 10

Practical rule: if a type has any method with a pointer receiver, use *T everywhere (fields, slices, interface assignments). Mixing value and pointer receivers is a common source of interface satisfaction failures.

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