I have to admit that "cannot use type p.T as type p.T" is a
bit weak.  8g gives a similar error ("cannot use v1 (type p.T)
as type p.T in assignment").

bug3.go:37:5: error: incompatible type in initialization (incompatible type for method ‘M’ (different parameter types))
bug3.go:38:5: error: incompatible type in initialization (incompatible type for method ‘M’ (different parameter types))
bug3.go:43:5: error: incompatible type in initialization (incompatible type for method ‘M’ (different parameter types))
bug3.go:44:5: error: incompatible type in initialization (incompatible type for method ‘M’ (different parameter types))
bug3.go:49:5: error: incompatible types in assignment (cannot use type p.T as type p.T)
bug3.go:50:5: error: incompatible types in assignment (cannot use type p.T as type p.T)
bug3.go:55:5: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:56:5: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:57:6: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:58:6: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:59:5: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:60:5: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:61:6: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))
bug3.go:62:6: error: incompatible types in assignment (incompatible type for method ‘M’ (different parameter types))

R=rsc
CC=golang-dev
https://golang.org/cl/199044