This would imply that you won't be able to shadow type names anymore.

type foo struct { 
 x int
}

binds the type struct { x int } to the identifier foo and identifiers may be redeclared in an inner block. dog is an identifier.

This would also imply that if you add types to existing code, and there happens to be a collision such as

type i struct {
  x float64
}

func bar() {
  for i := 0; i < 10; i++ {
  ...
 }
}

then this would produce an error. What you're proposing isn't adding better error messages, it's adding a new kind of error.

The actual funny thing imo is that

prog.go:14:8: dog is not a type
prog.go:14:10: dog is not a type

it reports the same error twice on the same line?

As @FMNSSun notes, it is not an error for a variable name to shadow a type name as long as they are declared in different scopes. (Contrast https://play.golang.org/p/D-1VbXmGbfd.)

This diagnostic would probably be improved by a reference to the declaration in use, though:

prog.go:14:8: dog (declared at prog.go:12) is not a type
prog.go:14:10: dog (declared at prog.go:12) is not a type
prog.go:15:16: dog (declared at prog.go:12) is not a type

This seems like an exact duplicate of #23065.

Indeed.