Type checking doesn't enter the body of the third loop because the range operand has the wrong type and the var y was declared with := not assigned with =. It should probably try harder; e.g., declare the variable y with type Invalid. (See also Google internal bug #19639588.)

package main

func main() {
for y := range "" {
_ = "" + 1 // type error => body was type-checked
}
for range 1 {
_ = "" + 1 // type error => body was type-checked
}
for y := range 1 {
_ = "" + 1 // no type error => body not type-checked
}
}