SwiftUI: How to Get a View with Perfectly Rounded Corners

You need to define it as a square and then put rounding on corners, like this:

Button(action: {
// ...
}) {
Text("I'm a Button")
    .frame(width:150, height:150)
    .background(Color.red)
    .cornerRadius(.infinity)

}

PS. Added some background color for visibility

Another solution to this is to use shapes, in this case the Capsule shape, and use the clipShape modifier

Taking the example already mentioned, it would be like this:

Button(action: {
// ...
}) {
Text("I'm a Button")
    .padding(.horizontal, 10)
    .background(Color.red)
    .clipShape(Capsule())
}

The padding in there can be adjusted so that your view looks how you want it to. The point it that capsule will always have the ends perfectly rounded. In this case I didn't want the text to be too close to the rounded edges, so I applied some padding to the sides.

A note to remember is that in SwiftUI the order the modifiers are applied in is very important.