Both 2^^30 and 2^^31 are integers positive power of 2, but forbidden.

----
// OK.
align(2 ^^ 30) struct S1 {};
pragma(msg, S1.sizeof);  // 1073741824LU

// Error: alignment must be an integer positive power of 2, not -2147483648.
align(2 ^^ 31) struct S2 {};

// Error: alignment must be an integer positive power of 2, not 0.
align(2 ^^ 32) struct S3 {};
---

2^^32 + 1 is not an integer positive power of 2, but allowed.

---
align(2 ^^ 32 + 1) struct S4 {};
pragma(msg, S4.sizeof);  // 1LU
----