Added static_assert to check that base_blob is using whole bytes. #27929

Prior to this commit it was possible to create base_blobs with any arbitrary amount of bits, like base_blob<9>. One could assume that this would be a valid way to create a bit field that guarantees to have at least 9 bits. However, in such a case, base_blob would not behave as expected because the WIDTH is rounded down to the closest whole byte (simple integer division by 8). This commit makes sure that this oddity is detected and blocked by the compiler.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #27551 (Reduce calls to various SetNull methods by Bushstar)

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lgtm ACK 5fc4939e17509534eb36727b27ac0afb941e44f7

ACK 5fc4939e17509534eb36727b27ac0afb941e44f7

I doubt that we would ever instantiate base_blob with a template parameter that is not a multiple of 8, but of course it doesn't hurt to check this.

Tested with

diff --git a/src/uint256.cpp b/src/uint256.cpp
index 7f81c3c448..802f6d5642 100644
--- a/src/uint256.cpp
+++ b/src/uint256.cpp
@@ -63,6 +63,9 @@ template std::string base_blob<160>::ToString() const;
 template void base_blob<160>::SetHex(const char*);
 template void base_blob<160>::SetHex(const std::string&);

+// Test [#27929](/bitcoin-bitcoin/27929/)
+template std::string base_blob<9>::GetHex() const;
+
 // Explicit instantiations for base_blob<256>
 template std::string base_blob<256>::GetHex() const;
 template std::string base_blob<256>::ToString() const;

leading to the failed static_assert as expected:

In file included from uint256.cpp:6:
./uint256.h:25:5: error: static_assert failed due to requirement '9U % 8 == 0' "base_blob currently only supports whole bytes."
    static_assert(BITS % 8 == 0, "base_blob currently only supports whole bytes.");
    ^             ~~~~~~~~~~~~~
uint256.cpp:67:22: note: in instantiation of template class 'base_blob<9>' requested here
template std::string base_blob<9>::GetHex() const;
                     ^
1 error generated.

Good find! LGTM ACK.

ACK 5fc4939e17509534eb36727b27ac0afb941e44f7