user89270: Looking at the code referenced in bip-0173, specifically for example here:
https://github.com/sipa/bech32/blob/master/ref/javascript/segwit_addr.js
  function decode (hrp, addr) {
      var dec = bech32.decode(addr);
      if (dec === null || dec.hrp !== hrp || dec.data.length < 1 || dec.data[0] > 16) {
        return null;
      }
      ...

I understand that this is an error checking statement, but why check a (user?) inputted hrp vs the decoded hrp from bech32.decode?
Also I'm assuming that the "dec.data[0] > 16" check is to make sure that the byte at index zero doesn't exceed a value of 16, which would be invalid hex.  Is this correct?

user89270: From BIP173 > Specification > Segwit address format > Decoding:

Software interpreting a segwit address:

MUST verify that the human-readable part is "bc" for mainnet and "tb"    for testnet. 
MUST verify that the first decoded data value (the    witness version) is between 0 and 16, inclusive



user52021: 
I'm assuming that the "dec.data[0] > 16" check is to make sure that the byte at index zero doesn't exceed a value of 16, which would be invalid hex. Is this correct?

No, it's because only versions 0 - 16 are specified. Other versions might behave entirely differently. (As an aside, if it were a comparison for the sake of clamping to the range of a single hex character the test would be >15 not >16).

user208: The HRP is part of the bech32 encoded string, and in the bech32 decoding API, it is returned along with the payload by the decoder, after checking the checksum.
We still want to compare it with the expected HRP in BIP173, which encodes the chain the software is operating on.
Otherwise you could have a testnet node that accepts mainnet BIP173 addresses or the other way around.

89270: <p>Looking at the code referenced in bip-0173, specifically for example here:
<a href="https://github.com/sipa/bech32/blob/master/ref/javascript/segwit_addr.js" rel="nofollow noreferrer">https://github.com/sipa/bech32/blob/master/ref/javascript/segwit_addr.js</a></p>

<pre><code>  function decode (hrp, addr) {
      var dec = bech32.decode(addr);
      if (dec === null || dec.hrp !== hrp || dec.data.length &lt; 1 || dec.data[0] &gt; 16) {
        return null;
      }
      ...
</code></pre>

<p>I understand that this is an error checking statement, but why check a (user?) inputted hrp vs the decoded hrp from bech32.decode?</p>

<p>Also I'm assuming that the "dec.data[0] > 16" check is to make sure that the byte at index zero doesn't exceed a value of 16, which would be invalid hex.  Is this correct?</p>
 89270: <p>From <a href="https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#Segwit_address_format" rel="nofollow noreferrer">BIP173</a> > Specification > Segwit address format > Decoding:</p>

<blockquote>
  <p>Software interpreting a segwit address:</p>
  
  <ul>
  <li>MUST verify that the human-readable part is "bc" for mainnet and "tb"    for testnet. </li>
  <li>MUST verify that the first decoded data value (the    witness version) is between 0 and 16, inclusive</li>
  </ul>
</blockquote>
 52021: <blockquote>
  <p>I'm assuming that the "dec.data[0] > 16" check is to make sure that the byte at index zero doesn't exceed a value of 16, which would be invalid hex. Is this correct?</p>
</blockquote>

<p>No, it's because only versions 0 - 16 are specified. Other versions might behave entirely differently. (As an aside, if it were a comparison for the sake of clamping to the range of a single hex character the test would be >15 not >16).</p>
 208: <p>The HRP is part of the bech32 encoded string, and in the bech32 decoding API, it is returned along with the payload by the decoder, after checking the checksum.</p>

<p>We still want to compare it with the expected HRP in BIP173, which encodes the chain the software is operating on.</p>

<p>Otherwise you could have a testnet node that accepts mainnet BIP173 addresses or the other way around.</p>
