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A tr() internal key, or a pk() tapscript leaf key, can resolve to a 33-byte compressed point when it is derived from an extended key or written as a raw compressed key. Taproot uses x-only public keys, so such a key is reduced to its 32-byte x-coordinate before it reaches taproot_output_script, dropping the parity byte as Bitcoin Core does. This is correct for either parity, since taproot keys are lifted to even-y when tweaked. A key already in x-only form is left unchanged.
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tr()descriptor whose internal key — or a tapscript leaf key — resolves to a 33-byte compressed point cannot be expanded.TRDescriptor.expandpasses the 33-byte value totaproot_output_script, whoseassert len(pubkey32) == 32fails; a tapscript leaf key silently produces a non-standard 33-byte-push script.So
tr(<xpub>/<path>)— the natural single-signer taproot form — is unusable, and a raw compressed key intr()expands to the wrong script.Bitcoin Core derives the x-only key from a compressed key by dropping the parity byte at script construction (
XOnlyPubKey(CPubKey)). This does the same: a small helper reduces a 33-byte compressed key to its 32-byte x-only form before it reaches the taproot output script — for both the internal key and tapscript leaf keys (pk(), the only function permitted as a leaf), covering both the extended-key and raw-compressed paths. A 32-byte key passes through unchanged, and the signing paths are untouched. The test checks the resulting output against the BIP-0386 x-only vector.