CONVERT(x USING binary) declares its result width in characters: materialization truncates multi-byte values

CREATE TABLE c (v VARCHAR(2) CHARACTER SET utf8mb4);

INSERT INTO c VALUES (0xF09F9880F09F9880);            -- two emoji, 8 bytes

SELECT LENGTH(CONVERT(v USING binary)) FROM c;         -- 8  (correct)

SELECT LENGTH(b) FROM

  (SELECT DISTINCT CONVERT(v USING binary) AS b FROM c) d;   -- 2  (truncated)

SET sql_mode='';

CREATE TABLE t AS SELECT CONVERT(v USING binary) AS b FROM c;

-- information_schema says varbinary(2); the stored value is 2 bytes