V + 什么(就)是什么: whatever you say goes
【六61】口语格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
V + 什么 (就) 是 什么 repeats the same question word before and after the verb to say: "whatever V is, that is what counts". With 说 (or 定, 想, 要), 你说什么(就)是什么 means "what you say goes, I agree to anything". The variant V + 什么 + 就 + V + 什么 repeats the verb: 想吃什么就吃什么 ("eat whatever you like"), 有什么说什么 ("say frankly what you think").
The 就 is optional in 你说什么是什么; 说什么就是什么 is a little more emphatic. The tone depends on the context: generous or trusting (今天听你的，你说什么是什么), but also critical when you speak of someone who decides everything (爸爸说什么就是什么), or in a reproachful question: 你以为你说什么就是什么吗？ ("do you think your word is law?").
Don't confuse it with 什么都 (hsk2-g52, hsk3-g40), which generalises (你说什么我都听: "whatever you say, I'll listen"). Here 什么 is repeated and takes no 都. The same mechanism works with other question words: 想去哪里就去哪里, 想怎么做就怎么做. Register: spoken, familiar.
Examples
今天你过生日，想吃什么就吃什么。
Jīntiān nǐ guò shēngrì, xiǎng chī shénme jiù chī shénme.It's your birthday: eat whatever you like.
行，你说什么是什么，我都听你的。
Xíng, nǐ shuō shénme shì shénme, wǒ dōu tīng nǐ de.Fine, whatever you say goes, I'll do as you like.
晚上去哪里吃？你定吧，你定什么就是什么。
Wǎnshang qù nǎlǐ chī? Nǐ dìng ba, nǐ dìng shénme jiù shì shénme.Where shall we eat tonight? You decide, whatever you pick is fine.
我这个人说话直，有什么说什么。
Wǒ zhège rén shuōhuà zhí, yǒu shénme shuō shénme.I'm a straight talker: I say whatever's on my mind.
在这个家里，爸爸说什么就是什么，谁也不敢反对。
Zài zhège jiā lǐ, bàba shuō shénme jiù shì shénme, shéi yě bù gǎn fǎnduì.In this home, whatever Dad says goes, and nobody dares object.
你以为你说什么就是什么吗？这件事得大家一起商量。
Nǐ yǐwéi nǐ shuō shénme jiù shì shénme ma? Zhè jiàn shì děi dàjiā yìqǐ shāngliang.Do you think what you say is law? We have to talk this over together.
Common mistakes
✗ 你说什么就是 → ✓ 你说什么就是什么 (什么 comes back at the end of the pattern, as its second term).
✗ 想吃什么就吃那个 → ✓ 想吃什么就吃什么 (the same question word returns, not a specific word).
✗ 想吃什么都吃什么 → ✓ 想吃什么就吃什么 (the repeated 什么 goes with 就; 都 belongs to 什么都吃, hsk2-g52).