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Disallow impl in match_first twice

proposals/p007493-disallow-impl-in-match-first-twice.md

0.0.0-0.nightly.2026.07.142.2 KB
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Disallow impl in match_first twice

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Pull request

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Table of contents

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Abstract

Disallow declaring an impl in the same match_first block more than once.

Problem

In proposal 5337 we have the rule "An impl may appear in at most one match_first block." This allows an impl to appear in one match_first block more than once, which creates ambiguity about its ordering.

carbon
match_first {
impl forall [T: X] T as Z {}
impl forall [T: Y] T as Z {}
impl forall [T: X] T as Z {}
}

The rules say that the first impl in the block should be selected, so given a type that implements X & Y, the first impl (with T: X) should be chosen. But there is also a T: Y impl that wins against the last T: X impl. While the order of is clear, this leads to more confusing code than is necessary. There is no need to write an impl twice in a match_first block.

Background

Proposal

Modify the rule "An impl may appear in at most one match_first block" to instead say "An impl may appear in at most one match_first block, and only once within it".

Details

This will disagnose the case where an impl is written multiple times in a match_first block, but the later ones would have been ignored. This allows the user to know about code they've written that isn't doing anything.

Rationale

This advances the Code that is easy to read, understand, and write goal, by avoiding the case where the user can write code that is never used. It may help them catch mistakes such as copy/paste errors.