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Document that you should not overload promote_type directly #41386

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Jun 29, 2021
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Document that you should not overload promote_type directly #41386

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5 changes: 5 additions & 0 deletions base/promotion.jl
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Expand Up @@ -223,6 +223,11 @@ Float16
julia> promote_type(Int8, UInt16)
UInt16
```

!!! warning "Don't overload this directly"
To overload promotion for your own types you should overload [`promote_rule`](@ref).
`promote_type` calls `promote_rule` internally to determine the type.
Overloading `promote_type` directly can cause ambiguity errors.
"""
function promote_type end

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10 changes: 8 additions & 2 deletions doc/src/manual/conversion-and-promotion.md
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Expand Up @@ -319,9 +319,15 @@ julia> promote_type(Int8, Int64)
Int64
```

Note that we do **not** overload `promote_type` directly: we overload `promote_rule` instead.
`promote_type` uses `promote_rule`, and adds the symmetry.
Overloading it directly can cause ambiguity errrors.
We overload `promote_rule` to define how things should be promoted, and we use `promote_type`
to query that.

Internally, `promote_type` is used inside of `promote` to determine what type argument values
should be converted to for promotion. It can, however, be useful in its own right. The curious
reader can read the code in [`promotion.jl`](https://github.com/JuliaLang/julia/blob/master/base/promotion.jl),
should be converted to for promotion. The curious reader can read the code in
[`promotion.jl`](https://github.com/JuliaLang/julia/blob/master/base/promotion.jl),
which defines the complete promotion mechanism in about 35 lines.

### Case Study: Rational Promotions
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