treat Pair as broadcast scalar (#32209)

Author: stevengj

NEWS.md

@@ -46,6 +46,7 @@ Standard library changes
   Previously, the functions  `+`, `-`, `*`, `/`, `conj`, `real` and `imag` returned the unwrapped element
   when operating over zero-dimensional arrays ([#32122]).
 * `IPAddr` subtypes now behave like scalars when used in broadcasting ([#32133]).
+* `Pair` is now treated as a scalar for broadcasting ([#32209]).
 * `clamp` can now handle missing values ([#31066]).
 * `empty` now accepts a `NamedTuple` ([#32534]).
 * `mod` now accepts a unit range as the second argument to easily perform offset modular arithmetic to ensure the result is inside the range ([#32628]).

base/broadcast.jl

@@ -652,7 +652,7 @@ julia> Broadcast.broadcastable("hello") # Strings break convention of matching i
 Base.RefValue{String}("hello")
 ```
 """
-broadcastable(x::Union{Symbol,AbstractString,Function,UndefInitializer,Nothing,RoundingMode,Missing,Val,Ptr,Regex}) = Ref(x)
+broadcastable(x::Union{Symbol,AbstractString,Function,UndefInitializer,Nothing,RoundingMode,Missing,Val,Ptr,Regex,Pair}) = Ref(x)
 broadcastable(::Type{T}) where {T} = Ref{Type{T}}(T)
 broadcastable(x::Union{AbstractArray,Number,Ref,Tuple,Broadcasted}) = x
 # Default to collecting iterables — which will error for non-iterables

base/pair.jl

@@ -21,7 +21,7 @@ const => = Pair
 
 Construct a `Pair` object with type `Pair{typeof(x), typeof(y)}`. The elements
 are stored in the fields `first` and `second`. They can also be accessed via
-iteration.
+iteration (but a `Pair` is treated as a single "scalar" for broadcasting operations).
 
 See also: [`Dict`](@ref)
 

test/broadcast.jl

@@ -831,3 +831,7 @@ let a = rand(5), b = rand(5), c = copy(a)
     x[[1,1]] .+= 1
     @test x == [2]
 end
+
+# treat Pair as scalar:
+@test replace.(split("The quick brown fox jumps over the lazy dog"), r"[aeiou]"i => "_") ==
+      ["Th_", "q__ck", "br_wn", "f_x", "j_mps", "_v_r", "th_", "l_zy", "d_g"]