Restrict merge(::NamedTuple, ::Pair{..., NamedTuple}) optimization

This restricts this optimization to apply only to "fully-iterated"
NamedTuple Pairs, which are the only kind generated by lowering and
`Base.pairs`.

Resolves #59292.

Author: topolarity

base/namedtuple.jl

@@ -343,7 +343,7 @@ merge(a::NamedTuple,     b::NamedTuple{()}) = a
 merge(a::NamedTuple{()}, b::NamedTuple{()}) = a
 merge(a::NamedTuple{()}, b::NamedTuple)     = b
 
-merge(a::NamedTuple, b::Iterators.Pairs{<:Any,<:Any,<:Any,<:NamedTuple}) = merge(a, getfield(b, :data))
+merge(a::NamedTuple, b::Iterators.Pairs{<:Any,<:Any,Nothing,<:NamedTuple}) = merge(a, getfield(b, :data))
 
 merge(a::NamedTuple, b::Iterators.Zip{<:Tuple{Any,Any}}) = merge(a, NamedTuple{Tuple(b.is[1])}(b.is[2]))
 

test/namedtuple.jl

@@ -163,6 +163,10 @@ end
 @test merge(NamedTuple(), [:a=>1, :b=>2, :c=>3, :a=>4, :c=>5]) == (a=4, b=2, c=5)
 @test merge((c=0, z=1), [:a=>1, :b=>2, :c=>3, :a=>4, :c=>5]) == (c=5, z=1, a=4, b=2)
 
+# https://github.com/JuliaLang/julia/issues/59292
+@test merge((; a = 1), Base.Pairs((; b = 2, c = 3), (:b,))) == (a = 1, b = 2)
+@test merge((; a = 1), Base.pairs((; b = 2, c = 3))) == (a = 1, b = 2, c = 3)
+
 @test keys((a=1, b=2, c=3)) == (:a, :b, :c)
 @test keys(NamedTuple()) == ()
 @test keys((a=1,)) == (:a,)