Fix reduce(vcat,...) type inference (#40277) (#40294)

(cherry picked from commit 03abc45)

Author: dlfivefifty

base/abstractarray.jl

@@ -1489,7 +1489,7 @@ AbstractVecOrTuple{T} = Union{AbstractVector{<:T}, Tuple{Vararg{T}}}
 
 _typed_vcat_similar(V, ::Type{T}, n) where T = similar(V[1], T, n)
 _typed_vcat(::Type{T}, V::AbstractVecOrTuple{AbstractVector}) where T =
-    _typed_vcat!(_typed_vcat_similar(V, T, mapreduce(length, +, V)), V)
+    _typed_vcat!(_typed_vcat_similar(V, T, sum(map(length, V))), V)
 
 function _typed_vcat!(a::AbstractVector{T}, V::AbstractVecOrTuple{AbstractVector}) where T
     pos = 1

test/abstractarray.jl

@@ -1230,3 +1230,9 @@ end
     @test [1; 1:BigInt(5)] == [1; 1:5]
     @test [1:BigInt(5); 1] == [1:5; 1]
 end
+
+@testset "reduce(vcat, ...) inferrence #40277" begin
+    x_vecs = ([5, ], [1.0, 2.0, 3.0])
+    @test @inferred(reduce(vcat, x_vecs)) == [5.0, 1.0, 2.0, 3.0]
+    @test @inferred(reduce(vcat, ([10.0], [20.0], Bool[]))) == [10.0, 20.0]
+end