Fix and test lcm([1//2, 1//2]) == 1//1 (#56423)

Author: LilithHafner

base/intfuncs.jl

@@ -150,7 +150,16 @@ gcd(a::T, b::T) where T<:Real = throw(MethodError(gcd, (a,b)))
 lcm(a::T, b::T) where T<:Real = throw(MethodError(lcm, (a,b)))
 
 gcd(abc::AbstractArray{<:Real}) = reduce(gcd, abc; init=zero(eltype(abc)))
-lcm(abc::AbstractArray{<:Real}) = reduce(lcm, abc; init=one(eltype(abc)))
+function lcm(abc::AbstractArray{<:Real})
+    # Using reduce with init=one(eltype(abc)) is buggy for Rationals.
+    l = length(abc)
+    if l == 0
+        eltype(abc) <: Integer && return one(eltype(abc))
+        throw(ArgumentError("lcm has no identity for $(eltype(abc))"))
+    end
+    l == 1 && return abs(only(abc))
+    return reduce(lcm, abc)
+end
 
 function gcd(abc::AbstractArray{<:Integer})
     a = zero(eltype(abc))

test/intfuncs.jl

@@ -191,6 +191,13 @@ end
 
         @test lcm(T[2, 4, 6]) ⟷ T(12)
     end
+
+    # Issue #55379
+    @test lcm([1//2; 1//2]) === lcm([1//2, 1//2]) === lcm(1//2, 1//2) === 1//2
+    @test gcd(Int[]) === 0
+    @test lcm(Int[]) === 1
+    @test gcd(Rational{Int}[]) === 0//1
+    @test_throws ArgumentError("lcm has no identity for Rational{$Int}") lcm(Rational{Int}[])
 end
 
 ⟷(a::Tuple{T, T, T}, b::Tuple{T, T, T}) where T <: Union{Int8, UInt8, Int16, UInt16, Int32, UInt32, Int64, UInt64, Int128, UInt128} = a === b