Handle the case when alpha and beta are zero in the betacdf

src/distrs/beta.jl

@@ -28,7 +28,7 @@ end
 
 function betacdf(α::Real, β::Real, x::Real)
     # Handle a degenerate case
-    if iszero(α)
+    if iszero(α) && β > 0
         return float(last(promote(α, β, x, x >= 0)))
     end
 
@@ -37,7 +37,7 @@ end
 
 function betaccdf(α::Real, β::Real, x::Real)
     # Handle a degenerate case
-    if iszero(α)
+    if iszero(α) && β > 0
         return float(last(promote(α, β, x, x < 0)))
     end
 
@@ -48,7 +48,7 @@ end
 # to an implementation based on the hypergeometric function ₂F₁ to avoid underflow.
 function betalogcdf(α::T, β::T, x::T) where {T<:Real}
     # Handle a degenerate case
-    if iszero(α)
+    if iszero(α) && β > 0
         return log(last(promote(x, x >= 0)))
     end
 
@@ -67,7 +67,7 @@ betalogcdf(α::Real, β::Real, x::Real) = betalogcdf(promote(α, β, x)...)
 
 function betalogccdf(α::Real, β::Real, x::Real)
     # Handle a degenerate case
-    if iszero(α)
+    if iszero(α) && β > 0
         return log(last(promote(α, β, x, x < 0)))
     end
 

test/rmath.jl

@@ -173,6 +173,10 @@ end
         # Test p=0.48 separately since R fails. (It's pretty slow, though, caused by the cdf being 9.0797754e-317)
         @test betainvcdf(1000, 2, betacdf(1000, 2, 0.48)) ≈ 0.48
     end
+    @testset "$(f)(0, 0, $x) should error" for f in (betacdf, betaccdf, betalogcdf, betalogccdf),
+        x in (0.0, 0.5, 1.0)
+        @test_throws DomainError f(0.0, 0.0, x)
+    end
 
     rmathcomp_tests("binom", [
         ((1, 0.5), 0.0:1.0),