Fix /(::Real,::Complex) (#19279)

Fixes issue #19240.

Author: simonbyrne

base/complex.jl

@@ -236,7 +236,7 @@ muladd(z::Complex, w::Complex, x::Real) =
     Complex(muladd(real(z), real(w), x) - imag(z)*imag(w), # TODO: use mulsub given #15985
             muladd(real(z), imag(w), imag(z) * real(w)))
 
-/(a::Real, z::Complex) = a*inv(z)
+/{R<:Real,S<:Complex}(a::R, z::S) = (T = promote_type(R,S); a*inv(T(z)))
 /(z::Complex, x::Real) = Complex(real(z)/x, imag(z)/x)
 
 function /{T<:Real}(a::Complex{T}, b::Complex{T})

test/complex.jl

@@ -55,6 +55,13 @@ end
             @test isequal(im * T(+1.0), Complex(T(+0.0), T(+1.0)))
             @test isequal(im * T(-1.0), Complex(T(-0.0), T(-1.0)))
         end
+
+        @testset "divide" begin
+            @test isequal(T(+0.0) / im, Complex(T(+0.0), T(-0.0)))
+            @test isequal(T(-0.0) / im, Complex(T(-0.0), T(+0.0)))
+            @test isequal(T(+1.0) / im, Complex(T(+0.0), T(-1.0)))
+            @test isequal(T(-1.0) / im, Complex(T(-0.0), T(+1.0)))
+        end
     end
     @test isequal(true + complex(true,false), complex(true,false) + complex(true,false))
     @test isequal(complex(true,false) + true, complex(true,false) + complex(true,false))
@@ -925,7 +932,7 @@ end
     @test Float16(1)+Float16(1)im === Complex32(1, 1)
     @test Float16(1)-Float16(1)im === Float16(1)+Float16(-1)im === Complex32(1, -1)
     @test Float16(1)*im === Complex32(im)
-    @test Float16(1)/im === 1.0f0/im === Complex(0.0, -1.0)
+    @test Float16(1)/im === Complex32(0,-1)
     @test Float16(1)^im === Complex32(1) === Float16(1)+Float16(0)im
 end
 
@@ -959,3 +966,6 @@ end
         @inferred expm1(x)
     end
 end
+
+# issue #19240
+@test big(1)/(10+10im) ≈ (5-5im)/big(100) ≈ big"0.05" - big"0.05"*im