Tightened CircularBuffer push! arg types to avoid bad state bug

Author: Robert Wright

src/circular_buffer.jl

@@ -101,7 +101,7 @@ end
 
 Add an element to the back and overwrite front if full.
 """
-@inline function Base.push!(cb::CircularBuffer, data)
+@inline function Base.push!(cb::CircularBuffer{T}, data::S) where {T, S <: T}
     # if full, increment and overwrite, otherwise push
     if cb.length == cb.capacity
         cb.first = (cb.first == cb.capacity ? 1 : cb.first + 1)

test/test_circular_buffer.jl

@@ -164,4 +164,12 @@
             @test Array(cb) == [21, 42, 42]
         end
     end
+
+    @testset "Issue 754" begin
+        cb = CircularBuffer{Int}(5)
+        @test size(cb) == (0,)
+        @test_throws MethodError push!(cb, 1.5)
+        @test size(cb) == (0,)
+    end
+
 end