fix decchar handling in writecell() for AbstractFloat

src/write.jl

@@ -454,9 +454,16 @@ function writecell(buf, pos, len, io, x::AbstractFloat, opts)
     bytes = codeunits(string(x))
     sz = sizeof(bytes)
     @check sz
-    for i = 1:sz
-        @inbounds buf[pos] = bytes[i]
-        pos += 1
+    if opts.decimal != UInt8('.')
+        for i = 1:sz
+            @inbounds buf[pos] = bytes[i] == UInt8('.') ? opts.decimal : bytes[i]
+            pos += 1
+        end
+    else
+        for i = 1:sz
+            @inbounds buf[pos] = bytes[i]
+            pos += 1
+        end
     end
     return pos
 end

test/write.jl

@@ -23,6 +23,10 @@ struct StructType
     aumber::Union{Real, Nothing}
 end
 
+struct AF <: AbstractFloat
+    f::Float64
+end
+Base.string(x::AF) = string(x.f)
 
 @testset "CSV.write" begin
 
@@ -156,6 +160,12 @@ end
             (delim='\t', decimal=','),
             "col1\tcol2\tcol3\n1,1\t4\t7\n2,2\t5\t8\n3,3\t6\t9\n"
         ),
+        # custom abstract float decimal: #1108
+        (
+            (col1=AF.([1.1,2.2,3.3]), col2=[4,5,6], col3=[7,8,9]),
+            (delim='\t', decimal=','),
+            "col1\tcol2\tcol3\n1,1\t4\t7\n2,2\t5\t8\n3,3\t6\t9\n"
+        ),
         # issue 515
         (
             (col1=[""],),