diff --git a/base/strings/unicode.jl b/base/strings/unicode.jl
index cc02412925d14..b659ec080680b 100644
--- a/base/strings/unicode.jl
+++ b/base/strings/unicode.jl
@@ -157,15 +157,15 @@ function utf8proc_decompose(str, options, buffer, nwords, chartransform::typeof(
     ret < 0 && utf8proc_error(ret)
     return ret
 end
-function utf8proc_decompose(str, options, buffer, nwords, chartransform::T) where T
-    ret = ccall(:utf8proc_decompose_custom, Int, (Ptr{UInt8}, Int, Ptr{UInt8}, Int, Cint, Ptr{Cvoid}, Ref{T}),
+function utf8proc_decompose(str, options, buffer, nwords, chartransform::F) where F
+    ret = ccall(:utf8proc_decompose_custom, Int, (Ptr{UInt8}, Int, Ptr{UInt8}, Int, Cint, Ptr{Cvoid}, Ref{F}),
                 str, sizeof(str), buffer, nwords, options,
-                @cfunction(utf8proc_custom_func, UInt32, (UInt32, Ref{T})), chartransform)
+                @cfunction(utf8proc_custom_func, UInt32, (UInt32, Ref{F})), chartransform)
     ret < 0 && utf8proc_error(ret)
     return ret
 end
 
-function utf8proc_map(str::Union{String,SubString{String}}, options::Integer, chartransform=identity)
+function utf8proc_map(str::Union{String,SubString{String}}, options::Integer, chartransform::F = identity) where F
     nwords = utf8proc_decompose(str, options, C_NULL, 0, chartransform)
     buffer = Base.StringVector(nwords*4)
     nwords = utf8proc_decompose(str, options, buffer, nwords, chartransform)
@@ -184,7 +184,7 @@ const _julia_charmap = Dict{UInt32,UInt32}(
     0x210F => 0x0127,
 )
 
-utf8proc_map(s::AbstractString, flags::Integer, chartransform=identity) = utf8proc_map(String(s), flags, chartransform)
+utf8proc_map(s::AbstractString, flags::Integer, chartransform::F = identity) where F = utf8proc_map(String(s), flags, chartransform)
 
 # Documented in Unicode module
 function normalize(