diff --git a/base/strings/substring.jl b/base/strings/substring.jl
index 7cc4c53a6b661..5142cf65fe9c5 100644
--- a/base/strings/substring.jl
+++ b/base/strings/substring.jl
@@ -214,10 +214,16 @@ end
 function string(a::Union{Char, String, SubString{String}, Symbol}...)
     n = 0
     for v in a
+        # 4 types is too many for automatic Union-splitting, so we split manually
+        # and allow one specializable call site per concrete type
         if v isa Char
             n += ncodeunits(v)
-        else
+        elseif v isa String
+            n += sizeof(v)
+        elseif v isa SubString{String}
             n += sizeof(v)
+        else
+            n += sizeof(v::Symbol)
         end
     end
     out = _string_n(n)