add back isvalid(::String), fixes JuliaLang#17467

Author: stevengj

base/strings/basic.jl

@@ -289,6 +289,7 @@ byte_string_classify(s::String) = byte_string_classify(s.data)
     # 2: valid UTF-8
 
 isvalid(::Type{String}, s::Union{Vector{UInt8},String}) = byte_string_classify(s) != 0
+isvalid(s::String) = isvalid(String, s)
 
 ## uppercase and lowercase transformations ##
 uppercase(s::AbstractString) = map(uppercase, s)

test/strings/basic.jl

@@ -287,11 +287,11 @@ for (val, pass) in (
         (b"\udc00\u0100", false),
         (b"\udc00\ud800", false)
         )
-    @test isvalid(String, val) == pass
+    @test isvalid(String, val) == pass == isvalid(String(val))
 end
 
 # Issue #11203
-@test isvalid(String, UInt8[]) == true
+@test isvalid(String, UInt8[]) == true == isvalid("")
 
 # Check UTF-8 characters
 # Check ASCII range (true),