incorrect ambiguity error

[JeffBezanson]

This is reduced from a failure in StrBase on backports-1.2.0:

struct Str{C} <: AbstractString
end
	 
struct CSE{X}
end

UTF16CSE = CSE{1}
UTF16Str = Str{UTF16CSE}

ASCIICSE = CSE{2}
ASCIIStr = Str{ASCIICSE}

conv(::Type{<:Str{UTF16CSE}}, str::AbstractString) = 42
conv(::Type{<:Str{C}}, str::Str{C}) where {C<:CSE} = str

conv(UTF16Str, ASCIIStr())

julia> conv(UTF16Str, ASCIIStr())
ERROR: MethodError: conv(::Type{Str{CSE{1}}}, ::Str{CSE{2}}) is ambiguous. Candidates:
  conv(::Type{#s1} where #s1<:Str{C}, str::Str{C}) where C<:CSE in Main at REPL[9]:1
  conv(::Type{#s1} where #s1<:Str{CSE{1}}, str::AbstractString) in Main at REPL[8]:1
Possible fix, define
  conv(::Type{#s1} where #s1<:Str{CSE{1}}, ::Str{CSE{1}})
Stacktrace:
 [1] top-level scope at REPL[10]:1

The methods are indeed ambiguous, but not at the arguments passed in this case. Works on 1.1.