make convert(Union{},x) directly ambiguous

This should make it impossible to accidentally define or call this
method on foreign types.

Refs: #31602
Fixes: #45837
Closes: #45051

base/array.jl

@@ -612,7 +612,6 @@ oneunit(x::AbstractMatrix{T}) where {T} = _one(oneunit(T), x)
 ## Conversions ##
 
 convert(::Type{T}, a::AbstractArray) where {T<:Array} = a isa T ? a : T(a)
-convert(::Type{Union{}}, a::AbstractArray) = throw(MethodError(convert, (Union{}, a)))
 
 promote_rule(a::Type{Array{T,n}}, b::Type{Array{S,n}}) where {T,n,S} = el_same(promote_type(T,S), a, b)
 

base/essentials.jl

@@ -279,7 +279,14 @@ See also: [`round`](@ref), [`trunc`](@ref), [`oftype`](@ref), [`reinterpret`](@r
 """
 function convert end
 
-convert(::Type{Union{}}, @nospecialize x) = throw(MethodError(convert, (Union{}, x)))
+# make convert(::Type{<:Union{}}, x::T) intentionally ambiguous for all T
+# so it will never get called or invalidated by loading packages
+# with carefully chosen types that won't have any other convert methods defined
+convert(T::Type{<:Core.IntrinsicFunction}, x) = throw(MethodError(convert, (T, x)))
+convert(T::Type{<:Nothing}, x) = throw(MethodError(convert, (Nothing, x)))
+convert(::Type{T}, x::T) where {T<:Core.IntrinsicFunction} = x
+convert(::Type{T}, x::T) where {T<:Nothing} = x
+
 convert(::Type{Type}, x::Type) = x # the ssair optimizer is strongly dependent on this method existing to avoid over-specialization
                                    # in the absence of inlining-enabled
                                    # (due to fields typed as `Type`, which is generally a bad idea)

base/filesystem.jl

@@ -36,7 +36,6 @@ export File,
        # open,
        futime,
        write,
-       JL_O_ACCMODE,
        JL_O_WRONLY,
        JL_O_RDONLY,
        JL_O_RDWR,

base/some.jl

@@ -34,8 +34,6 @@ end
 
 convert(::Type{T}, x::T) where {T>:Nothing} = x
 convert(::Type{T}, x) where {T>:Nothing} = convert(nonnothingtype_checked(T), x)
-convert(::Type{Nothing}, x) = throw(MethodError(convert, (Nothing, x)))
-convert(::Type{Nothing}, ::Nothing) = nothing
 convert(::Type{Some{T}}, x::Some{T}) where {T} = x
 convert(::Type{Some{T}}, x::Some) where {T} = Some{T}(convert(T, x.value))
 

stdlib/Test/src/Test.jl

@@ -1734,11 +1734,11 @@ function detect_ambiguities(mods::Module...;
     ambs = Set{Tuple{Method,Method}}()
     mods = collect(mods)::Vector{Module}
     function sortdefs(m1::Method, m2::Method)
-        ord12 = m1.file < m2.file
-        if !ord12 && (m1.file == m2.file)
-            ord12 = m1.line < m2.line
+        ord12 = cmp(m1.file, m2.file)
+        if ord12 == 0
+            ord12 = cmp(m1.line, m2.line)
         end
-        return ord12 ? (m1, m2) : (m2, m1)
+        return ord12 <= 0 ? (m1, m2) : (m2, m1)
     end
     function examine(mt::Core.MethodTable)
         for m in Base.MethodList(mt)

test/ambiguous.jl

@@ -100,10 +100,6 @@ ambig(x::Union{Char, Int16}) = 's'
 const allowed_undefineds = Set([
     GlobalRef(Base, :active_repl),
     GlobalRef(Base, :active_repl_backend),
-    GlobalRef(Base.Filesystem, :JL_O_TEMPORARY),
-    GlobalRef(Base.Filesystem, :JL_O_SHORT_LIVED),
-    GlobalRef(Base.Filesystem, :JL_O_SEQUENTIAL),
-    GlobalRef(Base.Filesystem, :JL_O_RANDOM),
 ])
 
 let Distributed = get(Base.loaded_modules,
@@ -167,7 +163,7 @@ using LinearAlgebra, SparseArrays, SuiteSparse
 # Test that Core and Base are free of ambiguities
 # not using isempty so this prints more information when it fails
 @testset "detect_ambiguities" begin
-    let ambig = Set{Any}(((m1.sig, m2.sig) for (m1, m2) in detect_ambiguities(Core, Base; recursive=true, ambiguous_bottom=false, allowed_undefineds)))
+    let ambig = Set(detect_ambiguities(Core, Base; recursive=true, ambiguous_bottom=false, allowed_undefineds))
         good = true
         for (sig1, sig2) in ambig
             @test sig1 === sig2 # print this ambiguity
@@ -178,6 +174,9 @@ using LinearAlgebra, SparseArrays, SuiteSparse
 
     # some ambiguities involving Union{} type parameters are expected, but not required
     let ambig = Set(detect_ambiguities(Core; recursive=true, ambiguous_bottom=true))
+        m1 = which(Core.Compiler.convert, Tuple{Type{<:Core.IntrinsicFunction}, Any})
+        m2 = which(Core.Compiler.convert, Tuple{Type{<:Nothing}, Any})
+        pop!(ambig, (m1, m2))
         @test !isempty(ambig)
     end