Fix some incorrect piracy flagging

src/piracy.jl

@@ -110,11 +110,37 @@ is_foreign(@nospecialize(T::TypeVar), pkg::Base.PkgId) = is_foreign(T.ub, pkg)
 end
 
 function is_foreign(@nospecialize(U::Union), pkg::Base.PkgId)
-    # Even if Foo is local, overloading f(::Union{Foo, Int}) with foreign f 
+    # Even if Foo is local, overloading f(::Union{Foo, Int}) with foreign f
     # is piracy.
     any(T -> is_foreign(T, pkg), Base.uniontypes(U))
 end
 
+function is_foreign_method(@nospecialize(U::Union), pkg::Base.PkgId)
+    # When installing a method for a union type, then we only consider it as
+    # foreign if *all* parameters of the union are foreign, i.e. overloading
+    # Union{Foo, Int}() is not piracy.
+    all(T -> is_foreign(T, pkg), Base.uniontypes(U))
+end
+
+function is_foreign_method(@nospecialize(x::Any), pkg::Base.PkgId)
+    is_foreign(x, pkg)
+end
+
+function is_foreign_method(@nospecialize(T::DataType), pkg::Base.PkgId)
+    params = T.parameters
+    # For Type{Foo}, we consider it to originate from the same as Foo
+    C = getfield(parentmodule(T), nameof(T))
+    if C === Type
+        @assert length(params) == 1
+        U = first(params)
+        return is_foreign_method(first(params), pkg)
+    end
+
+    # fallback to general code
+    return is_foreign(T, pkg)
+end
+
+
 function is_pirate(meth::Method)
     method_pkg = Base.PkgId(meth.module)
 
@@ -123,6 +149,10 @@ function is_pirate(meth::Method)
         signature = signature.body
     end
 
+    # the first parameter in the signature is the function type, and it
+    # follows slightly other rules if it happens to be a Union type
+    is_foreign_method(signature.parameters[1], method_pkg) || return false
+
     all(param -> is_foreign(param, method_pkg), signature.parameters)
 end
 

test/test_piracy.jl

@@ -29,6 +29,12 @@ Base.findlast(::Type{Val{Foo}}, x::Int) = x + 1
 Base.findlast(::Tuple{Vararg{Bar{Set{Int}}}}, x::Int) = x + 1
 Base.findlast(::Val{:foo}, x::Int) = x + 1
 
+# Not piracy
+const MyUnion = Union{Int,Foo}
+MyUnion(x::Int) = x
+
+export MyUnion
+
 # Piracy
 Base.findfirst(::Set{Vector{Char}}, ::Int) = 1
 Base.findfirst(::Union{Foo,Bar{Set{Unsigned}},UInt}, ::Tuple{Vararg{String}}) = 1
@@ -48,9 +54,10 @@ end
 
 # 2 Foo constructors
 # 2 from f
+# 1 from MyUnion
 # 5 from findlast
 # 3 from findfirst
-@test length(meths) == 2 + 2 + 5 + 3
+@test length(meths) == 2 + 2 + 1 + 5 + 3
 
 # Test what is foreign
 BasePkg = Base.PkgId(Base)