Make it easier to avoid StackOverflowErrors with promotion and conversion

promoted_noncircular is somewhat similar to (but more general than) promote_to_supertype. It is not quite as atsign-pure, so the existing mechanism is also retained.

Fixes #12007, fixes #10326, fixes #15736

Author: timholy

base/dates/types.jl

@@ -239,7 +239,7 @@ Base.eltype{T<:Period}(::Type{T}) = T
 Base.promote_rule(::Type{Date},x::Type{DateTime}) = DateTime
 Base.isless(x::Date,y::Date) = isless(value(x),value(y))
 Base.isless(x::DateTime,y::DateTime) = isless(value(x),value(y))
-Base.isless(x::TimeType,y::TimeType) = isless(promote(x,y)...)
+Base.isless(x::TimeType,y::TimeType) = isless(Base.promote_noncircular(x,y)...)
 ==(x::TimeType,y::TimeType) = ===(promote(x,y)...)
 
 import Base: sleep,Timer,timedwait

base/int.jl

@@ -72,7 +72,7 @@ signbit(x::Unsigned) = false
 
 flipsign{T<:BitSigned}(x::T, y::T) = box(T,flipsign_int(unbox(T,x),unbox(T,y)))
 
-flipsign(x::Signed, y::Signed)  = convert(typeof(x), flipsign(promote(x,y)...))
+flipsign(x::Signed, y::Signed)  = convert(typeof(x), flipsign(promote_noncircular(x,y)...))
 flipsign(x::Signed, y::Float16) = flipsign(x, reinterpret(Int16,y))
 flipsign(x::Signed, y::Float32) = flipsign(x, reinterpret(Int32,y))
 flipsign(x::Signed, y::Float64) = flipsign(x, reinterpret(Int64,y))

base/operators.jl

@@ -927,7 +927,7 @@ for f in (:+, :-)
 
         $f(r1::Union{FloatRange, OrdinalRange, LinSpace},
            r2::Union{FloatRange, OrdinalRange, LinSpace}) =
-               $f(promote(r1, r2)...)
+               $f(promote_noncircular(r1, r2)...)
     end
 end
 

base/promotion.jl

@@ -196,6 +196,41 @@ promote_to_supertype{T<:Number,S<:Number}(::Type{T}, ::Type{S}, ::Type{S}) = (@_
 promote_to_supertype{T<:Number,S<:Number}(::Type{T}, ::Type{S}, ::Type) =
     error("no promotion exists for ", T, " and ", S)
 
+# promotion with a check for circularity. Can be used to catch what
+# would otherwise become StackOverflowErrors.
+function promote_noncircular(x, y)
+    @_inline_meta
+    px, py = promote(x, y)
+    not_all_sametype((x,px), (y,py))
+    px, py
+end
+function promote_noncircular(x, y, z)
+    @_inline_meta
+    px, py, pz = promote(x, y, z)
+    not_all_sametype((x,px), (y,py), (z,pz))
+    px, py, pz
+end
+function promote_noncircular(x, y, z, a...)
+    p = promote(x, y, z, a...)
+    not_all_sametype(map(identity, (x, y, z, a...), p))
+    p
+end
+not_all_sametype(x, y) = nothing
+not_all_sametype(x, y, z) = nothing
+not_all_sametype{S,T}(x::Tuple{S,S}, y::Tuple{T,T}) = sametype_error(x[1], y[1])
+not_all_sametype{R,S,T}(x::Tuple{R,R}, y::Tuple{S,S}, z::Tuple{T,T}) = sametype_error(x[1], y[1], z[1])
+function not_all_sametype{R,S,T}(::Tuple{R,R}, y::Tuple{S,S}, z::Tuple{T,T}, args...)
+    @_inline_meta
+    not_all_sametype(y, z, args...)
+end
+not_all_sametype() = error("promotion failed to change any input types")
+function sametype_error(input...)
+    @_noinline_meta
+    error("circular method definition: promotion of types ",
+          join(map(x->string(typeof(x)), input), ", ", " and "),
+          " failed to change any input types")
+end
+
 +(x::Number, y::Number) = +(promote(x,y)...)
 *(x::Number, y::Number) = *(promote(x,y)...)
 -(x::Number, y::Number) = -(promote(x,y)...)

base/sysimg.jl

@@ -23,6 +23,7 @@ eval(x) = Core.eval(Base, x)
 eval(m, x) = Core.eval(m, x)
 (::Type{T}){T}(arg) = convert(T, arg)::T
 (::Type{VecElement{T}}){T}(arg) = VecElement{T}(convert(T, arg))
+(::Type{Union{}})(arg) = error("cannot convert type ", typeof(arg), " to Union{}")
 convert{T<:VecElement}(::Type{T}, arg) = T(arg)
 convert{T<:VecElement}(::Type{T}, arg::T) = arg
 

test/core.jl

@@ -4826,3 +4826,24 @@ end
 f19599{T}(x::((S)->Vector{S})(T)...) = 1
 @test f19599([1],[1]) == 1
 @test_throws MethodError f19599([1],[1.0])
+
+# avoiding StackOverflowErrors (issues #12007, #10326, #15736)
+module SOE
+type Sgnd <: Signed
+    v::Int
+end
+using Base.Test
+@test_throws ErrorException abs(Sgnd(1))       #12007
+io = IOBuffer()
+@test_throws ErrorException show(io, Sgnd(1))  #12007
+
+@test_throws ErrorException convert(Union{}, 1) #10326
+
+@test_throws ErrorException permutedims(rand(()), ()) #15736
+
+immutable MyTime <: Dates.TimeType
+    value::Int
+end
+@test_throws ErrorException isless(MyTime(1), now())
+
+end