RFC: make promote throw an error if no arguments can be changed

NEWS.md

@@ -178,6 +178,11 @@ This section lists changes that do not have deprecation warnings.
     of the output was shrunk to fit the union of the type of each element in the input.
     ([#22696])
 
+  * The `promote` function now raises an error if its arguments are of different types
+    and if attempting to convert them to a common type fails to change any of their types.
+    This avoids stack overflows in the common case of definitions like
+    `f(x, y) = f(promote(x, y)...)` ([#22801]).
+
 Library improvements
 --------------------
 

base/dates/types.jl

@@ -340,7 +340,7 @@ Base.typemin(::Union{Time, Type{Time}}) = Time(0)
 Base.eltype(::Type{T}) where {T<:Period} = T
 Base.promote_rule(::Type{Date}, x::Type{DateTime}) = DateTime
 Base.isless(x::T, y::T) where {T<:TimeType} = isless(value(x), value(y))
-Base.isless(x::TimeType, y::TimeType) = isless(Base.promote_noncircular(x, y)...)
+Base.isless(x::TimeType, y::TimeType) = isless(promote(x, y)...)
 (==)(x::T, y::T) where {T<:TimeType} = (==)(value(x), value(y))
 function ==(a::Time, b::Time)
     return hour(a) == hour(b) && minute(a) == minute(b) &&

base/deprecated.jl

@@ -1752,6 +1752,8 @@ end
 @deprecate selectperm partialsortperm
 @deprecate selectperm! partialsortperm!
 
+@deprecate promote_noncircular promote false
+
 # END 0.7 deprecations
 
 # BEGIN 1.0 deprecations

base/int.jl

@@ -84,7 +84,7 @@ signbit(x::Unsigned) = false
 flipsign(x::T, y::T) where {T<:BitSigned} = flipsign_int(x, y)
 flipsign(x::BitSigned, y::BitSigned) = flipsign_int(promote(x, y)...) % typeof(x)
 
-flipsign(x::Signed, y::Signed)  = convert(typeof(x), flipsign(promote_noncircular(x, y)...))
+flipsign(x::Signed, y::Signed)  = convert(typeof(x), flipsign(promote(x, y)...))
 flipsign(x::Signed, y::Float16) = flipsign(x, bitcast(Int16, y))
 flipsign(x::Signed, y::Float32) = flipsign(x, bitcast(Int32, y))
 flipsign(x::Signed, y::Float64) = flipsign(x, bitcast(Int64, y))

base/promotion.jl

@@ -187,7 +187,8 @@ promote_result(::Type{T},::Type{S},::Type{Bottom},::Type{Bottom}) where {T,S} =
 """
     promote(xs...)
 
-Convert all arguments to their common promotion type (if any), and return them all (as a tuple).
+Convert all arguments to a common type, and return them all (as a tuple).
+If no arguments can be converted, an error is raised.
 
 # Examples
 ```jldoctest
@@ -197,21 +198,19 @@ julia> promote(Int8(1), Float16(4.5), Float32(4.1))
 """
 function promote end
 
-promote() = ()
-promote(x) = (x,)
-function promote(x::T, y::S) where {T,S}
+function _promote(x::T, y::S) where {T,S}
     @_inline_meta
     (convert(promote_type(T,S),x), convert(promote_type(T,S),y))
 end
 promote_typeof(x) = typeof(x)
 promote_typeof(x, xs...) = (@_inline_meta; promote_type(typeof(x), promote_typeof(xs...)))
-function promote(x, y, z)
+function _promote(x, y, z)
     @_inline_meta
     (convert(promote_typeof(x,y,z), x),
      convert(promote_typeof(x,y,z), y),
      convert(promote_typeof(x,y,z), z))
 end
-function promote(x, y, zs...)
+function _promote(x, y, zs...)
     @_inline_meta
     (convert(promote_typeof(x,y,zs...), x),
      convert(promote_typeof(x,y,zs...), y),
@@ -240,39 +239,38 @@ promote_to_supertype(::Type{T}, ::Type{S}, ::Type{S}) where {T<:Number,S<:Number
 promote_to_supertype(::Type{T}, ::Type{S}, ::Type) where {T<:Number,S<:Number} =
     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)
+promote() = ()
+promote(x) = (x,)
+
+function promote(x, y)
     @_inline_meta
-    px, py = promote(x, y)
-    not_all_sametype((x,px), (y,py))
+    px, py = _promote(x, y)
+    not_sametype((x,y), (px,py))
     px, py
 end
-function promote_noncircular(x, y, z)
+function promote(x, y, z)
     @_inline_meta
-    px, py, pz = promote(x, y, z)
-    not_all_sametype((x,px), (y,py), (z,pz))
+    px, py, pz = _promote(x, y, z)
+    not_sametype((x,y,z), (px,py,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))
+function promote(x, y, z, a...)
+    p = _promote(x, y, z, a...)
+    not_sametype((x, y, z, a...), p)
     p
 end
-not_all_sametype(x, y) = nothing
-not_all_sametype(x, y, z) = nothing
-not_all_sametype(x::Tuple{S,S}, y::Tuple{T,T}) where {S,T} = sametype_error(x[1], y[1])
-not_all_sametype(x::Tuple{R,R}, y::Tuple{S,S}, z::Tuple{T,T}) where {R,S,T} = sametype_error(x[1], y[1], z[1])
-function not_all_sametype(::Tuple{R,R}, y::Tuple{S,S}, z::Tuple{T,T}, args...) where {R,S,T}
-    @_inline_meta
-    not_all_sametype(y, z, args...)
-end
-not_all_sametype() = error("promotion failed to change any input types")
-function sametype_error(input...)
+
+promote(x::T, y::T, zs::T...) where {T} = (x, y, zs...)
+
+not_sametype(x::T, y::T) where {T} = sametype_error(x)
+
+not_sametype(x, y) = nothing
+
+function sametype_error(input)
     @_noinline_meta
-    error("circular method definition: promotion of types ",
+    error("promotion of types ",
           join(map(x->string(typeof(x)), input), ", ", " and "),
-          " failed to change any input types")
+          " failed to change any arguments")
 end
 
 +(x::Number, y::Number) = +(promote(x,y)...)
@@ -389,3 +387,5 @@ minmax(x::Real) = (x, x)
 max(x::T, y::T) where {T<:Real} = select_value(y < x, x, y)
 min(x::T, y::T) where {T<:Real} = select_value(y < x, y, x)
 minmax(x::T, y::T) where {T<:Real} = y < x ? (y, x) : (x, y)
+
+flipsign(x::T, y::T) where {T<:Signed} = no_op_err("flipsign", T)

base/range.jl

@@ -241,7 +241,7 @@ julia> linspace(1.3,2.9,9)
 1.3:0.2:2.9
 ```
 """
-linspace(start, stop, len::Real=50) = linspace(promote_noncircular(start, stop)..., Int(len))
+linspace(start, stop, len::Real=50) = linspace(promote(start, stop)..., Int(len))
 linspace(start::T, stop::T, len::Real=50) where {T} = linspace(start, stop, Int(len))
 
 linspace(start::Real, stop::Real, len::Integer) = linspace(promote(start, stop)..., len)
@@ -947,7 +947,7 @@ function _define_range_op(@nospecialize f)
 
         $f(r1::Union{StepRangeLen, OrdinalRange, LinSpace},
            r2::Union{StepRangeLen, OrdinalRange, LinSpace}) =
-               $f(promote_noncircular(r1, r2)...)
+               $f(promote(r1, r2)...)
     end
 end
 _define_range_op(:+)

base/twiceprecision.jl

@@ -84,7 +84,7 @@ function add12(x::T, y::T) where {T}
     x, y = ifelse(abs(y) > abs(x), (y, x), (x, y))
     canonicalize2(x, y)
 end
-add12(x, y) = add12(promote_noncircular(x, y)...)
+add12(x, y) = add12(promote(x, y)...)
 
 """
     zhi, zlo = mul12(x, y)
@@ -116,7 +116,7 @@ function mul12(x::T, y::T) where {T<:AbstractFloat}
     ifelse(iszero(h) | !isfinite(h), (h, h), canonicalize2(h, fma(x, y, -h)))
 end
 mul12(x::T, y::T) where {T} = (p = x * y; (p, zero(p)))
-mul12(x, y) = mul12(promote_noncircular(x, y)...)
+mul12(x, y) = mul12(promote(x, y)...)
 
 """
     zhi, zlo = div12(x, y)
@@ -152,7 +152,7 @@ function div12(x::T, y::T) where {T<:AbstractFloat}
     ifelse(iszero(r) | !isfinite(r), (r, r), (ldexp(rh, xe-ye), ldexp(rl, xe-ye)))
 end
 div12(x::T, y::T) where {T} = (p = x / y; (p, zero(p)))
-div12(x, y) = div12(promote_noncircular(x, y)...)
+div12(x, y) = div12(promote(x, y)...)
 
 
 ## TwicePrecision
@@ -269,7 +269,7 @@ function +(x::TwicePrecision{T}, y::TwicePrecision{T}) where T
     s = abs(x.hi) > abs(y.hi) ? (((x.hi - r) + y.hi) + y.lo) + x.lo : (((y.hi - r) + x.hi) + x.lo) + y.lo
     TwicePrecision(canonicalize2(r, s)...)
 end
-+(x::TwicePrecision, y::TwicePrecision) = +(promote_noncircular(x, y)...)
++(x::TwicePrecision, y::TwicePrecision) = +(promote(x, y)...)
 
 -(x::TwicePrecision, y::TwicePrecision) = x + (-y)
 -(x::TwicePrecision, y::Number) = x + (-y)
@@ -292,7 +292,7 @@ function *(x::TwicePrecision{T}, y::TwicePrecision{T}) where {T}
     ret = TwicePrecision{T}(canonicalize2(zh, (x.hi * y.lo + x.lo * y.hi) + zl)...)
     ifelse(iszero(zh) | !isfinite(zh), TwicePrecision{T}(zh, zh), ret)
 end
-*(x::TwicePrecision, y::TwicePrecision) = *(promote_noncircular(x, y)...)
+*(x::TwicePrecision, y::TwicePrecision) = *(promote(x, y)...)
 
 function /(x::TwicePrecision, v::Number)
     x / TwicePrecision{typeof(x.hi/v)}(v)