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Informative error in promotion with conflicting promote_rules #57507

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978338a
Imformative error in promotion with conflicting promote_rules
jishnub Feb 23, 2025
ece1461
Test type of error
jishnub Feb 23, 2025
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15 changes: 15 additions & 0 deletions base/promotion.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -334,10 +334,25 @@ promote_rule(::Type{Bottom}, ::Type{Bottom}, slurp...) = Bottom # not strictly n
promote_rule(::Type{Bottom}, ::Type{T}, slurp...) where {T} = T
promote_rule(::Type{T}, ::Type{Bottom}, slurp...) where {T} = T

# if both the arguments are identical, or if both the orderings in promote_rule
# are defined to return identical results, we may return the result directly
promote_result(::Type{T},::Type{T},::Type{T},::Type{T}) where {T} = T
promote_result(::Type,::Type,::Type{T},::Type{T}) where {T} = T
# If only one promote_rule is defined, use the definition directly
promote_result(::Type,::Type,::Type{T},::Type{Bottom}) where {T} = T
promote_result(::Type,::Type,::Type{Bottom},::Type{T}) where {T} = T
Comment on lines +340 to +343
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I think these methods are not necessary

Suggested change
promote_result(::Type,::Type,::Type{T},::Type{T}) where {T} = T
# If only one promote_rule is defined, use the definition directly
promote_result(::Type,::Type,::Type{T},::Type{Bottom}) where {T} = T
promote_result(::Type,::Type,::Type{Bottom},::Type{T}) where {T} = T

# if multiple promote_rules are defined, try to promote the results
promote_result(::Type,::Type,::Type{T},::Type{S}) where {T,S} = (@inline; promote_type(T,S))
# If no promote_rule is defined, both directions give Bottom. In that
# case use typejoin on the original types instead.
promote_result(::Type{T},::Type{S},::Type{Bottom},::Type{Bottom}) where {T,S} = (@inline; typejoin(T, S))
# avoid recursion if the types don't change under promote_rule, and throw an informative error instead
function _throw_promote_type_fail(A::Type, B::Type)
throw(ArgumentError(LazyString("promote_type(", A, ", ", B, ") failed, as conflicting promote_rule definitions were ",
"detected with both ", A, " and ", B, " being possible results.")))
end
promote_result(::Type{T},::Type{S},::Type{T},::Type{S}) where {T,S} = _throw_promote_type_fail(T, S)
promote_result(::Type{T},::Type{S},::Type{S},::Type{T}) where {T,S} = _throw_promote_type_fail(T, S)

"""
promote(xs...)
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23 changes: 23 additions & 0 deletions test/core.jl
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Expand Up @@ -8471,3 +8471,26 @@ module GlobalAssign57446
(@__MODULE__).theglobal = 1
@test theglobal == 1
end

@testset "conflicting promote_rule error" begin
struct PromoteA end
struct PromoteB end
struct PromoteC end

@testset "error with conflicting promote_rules" begin
Base.promote_rule(::Type{PromoteA}, ::Type{PromoteB}) = PromoteA
Base.promote_rule(::Type{PromoteB}, ::Type{PromoteA}) = PromoteB
@test_throws ArgumentError promote_type(PromoteA, PromoteB)
@test_throws ArgumentError promote_type(PromoteB, PromoteA)
end
@testset "unambiguous cases" begin
@test promote_type(PromoteA, PromoteA) == PromoteA
@test promote_type(PromoteB, PromoteB) == PromoteB

Base.promote_rule(::Type{PromoteC}, ::Type{PromoteA}) = PromoteC
Base.promote_rule(::Type{PromoteB}, ::Type{PromoteC}) = PromoteC

@test promote_type(PromoteA, PromoteC) == PromoteC
@test promote_type(PromoteC, PromoteB) == PromoteC
end
end