Use checked_mul in length(::ProductIterator) (#43429)

Co-authored-by: Steven G. Johnson <stevenj@mit.edu>

base/iterators.jl

@@ -14,13 +14,17 @@ using .Base:
     @propagate_inbounds, @isdefined, @boundscheck, @inbounds, Generator,
     AbstractRange, AbstractUnitRange, UnitRange, LinearIndices,
     (:), |, +, -, *, !==, !, ==, !=, <=, <, >, >=, missing,
-    any, _counttuple, eachindex, ntuple, zero, prod, in, firstindex, lastindex,
+    any, _counttuple, eachindex, ntuple, zero, prod, reduce, in, firstindex, lastindex,
     tail, fieldtypes, min, max, minimum, zero, oneunit, promote, promote_shape
 using Core: @doc
 
 if Base !== Core.Compiler
 using .Base:
     cld, fld, SubArray, view, resize!, IndexCartesian
+using .Base.Checked: checked_mul
+else
+    # Checked.checked_mul is not available during bootstrapping:
+    const checked_mul = *
 end
 
 import .Base:
@@ -1055,7 +1059,7 @@ _prod_axes1(a, A) =
     throw(ArgumentError("Cannot compute indices for object of type $(typeof(a))"))
 
 ndims(p::ProductIterator) = length(axes(p))
-length(P::ProductIterator) = prod(size(P))
+length(P::ProductIterator) = reduce(checked_mul, size(P); init=1)
 
 IteratorEltype(::Type{ProductIterator{Tuple{}}}) = HasEltype()
 IteratorEltype(::Type{ProductIterator{Tuple{I}}}) where {I} = IteratorEltype(I)

test/iterators.jl

@@ -336,21 +336,25 @@ let a = 1:2,
     c = Int32(1):Int32(0)
 
     # length
+    @test length(product())        == 1
     @test length(product(a))       == 2
     @test length(product(a, b))    == 20
     @test length(product(a, b, c)) == 0
 
     # size
+    @test size(product())          == tuple()
     @test size(product(a))         == (2,)
     @test size(product(a, b))      == (2, 10)
     @test size(product(a, b, c))   == (2, 10, 0)
 
     # eltype
+    @test eltype(product())        == Tuple{}
     @test eltype(product(a))       == Tuple{Int}
     @test eltype(product(a, b))    == Tuple{Int, Float64}
     @test eltype(product(a, b, c)) == Tuple{Int, Float64, Int32}
 
     # ndims
+    @test ndims(product())         == 0
     @test ndims(product(a))        == 1
     @test ndims(product(a, b))     == 2
     @test ndims(product(a, b, c))  == 3
@@ -425,6 +429,8 @@ let a = 1:2,
         @test_throws ArgumentError   size(product(itr))
         @test_throws ArgumentError  ndims(product(itr))
     end
+
+    @test_throws OverflowError length(product(1:typemax(Int), 1:typemax(Int)))
 end
 
 # IteratorSize trait business