prevpow errors on some BigInt/BigFloat input combinations

[abigail-gentle]

Here is a pretty self explanatory MWE:

julia> prevpow(2, 5.1)
4

julia> prevpow(2, BigFloat(5.1))
ERROR: MethodError: no method matching typemax(::Type{BigInt})
The function `typemax` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  typemax(::Type{LibGit2.Consts.GIT_SUBMODULE_IGNORE})
   @ LibGit2 Enums.jl:217
  typemax(::Type{LibGit2.Consts.GIT_MERGE_FILE_FAVOR})
   @ LibGit2 Enums.jl:217
  typemax(::Type{Int128})
   @ Base int.jl:823
  ...

Stacktrace:
 [1] typemax(x::BigInt)
   @ Base ./float.jl:1039
 [2] prevpow(a::Int64, x::BigFloat)
   @ Base ./intfuncs.jl:570
 [3] top-level scope
   @ REPL[12]:1

julia> prevpow(BigFloat(2), 5.1)
4.0

julia> prevpow(BigInt(2), 5.1)
ERROR: MethodError: no method matching typemax(::Type{BigInt})
The function `typemax` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  typemax(::Type{LibGit2.Consts.GIT_SUBMODULE_IGNORE})
   @ LibGit2 Enums.jl:217
  typemax(::Type{LibGit2.Consts.GIT_MERGE_FILE_FAVOR})
   @ LibGit2 Enums.jl:217
  typemax(::Type{Int128})
   @ Base int.jl:823
  ...

Stacktrace:
 [1] typemax(x::BigInt)
   @ Base ./float.jl:1039
 [2] prevpow(a::BigInt, x::Float64)
   @ Base ./intfuncs.jl:570
 [3] top-level scope
   @ REPL[14]:1

julia> versioninfo()
Julia Version 1.11.4
Commit 8561cc3d68d (2025-03-10 11:36 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M1 Pro
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 

The error is coming from a typemax call in the prevpow code.

As far as fixing it goes, adding (x isa BigInt) || ... pushes the problem downstream, where mul_with_overflow isn't defined for a mixture of Int and BigInt, this approach would require changing that line to mul_with_overflow(promote(a,p)...) I do not know if this fits the style, or if it is performant.

First and foremost I think there needs to be more test cases to catch things like this, here is the summary of combinations I have right now, with coverage of the special case a == 2

@testset "prevpow input types" begin
    @test prevpow(3,7) == 3
    @test prevpow(3, BigInt(7)) == 3                broken = true
    @test prevpow(3, BigFloat(7)) == 3              broken = true
    @test prevpow(BigInt(3), 7) ==3                 broken = true
    @test prevpow(BigInt(3), BigInt(7)) == 3        broken = true
    @test prevpow(BigInt(3), BigFloat(7)) == 3      broken = true
    @test prevpow(BigFloat(3), 7) == 3
    @test prevpow(BigFloat(3), BigInt(7)) == 3
    @test prevpow(BigFloat(3), BigFloat(7)) == 3

    @test prevpow(BigInt(2), 7) == 4 
    @test prevpow(BigInt(2), BigInt(7)) == 4
    @test prevpow(BigFloat(2), 7) == 4
    @test prevpow(2, BigInt(7)) == 4
    @test prevpow(2, BigFloat(7)) == 4              broken = true
end

[nsajko]

Regarding BigInt, there's PR #49669, which I forgot about until now.

[abigail-gentle]

Ah , thank you! I missed this when searching somehow