@code_warntype shows the following code is type unstable

[atbug]

Running the following code with julia test.jl shows type instability, but I believe the code is type stable.

test.jl

using LinearAlgebra
using InteractiveUtils

function hey(q)
    A = kron([1 1; 1 1], [1 1; 1 1])
    B = kron([1 1; 1 1], [1 0; 0 1])
    if norm(q) > 1.0
        return A
    else
        return A - B
    end
end

function foo(f)
    return f.([[1.0, 1.0], [1.0, 0.0]])
end

# hey([1.0, 0.0])

@code_warntype foo(hey)

The output is

MethodInstance for foo(::typeof(hey))
  from foo(f) @ Main /path/to/test.jl:14
Arguments
  #self#::Core.Const(foo)
  f::Core.Const(hey)
Body::Any
1 ─ %1 = Base.vect(1.0, 1.0)::Vector{Float64}
│   %2 = Base.vect(1.0, 0.0)::Vector{Float64}
│   %3 = Base.vect(%1, %2)::Vector{Vector{Float64}}
│   %4 = Base.broadcasted(f, %3)::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1}, Nothing, typeof(hey), Tuple{Vector{Vector{Float64}}}}
│   %5 = Base.materialize(%4)::Any
└──      return %5

However, uncomment the line hey([1.0, 0.0]) fix the issue.

This has been tested on 1.9.0, 1.8.5 and 1.6.7.

[inkydragon]

The interesting thing is that if you call hey first, foo becomes type stable.
v1.9.0

using LinearAlgebra

function hey(q)
    A = kron([1 1; 1 1], [1 1; 1 1])
    B = kron([1 1; 1 1], [1 0; 0 1])
    if norm(q) > 1.0
        return A
    else
        return A - B
    end
end
hey([1.0, 1.0]);
# @code_warntype hey([1.0, 1.0])
# @code_warntype hey([1.0, 0.0])

function foo(f)
    return f.([[1.0, 1.0], [1.0, 0.0]])
end
@code_warntype foo(hey)

julia> @code_warntype foo(hey)
MethodInstance for foo(::typeof(hey))
  from foo(f) @ Main REPL[6]:1
Arguments
  #self#::Core.Const(foo)
  f::Core.Const(hey)
Body::Vector{Matrix{Int64}}
1 ─ %1 = Base.vect(1.0, 1.0)::Vector{Float64}
│   %2 = Base.vect(1.0, 0.0)::Vector{Float64}
│   %3 = Base.vect(%1, %2)::Vector{Vector{Float64}}
│   %4 = Base.broadcasted(f, %3)::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1}, Nothing, typeof(hey), Tuple{Vector{Vector{Float64}}}}
│   %5 = Base.materialize(%4)::Vector{Matrix{Int64}}
└──      return %5

Or add a type assert

function hey(q) :: Matrix{Int64}
...

[KristofferC]

Probably similar to #35800