Avoid StackOverflowError of generic norm on AbstractChar (JuliaLang#54122)

Author: jd-foster

stdlib/LinearAlgebra/src/generic.jl

@@ -668,6 +668,9 @@ true
 """
 Base.@constprop :aggressive function norm(itr, p::Real=2)
     isempty(itr) && return float(norm(zero(eltype(itr))))
+    v, s = iterate(itr)
+    !isnothing(s) && !ismissing(v) && v == itr && throw(ArgumentError(
+        "cannot evaluate norm recursively if the type of the initial element is identical to that of the container"))
     if p == 2
         return norm2(itr)
     elseif p == 1

stdlib/LinearAlgebra/test/generic.jl

@@ -571,6 +571,13 @@ end
     @test_broken ismissing(norm(x, 0))
 end
 
+@testset "avoid stackoverflow of norm on AbstractChar" begin
+    @test_throws ArgumentError norm('a')
+    @test_throws ArgumentError norm(['a', 'b'])
+    @test_throws ArgumentError norm("s")
+    @test_throws ArgumentError norm(["s", "t"])
+end
+
 @testset "peakflops" begin
     @test LinearAlgebra.peakflops(1024, eltype=Float32, ntrials=2) > 0
 end