Ensure bidiagonal setindex! does not read indices in error message (J…

…uliaLang#55342)

This fixes the error message if the matrix is uninitialized. This is
because a `Bidiagonal` with `uplo == 'L'` may still be `istriu` if the
subdiaognal is zero. We only care about the band index in the error
message, and not the values.

stdlib/LinearAlgebra/src/bidiag.jl

@@ -184,7 +184,7 @@ end
         @inbounds A.ev[A.uplo == 'U' ? i : j] = x
     elseif !iszero(x)
         throw(ArgumentError(LazyString(lazy"cannot set entry ($i, $j) off the ",
-            istriu(A) ? "upper" : "lower", " bidiagonal band to a nonzero value ", x)))
+            A.uplo == 'U' ? "upper" : "lower", " bidiagonal band to a nonzero value ", x)))
     end
     return x
 end

stdlib/LinearAlgebra/test/bidiag.jl

@@ -984,4 +984,9 @@ end
     @test Tridiagonal{Float64}(B) === Tridiagonal(evf, dvf, zf)
 end
 
+@testset "off-band indexing error" begin
+    B = Bidiagonal(Vector{BigInt}(undef, 4), Vector{BigInt}(undef,3), :L)
+    @test_throws "cannot set entry" B[1,2] = 4
+end
+
 end # module TestBidiagonal