JuliaLang / julia / #37937

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Committed by web-flow

Commit Message

Fix triu/tril for partly initialized matrices (#55312)

This fixes
```julia
julia> using LinearAlgebra, StaticArrays

julia> M = Matrix{BigInt}(undef, 2, 2); M[1,1] = M[2,2] = M[1,2] = 3;

julia> S = SizedMatrix{2,2}(M)
2×2 SizedMatrix{2, 2, BigInt, 2, Matrix{BigInt}} with indices SOneTo(2)×SOneTo(2):
   3     3
 #undef  3

julia> triu(S)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getindex
   @ ./essentials.jl:907 [inlined]
 [2] getindex
   @ ~/.julia/packages/StaticArrays/MSJcA/src/SizedArray.jl:92 [inlined]
 [3] copyto_unaliased!
   @ ./abstractarray.jl:1086 [inlined]
 [4] copyto!(dest::SizedMatrix{2, 2, BigInt, 2, Matrix{BigInt}}, src::SizedMatrix{2, 2, BigInt, 2, Matrix{BigInt}})
   @ Base ./abstractarray.jl:1066
 [5] copymutable
   @ ./abstractarray.jl:1200 [inlined]
 [6] triu(M::SizedMatrix{2, 2, BigInt, 2, Matrix{BigInt}})
   @ LinearAlgebra ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/LinearAlgebra/src/generic.jl:413
 [7] top-level scope
   @ REPL[11]:1
```
After this PR:
```julia
julia> triu(S)
2×2 SizedMatrix{2, 2, BigInt, 2, Matrix{BigInt}} with indices SOneTo(2)×SOneTo(2):
 3  3
 0  3
```
Only the indices that need to be copied are accessed, and the others are
written to without being read.

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