Unaliasing and short-circuiting in copytrito!

src/generic.jl

@@ -2088,26 +2088,32 @@ julia> copytrito!(B, A, 'L')
 function copytrito!(B::AbstractMatrix, A::AbstractMatrix, uplo::AbstractChar)
     require_one_based_indexing(A, B)
     BLAS.chkuplo(uplo)
+    B === A && return B
     m,n = size(A)
     A = Base.unalias(B, A)
     if uplo == 'U'
         LAPACK.lacpy_size_check(size(B), (n < m ? n : m, n))
+        # extract the parents for UpperTriangular matrices
+        Bv, Av = uppertridata(B), uppertridata(A)
         for j in axes(A,2), i in axes(A,1)[begin : min(j,end)]
-            # extract the parents for UpperTriangular matrices
-            Bv, Av = uppertridata(B), uppertridata(A)
             @inbounds Bv[i,j] = Av[i,j]
         end
     else # uplo == 'L'
         LAPACK.lacpy_size_check(size(B), (m, m < n ? m : n))
+        # extract the parents for LowerTriangular matrices
+        Bv, Av = lowertridata(B), lowertridata(A)
         for j in axes(A,2), i in axes(A,1)[j:end]
-            # extract the parents for LowerTriangular matrices
-            Bv, Av = lowertridata(B), lowertridata(A)
             @inbounds Bv[i,j] = Av[i,j]
         end
     end
     return B
 end
 # Forward LAPACK-compatible strided matrices to lacpy
 function copytrito!(B::StridedMatrixStride1{T}, A::StridedMatrixStride1{T}, uplo::AbstractChar) where {T<:BlasFloat}
+    require_one_based_indexing(A, B)
+    BLAS.chkuplo(uplo)
+    B === A && return B
+    A = Base.unalias(B, A)
     LAPACK.lacpy!(B, A, uplo)
+    return B
 end

test/generic.jl

@@ -879,4 +879,13 @@ end
     @test mul!(copy!(similar(v), v), v, 2, 2, 0) == 4v
 end
 
+@testset "aliasing in copytrito! for strided matrices" begin
+    M = rand(4, 1)
+    A = view(M, 1:3, 1:1)
+    A2 = copy(A)
+    B = view(M, 2:4, 1:1)
+    copytrito!(B, A, 'L')
+    @test B == A2
+end
+
 end # module TestGeneric