Fixes and stronger tests for SymTridiagonal setindex!.

Makes SymTridiagonal setindex! no longer support off-diagonal assignment of any form.
Strengthens tests for SymTridiagonal setindex!.

Author: Sacha0

base/linalg/tridiag.jl

@@ -375,13 +375,13 @@ function getindex{T}(A::SymTridiagonal{T}, i::Integer, j::Integer)
 end
 
 function setindex!(A::SymTridiagonal, x, i::Integer, j::Integer)
+    @boundscheck checkbounds(A, i, j)
     if i == j
-        A.dv[i] = x
-    elseif abs(i - j) == 1
-        A.ev[min(i,j)] = x
+        @inbounds A.dv[i] = x
     else
-        throw(ArgumentError("cannot set elements outside the sub, main, or super diagonals"))
+        throw(ArgumentError("cannot set off-diagonal entry ($i, $j)"))
     end
+    return x
 end
 
 ## Tridiagonal matrices ##

test/linalg/tridiag.jl

@@ -266,12 +266,12 @@ let n = 12 #Size of matrix problem to test
         @test A[1,1] == a[1]
 
         debug && println("setindex!")
-        @test_throws ArgumentError A[n,1] = 1
-        @test_throws ArgumentError A[1,n] = 1
-        A[3,3] = A[3,3]
-        A[2,3] = A[2,3]
-        A[3,2] = A[3,2]
-        @test A == fA
+        @test_throws BoundsError A[n + 1, 1] = 0 # test bounds check
+        @test_throws BoundsError A[1, n + 1] = 0 # test bounds check
+        @test ((A[3, 3] = A[3, 3]) == A[3, 3]; A == fA) # test assignment on the main diagonal
+        @test_throws ArgumentError A[3, 2] = 1 # test assignment on the subdiagonal
+        @test_throws ArgumentError A[2, 3] = 1 # test assignment on the superdiagonal
+        @test_throws ArgumentError A[1, 3] = 1 # test assignment off the main/sub/super diagonal
 
         debug && println("Diagonal extraction")
         @test diag(A,1) == b