Fix float block index causing hang due to infinite recursion (#492)

Calling `BlockIndex((1.0, 2.0), (1,2))` currently causes an infinite
growth of the tuple `b`, which this PR fixes by requiring tuple `a` to
have integer elements. This lead to the computer just working for a very
long time (at least 10 minutes) without any error message.

Author: KnutAM

src/blockindices.jl

@@ -172,7 +172,7 @@ end
 @inline BlockIndex(::Tuple{}, b::Tuple{}) = BlockIndex{0,Tuple{},Tuple{}}((), ())
 
 @inline BlockIndex(a::Integer, b) = BlockIndex((a,), (b,))
-@inline BlockIndex(a::Tuple, b) = BlockIndex(a, (b,))
+@inline BlockIndex(a::NTuple{N, Integer}, b) where {N} = BlockIndex(a, (b,))
 @inline BlockIndex(a::Integer, b::Tuple) = BlockIndex((a,), b)
 @inline BlockIndex() = BlockIndex((), ())
 

test/test_blockindices.jl

@@ -113,6 +113,7 @@ import BlockArrays: split_index, merge_indices
         @test copy(Block(1)[1:2]) === BlockIndexRange(Block(1),1:2)
         @test copy(Block(1)[[1,3]]) == BlockIndices(Block(1),[1,3])
         @test copy(Block(1)[[1,3]]) ≢ BlockIndices(Block(1),[1,3])
+        @test_throws MethodError BlockIndex((1.0, 2.0), (1, 2))
     end
 
     @testset "BlockIndices" begin