Give a better error than BoundsError in multiple assignment

[quinnj]

Seems like we could give a better error here:

In  [65]: a,b = 1,2

Out [65]: (1,2)

In  [66]: a,b = 1,2,3

Out [66]: (1,2,3)

In  [67]: a,b,c = 1,2

Out [67]: ERROR: BoundsError()
while loading In[67], in expression starting on line 1

This example is trivial, but a use case that was much more frustrating was along the lines of

julia> a,b,c = arrayA[x], methodB(y)

Out [67]: ERROR: BoundsError()
while loading In[67], in expression starting on line 1

In which case you spend too much time checking if x is inbounds, and then if methodB is somehow out of bounds, etc.

[tknopp]

see #4744 and #2144

I would love if in the case of an BoundsError it would be reported:

• The size of the underlying array / tuple ...
• The index at which the access has failed.

Debugging BoundsErrors is really tricky at the moment.

[quinnj]

The output for this is now

In  [3]: a,b,c = 1,2

Out [3]: ERROR: LoadError: BoundsError: attempt to access (1,2)
  at index [3]
while loading In[19], in expression starting on line 1

 in indexed_next at no file

which is much more helpful. Thanks @vtjnash!

[ssfrr]

This has maybe regressed? In 0.6:

julia> foo() = 1
foo (generic function with 1 method)

julia> x, y = foo()
ERROR: BoundsError
Stacktrace:
 [1] indexed_next(::Int64, ::Int64, ::Bool) at ./tuple.jl:56

[vtjnash]

Doesn't look like that function was ever fixed. This patch fixes that case, but needs tests.

diff --git a/base/tuple.jl b/base/tuple.jl
index 1ffc85f9c4..b08da0ca75 100644
--- a/base/tuple.jl
+++ b/base/tuple.jl
@@ -57,7 +57,7 @@ end
 # while reducing to plain next() for arbitrary iterables.
 indexed_next(t::Tuple, i::Int, state) = (t[i], i+1)
 indexed_next(a::Array, i::Int, state) = (a[i], i+1)
-indexed_next(I, i, state) = done(I,state) ? throw(BoundsError()) : next(I, state)
+indexed_next(I, i, state) = done(I, state) ? throw(BoundsError(I, state)) : next(I, state)
 
 # Use dispatch to avoid a branch in first
 first(::Tuple{}) = throw(ArgumentError("tuple must be non-empty"))