Treat bounds check in isassigned as, well, bounds check

The change to `isassigned(::SimpleVector, ...)` and `isassigned(::BitArray, ...)`
are currently no op since they will unlikely be inlined.
The `SimpleVector` version is also currently unsafe to inline.
These might change with a better inlining heuristic (#22210) and a smarter GC frame
allocation pass (#21888).

base/array.jl

@@ -96,7 +96,7 @@ sizeof(a::Array) = elsize(a) * length(a)
 function isassigned(a::Array, i::Int...)
     @_inline_meta
     ii = (sub2ind(size(a), i...) % UInt) - 1
-    ii < length(a) % UInt || return false
+    @boundscheck ii < length(a) % UInt || return false
     ccall(:jl_array_isassigned, Cint, (Any, UInt), a, ii) == 1
 end
 

base/essentials.jl

@@ -294,7 +294,7 @@ false
 function isassigned end
 
 function isassigned(v::SimpleVector, i::Int)
-    1 <= i <= length(v) || return false
+    @boundscheck 1 <= i <= length(v) || return false
     x = unsafe_load(convert(Ptr{Ptr{Void}},data_pointer_from_objref(v)) + i*sizeof(Ptr))
     return x != C_NULL
 end

base/multidimensional.jl

@@ -1229,7 +1229,7 @@ end
         @nexprs $N d->begin
             l = size(B,d)
             stride *= l
-            1 <= I_{d-1} <= l || return false
+            @boundscheck 1 <= I_{d-1} <= l || return false
             index += (I_d - 1) * stride
         end
         return isassigned(B, index)

test/boundscheck_exec.jl

@@ -209,4 +209,14 @@ else
     @test_throws BoundsError V1()
 end
 
+# This tests both the bounds check elision and the behavior of `jl_array_isassigned`
+# For `isbits` array the `ccall` should return a constant `true` and does not access
+# the array
+inbounds_isassigned(a, i) = @inbounds return isassigned(a, i)
+if bc_opt == bc_default || bc_opt == bc_off
+    @test inbounds_isassigned(Int[], 2) == true
+else
+    @test inbounds_isassigned(Int[], 2) == false
+end
+
 end