Inherit default allowscalar settings in child tasks.

src/host/indexing.jl

@@ -7,6 +7,10 @@ export allowscalar, @allowscalar, assertscalar
 
 @enum ScalarIndexing ScalarAllowed ScalarWarn ScalarWarned ScalarDisallowed
 
+# if the user explicitly calls allowscalar, use that setting for all new tasks
+# XXX: use context variables to inherit the parent task's setting, once available.
+const default_scalar_indexing = Ref{Union{Nothing,ScalarIndexing}}(nothing)
+
 """
     allowscalar() do
         # code that can use scalar indexing
@@ -24,7 +28,9 @@ function allowscalar(allow::Bool=true)
     if allow
         Base.depwarn("allowscalar([true]) is deprecated, use `allowscalar() do end` or `@allowscalar` to denote exactly which operations can use scalar operations.", :allowscalar)
     end
-    task_local_storage(:ScalarIndexing, allow ? ScalarAllowed : ScalarDisallowed)
+    setting = allow ? ScalarAllowed : ScalarDisallowed
+    task_local_storage(:ScalarIndexing, setting)
+    default_scalar_indexing[] = setting
     return
 end
 
@@ -36,11 +42,7 @@ error will be thrown ([`allowscalar`](@ref)).
 """
 function assertscalar(op = "operation")
     val = get!(task_local_storage(), :ScalarIndexing) do
-        if isinteractive()
-            ScalarWarn
-        else
-            ScalarDisallowed
-        end
+        something(default_scalar_indexing[], isinteractive() ? ScalarWarn : ScalarDisallowed)
     end
     desc = """Invocation of $op resulted in scalar indexing of a GPU array.
               This is typically caused by calling an iterating implementation of a method.
@@ -51,7 +53,7 @@ function assertscalar(op = "operation")
         error("""Scalar indexing is disallowed.
                  $desc""")
     elseif val == ScalarWarn
-        @warn("""Performing scalar indexing.
+        @warn("""Performing scalar indexing on task $(current_task()).
                  $desc""")
         task_local_storage(:ScalarIndexing, ScalarWarned)
     end