Spans's parent must be passed as Context instead of Span(Context)s.

CHANGELOG.md

@@ -64,6 +64,9 @@ Updates:
   ([#911](https://github.com/open-telemetry/opentelemetry-specification/pull/911))
 - Explicitly specify the SpanContext APIs IsValid and IsRemote as required
   ([#914](https://github.com/open-telemetry/opentelemetry-specification/pull/914))
+- A full `Context` is the only way to specify a parent of a `Span`.
+  `SpanContext` or even `Span` are not allowed anymore.
+  ([#875](https://github.com/open-telemetry/opentelemetry-specification/pull/875))
 - Remove obsolete `http.status_text` from semantic conventions
   ([#972](https://github.com/open-telemetry/opentelemetry-specification/pull/972))
 - SDK: Rename the `Decision` values for `SamplingResult`s to `DROP`, `RECORD_ONLY`

specification/trace/api.md

@@ -307,24 +307,20 @@ directly. All `Span`s MUST be created via a `Tracer`.
 
 There MUST NOT be any API for creating a `Span` other than with a [`Tracer`](#tracer).
 
-When creating a new `Span`, the `Tracer` MUST allow the caller to specify the
-new `Span`'s parent in the form of a `Span` or `SpanContext`. The `Tracer`
-SHOULD create each new `Span` as a child of its active `Span`, unless an
-explicit parent is provided or the option to create a span without a parent is
-selected.
-
 `Span` creation MUST NOT set the newly created `Span` as the currently
 active `Span` by default, but this functionality MAY be offered additionally
 as a separate operation.
 
 The API MUST accept the following parameters:
 
 - The span name. This is a required parameter.
-- The parent `Span` or a `Context` containing a parent `Span` or `SpanContext`,
-  and whether the new `Span` should be a root `Span`. API MAY also have an
-  option for implicit parenting from the current context as a default behavior.
-  See [Determining the Parent Span from a Context](#determining-the-parent-span-from-a-context)
-  for guidance on `Span` parenting from explicit and implicit `Context`s.
+- The parent `Context` or an indication that the new `Span` should be a root `Span`.
+  The API MAY also have an option for implicitly using
+  the current Context as parent as a default behavior.
+  This API MUST NOT accept a `Span` or `SpanContext` as parent, only a full `Context`.
+
+  The semantic parent of the Span MUST be determined according to the rules
+  described in [Determining the Parent Span from a Context](#determining-the-parent-span-from-a-context).
 - [`SpanKind`](#spankind), default to `SpanKind.Internal` if not specified.
 - [`Attributes`](../common/common.md#attributes). Additionally,
   these attributes may be used to make a sampling decision as noted in [sampling

specification/trace/sdk.md

@@ -276,19 +276,22 @@ in the SDK:
 
 ### Interface definition
 
-#### OnStart(Span)
+#### OnStart
 
 `OnStart` is called when a span is started. This method is called synchronously
 on the thread that started the span, therefore it should not block or throw
 exceptions.
 
 **Parameters:**
 
-* `Span` - a [read/write span object](#additional-span-interfaces) for the started span.
+* `span` - a [read/write span object](#additional-span-interfaces) for the started span.
   It SHOULD be possible to keep a reference to this span object and updates to the span
   SHOULD be reflected in it.
   For example, this is useful for creating a SpanProcessor that periodically
   evaluates/prints information about all active span from a background thread.
+* `parentContext` - the parent `Context` of the span that the SDK determined
+  (the explicitly passed `Context`, the current `Context` or an empty `Context`
+  if that was explicitly requested).
 
 **Returns:** `Void`