Fix contradiction with context constructors

The context constructor overloads in the `context` synopsis did not
match the description in the text of the specification.  We decided that
the overloads in the synopsis were what we intended, so change the text
to match.

We also discovered that two constructor overloads taking a `platform`
argument were present in SYCL 1.2.1 and were inadvertently dropped from
the SYCL 2020 specification.  Add them back.

Clarify the behavior of the constructors when the set of devices is
empty.  We decided that this should throw an exception, which is
consistent with OpenCL's `clCreateContext`.

Closes KhronosGroup#470
Closes KhronosGroup#474

adoc/chapters/programming_interface.adoc

@@ -1227,56 +1227,141 @@ include::{header_dir}/context.h[lines=4..-1]
 [[sec:context-ctors]]
 ==== Constructors
 
+All context constructors take a parameter named [code]#propList# which allows
+the application to pass zero or more properties.
+These properties may specify additional effects of the constructor and may also
+specify exceptions that the constructor throws.
+See <<sec:context-properties>> for the context properties that are defined by
+the <<core-spec>>.
+
+'''
+
 .[apititle]#Default constructor#
 [source,role=synopsis,id=api:context-ctor]
 ----
-explicit context(async_handler asyncHandler = {})
+explicit context(const property_list& propList = {})
 ----
 
-_Effects:_ Constructs a [code]#context# object using the
-[code]#default_selector_v# to determine the associated platform and devices.
-The associated platform is the platform which contains the device selected by
+_Effects:_ Constructs a [code]#context# object using the device selected by
 [code]#default_selector_v#.
-The <<device,devices>> that are associated with the constructed context are
-implementation-defined but must contain the device that is selected by
+The context's platform is the platform that contains this device.
+The context contains the selected device.
+The context may also contain other devices from the same platform.
+Whether this happens is implementation defined.
+
+'''
+
+.[apititle]#Constructor with async handler#
+[source,role=synopsis,id=api:context-ctor-async-handler]
+----
+explicit context(async_handler asyncHandler, const property_list& propList = {})
+----
+
+_Effects:_ Constructs a [code]#context# object using the device selected by
 [code]#default_selector_v#.
-The constructed context uses the [code]#asyncHandler# parameter to handle
-exceptions.
+The context's platform is the platform that contains this device.
+The context contains the selected device.
+The context may also contain other devices from the same platform.
+Whether this happens is implementation defined.
+The context has the asynchronous error handler [code]#asyncHandler#.
+
+'''
+
+.[apititle]#Constructor with device#
+[source,role=synopsis,id=api:context-ctor-device]
+----
+explicit context(const device& dev, const property_list& propList = {})
+----
+
+_Effects:_ Constructs a [code]#context# object that contains the device
+[code]#dev#.
+The context's platform is the platform that contains [code]#dev#.
+
+'''
+
+.[apititle]#Constructor with device and async handler#
+[source,role=synopsis,id=api:context-ctor-device-async-handler]
+----
+explicit context(const device& dev, async_handler asyncHandler,
+                 const property_list& propList = {})
+----
+
+_Effects:_ Constructs a [code]#context# object that contains the device
+[code]#dev#.
+The context's platform is the platform that contains [code]#dev#.
+The context has the asynchronous error handler [code]#asyncHandler#.
 
 '''
 
-.[apititle]#Construct from device#
-[source,role=synopsis,id=api:context-ctor-dev]
+.[apititle]#Constructor with platform#
+[source,role=synopsis,id=api:context-ctor-platform]
 ----
-explicit context(const device& dev, async_handler asyncHandler = {})
+explicit context(const platform &plt, const property_list &propList = {})
 ----
 
-_Effects:_ Constructs a [code]#context# object using the [code]#dev# parameter
-to determine the associated platform and device.
-The associated platform is the platform that contains [code]#dev#, and the
-associated device is [code]#dev#.
-The constructed context uses the [code]#asyncHandler# parameter to handle
-exceptions.
+_Effects:_ Constructs a [code]#context# object that contains all of the devices
+in the platform [code]#plt#.
+The context's platform is [code]#plt#.
+
+_Throws:_ An [code]#exception# with the [code]#errc::invalid# error code if the
+platform [code]#plt# contains no devices.
 
 '''
 
-.[apititle]#Construct from device list#
-[source,role=synopsis,id=api:context-ctor-dev-list]
+.[apititle]#Constructor with platform and async handler#
+[source,role=synopsis,id=api:context-ctor-platform-async-handler]
 ----
-explicit context(const std::vector<device>& deviceList,
-                 async_handler asyncHandler = {})
+explicit context(const platform &plt, async_handler asyncHandler,
+                 const property_list &propList = {})
+----
+
+_Effects:_ Constructs a [code]#context# object that contains all of the devices
+in the platform [code]#plt#.
+The context's platform is [code]#plt#.
+The context has the asynchronous error handler [code]#asyncHandler#.
+
+_Throws:_ An [code]#exception# with the [code]#errc::invalid# error code if the
+platform [code]#plt# contains no devices.
+
+'''
+
+.[apititle]#Constructor with device list#
+[source,role=synopsis,id=api:context-ctor-device-list]
+----
+explicit context(const std::vector<device>& deviceList, const property_list& propList = {})
 ----
 
 _Preconditions:_ All devices in [code]#deviceList# must be associated with the
 same platform.
 
-_Effects:_ Constructs a [code]#context# object using the [code]#deviceList#
-parameter to determine the associated platform and device.
-The associated platform is the platform that contains all of the devices in
+_Effects:_ Constructs a [code]#context# object that contains all of the devices
+in [code]#deviceList#.
+The context's platform is the platform that contains the devices in
 [code]#deviceList#.
-The associated devices are those devices in [code]#deviceList#.
-The constructed context uses the [code]#asyncHandler# parameter to handle
-exceptions.
+
+_Throws:_ An [code]#exception# with the [code]#errc::invalid# error code if
+[code]#deviceList# is empty.
+
+'''
+
+.[apititle]#Constructor with device list and async handler#
+[source,role=synopsis,id=api:context-ctor-device-list-async-handler]
+----
+explicit context(const std::vector<device>& deviceList, async_handler asyncHandler,
+                 const property_list& propList = {})
+----
+
+_Preconditions:_ All devices in [code]#deviceList# must be associated with the
+same platform.
+
+_Effects:_ Constructs a [code]#context# object that contains all of the devices
+in [code]#deviceList#.
+The context's platform is the platform that contains the devices in
+[code]#deviceList#.
+The context has the asynchronous error handler [code]#asyncHandler#.
+
+_Throws:_ An [code]#exception# with the [code]#errc::invalid# error code if
+[code]#deviceList# is empty.
 
 '''
 
@@ -1503,6 +1588,7 @@ At a minimum, each context must support [code]#memory_scope::work_item#,
 [[sec:context-properties]]
 ==== Properties
 
+The <<core-spec>> does not define any properties for the context constructors.
 The [code]#property_list# constructor parameters are present for extensibility.
 
 

adoc/headers/context.h

@@ -14,6 +14,11 @@ class context {
   explicit context(const device& dev, async_handler asyncHandler,
                    const property_list& propList = {});
 
+  explicit context(const platform &plt, const property_list &propList = {});
+
+  explicit context(const platform &plt, async_handler asyncHandler,
+                   const property_list &propList = {});
+
   explicit context(const std::vector<device>& deviceList,
                    const property_list& propList = {});