[Ada] Run-time exception on class-wide return of aggregate of an extended type

[Arnaud Charlet <charlet at adacore dot com>]

For a function with a limited class-wide result, the attempt to execute a
return statement with an expression given by an aggregate of an extended
type resulted in a Constraint_Error. This occurred because the expansion
of the aggregate results in individual assignments of the extended components
to the class-wide return object, and gigi generates the raise of an exception
with a discriminant-check failure (treating it like violation of a discriminant
of the target object). This is fixed by ensuring that a conversion to the
specific type is done for such component assignments when the target object
is class-wide. Also, the case of a simple return of an aggregate in a
class-wide function is now handled by creating a return object of the specific
type rather than the class-wide type, avoiding the potentially expensive
conversions in that case.

See gnat.dg/bip_aggregate_bug.adb

2008-08-04  Gary Dismukes  <dismukes@adacore.com>

	* exp_aggr.adb (Build_Record_Aggr_Code): Perform a conversion of the
	target to the type of the aggregate in the case where the target object
	is class-wide.

	* exp_ch5.adb (Expand_Simple_Function_Return): When the function's
	result type is class-wide and inherently limited, and the expression
	has a specific type, create a return object of the specific type, for
	more efficient handling of returns of build-in-place aggregates (avoids
	conversions of the class-wide return object to the specific type on
	component assignments).

	* sem_ch6.adb (Check_Return_Subtype_Indication): Suppress the error
	about a type mismatch for a class-wide function with a return object
	having a specific type when the object declaration doesn't come from
	source. Such an object can result from the expansion of a simple return.

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