[Ada] Crash building VSS with compiler built with assertions

When a tagged type T has aspect String_Literal, a derived type defines a
null extension T2, and the context to resolve the use of an object of
type T2 where the string literal is applicable is a class-wide type the
frontend crashes trying to evaluate if the object is a null extension.
This problem does not reproduce when the compiler is built with
assertions disabled.

gcc/ada/

	* sem_ch6.adb (Find_Corresponding_Spec): Avoid calling
	Is_Null_Extension with a class-wide type entity.
	(Overrides_Visible_Function): Handle alias entities.
	* sem_res.adb (Has_Applicable_User_Defined_Literal): Conversion
	not needed if the result type of the call is class-wide or if
	the result type matches the context type.
	* sem_util.ads (Is_Null_Extension): Adding documentation.
	(Is_Null_Extension_Of): Adding documentation.
	* sem_util.adb (Is_Null_Extension): Adding assertion.
	(Is_Null_Extension_Of): Adding assertions.

1 files changed, 2 insertions, 0 deletions

diff --git a/gcc/ada/sem_util.ads b/gcc/ada/sem_util.ads
index 6f28fe0..b6c70ca 100644
--- a/gcc/ada/sem_util.ads
+++ b/gcc/ada/sem_util.ads
@@ -2209,12 +2209,14 @@ package Sem_Util is
    --  Given a tagged type, returns True if argument is a type extension
    --  that introduces no new components (discriminant or nondiscriminant).
    --  Ignore_Privacy should be True for use in implementing dynamic semantics.
+   --  Cannot be called with class-wide types.
 
    function Is_Null_Extension_Of
      (Descendant, Ancestor : Entity_Id) return Boolean;
    --  Given two tagged types, the first a descendant of the second,
    --  returns True if every component of Descendant is inherited
    --  (directly or indirectly) from Ancestor. Privacy is ignored.
+   --  Cannot be called with class-wide types.
 
    function Is_Null_Record_Definition (Record_Def : Node_Id) return Boolean;
    --  Returns True for an N_Record_Definition node that has no user-defined