[Ada] Fix internal error on untagged record type with equality operator

[Pierre-Marie de Rodat <derodat@adacore.com>]

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After the binding interpretation issued under AI12-0413, a user-defined
primitive equality operator of an untagged record type hides the predefined
equality operator in an instantiation, but this does not apply if the
instantiation appears in the same declarative region as the type and
before the declaration of this user-defined operator.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_res.adb (Resolve_Equality_Op): Make sure that the user-defined
	operator of an untagged record type is declared ahead of an instance
	before using it to resolve the equality operator in the instance.

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diff --git a/gcc/ada/sem_res.adb b/gcc/ada/sem_res.adb
--- a/gcc/ada/sem_res.adb
+++ b/gcc/ada/sem_res.adb
@@ -8967,7 +8967,14 @@ package body Sem_Res is
                then
                   Eq := Get_User_Defined_Equality (T);
 
-                  if Present (Eq) then
+                  --  We need to make sure that the instance is not within the
+                  --  same declarative region as the type, or else that it lies
+                  --  after the declaration of the user-defined "=" operator.
+
+                  if Present (Eq)
+                    and then (not In_Same_Extended_Unit (Eq, N)
+                               or else Earlier_In_Extended_Unit (Eq, N))
+                  then
                      if Is_Abstract_Subprogram (Eq) then
                         Nondispatching_Call_To_Abstract_Operation (N, Eq);
                      else