[Ada] Bad warning: Size in Compile_Time_Error in nested instance

2019-12-16  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_attr.adb (Analyze_Attribute): Use Known_RM_Size. But we
	still need Size_Known_At_Compile_Time, because when the size
	really is known, sometimes only one or the other of these is
	True.

From-SVN: r279421

gcc/ada/ChangeLog

+2019-12-16  Bob Duff  <duff@adacore.com>
+
+	* sem_attr.adb (Analyze_Attribute): Use Known_RM_Size. But we
+	still need Size_Known_At_Compile_Time, because when the size
+	really is known, sometimes only one or the other of these is
+	True.
+
 2019-12-16  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* sem_util.adb (Is_Atomic_Or_VFA_Object): Also return true for

gcc/ada/sem_attr.adb

@@ -5841,13 +5841,16 @@ package body Sem_Attr is
          --  Time_Errors after the back end has been called and this occurrence
          --  of 'Size is known at compile time then it is safe to perform this
          --  evaluation. Needed to perform the static evaluation of the full
-         --  boolean expression of these pragmas.
+         --  boolean expression of these pragmas. Note that Known_RM_Size is
+         --  sometimes True when Size_Known_At_Compile_Time is False, when the
+         --  back end has computed it.
 
          if In_Compile_Time_Warning_Or_Error
            and then Is_Entity_Name (P)
            and then (Is_Type (Entity (P))
                       or else Ekind (Entity (P)) = E_Enumeration_Literal)
-           and then Size_Known_At_Compile_Time (Entity (P))
+           and then (Known_RM_Size (Entity (P))
+                       or else Size_Known_At_Compile_Time (Entity (P)))
          then
             declare
                Siz : Uint;