[gcc r12-1588] [Ada] Error in instance on incomplete actual passed to formal incomplete type

[gcc r12-1588] [Ada] Error in instance on incomplete actual passed to formal incomplete type

[Pierre-Marie de Rodat]

https://gcc.gnu.org/g:83a5b1df3da2d6c50efabdbf4767a32e80f0cab1

commit r12-1588-g83a5b1df3da2d6c50efabdbf4767a32e80f0cab1
Author: Gary Dismukes <dismukes@adacore.com>
Date:   Wed Mar 10 18:35:58 2021 -0500

    [Ada] Error in instance on incomplete actual passed to formal incomplete type
    
    gcc/ada/
    
            * sem_ch12.adb (Instantiate_Type): If the actual type for an
            incomplete formal type is also incomplete, but has a Full_View,
            use the Full_View of the actual type rather than the incomplete
            view.

Diff:
---
 gcc/ada/sem_ch12.adb | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/gcc/ada/sem_ch12.adb b/gcc/ada/sem_ch12.adb
index 2cd46895d28..06947e22878 100644
--- a/gcc/ada/sem_ch12.adb
+++ b/gcc/ada/sem_ch12.adb
@@ -14022,9 +14022,12 @@ package body Sem_Ch12 is
                      and then Ekind (Root_Type (Act_T)) = E_Incomplete_Type)
          then
             --  If the formal is an incomplete type, the actual can be
-            --  incomplete as well.
+            --  incomplete as well, but if an actual incomplete type has
+            --  a full view, then we'll retrieve that.
 
-            if Ekind (A_Gen_T) = E_Incomplete_Type then
+            if Ekind (A_Gen_T) = E_Incomplete_Type
+              and then not Present (Full_View (Act_T))
+            then
                null;
 
             elsif Is_Class_Wide_Type (Act_T)
@@ -14032,6 +14035,7 @@ package body Sem_Ch12 is
             then
                Error_Msg_N ("premature use of incomplete type", Actual);
                Abandon_Instantiation (Actual);
+
             else
                Act_T := Full_View (Act_T);
                Set_Entity (Actual, Act_T);