[Ada] Fix assertion failure for unfrozen mutably tagged type as actual in instance

[Eric Botcazou]

An instantiation is a freezing point for the actuals, so the mutably tagged 
type will be frozen by the instantiation, but this happens too late in the 
current implementation of mutably tagged types, because the declaration of 
their CW-equivalent type is not analyzed until after the type is frozen.

Tested on x86-64/Linux, applied on the mainline and 15 branch.


2025-12-27  Eric Botcazou  <ebotca...@adacore.com>

        PR ada/123306
        * sem_ch12.adb (Analyze_One_Association): Immediately freeze the
        root type of mutably tagged types used as actual type parameters.


2025-12-27  Eric Botcazou  <ebotca...@adacore.com>

        * gnat.dg/specs/mutably_tagged1.ads: New test.

-- 
Eric Botcazou
diff --git a/gcc/ada/sem_ch12.adb b/gcc/ada/sem_ch12.adb
index d2478d24ec8..729a6b145ac 100644
--- a/gcc/ada/sem_ch12.adb
+++ b/gcc/ada/sem_ch12.adb
@@ -2657,9 +2657,20 @@ package body Sem_Ch12 is
                if Ekind (Etype (Match)) /= E_Void
                  and then Is_Mutably_Tagged_Type (Etype (Match))
                then
+                  --  The declaration of the CW-equivalent type of a mutably
+                  --  tagged type is analyzed when the tagged type is frozen.
+
+                  if Nkind (N) /= N_Formal_Package_Declaration
+                    and then Ekind (Defining_Identifier (Assoc.An_Formal)) /=
+                                                              E_Incomplete_Type
+                  then
+                     Freeze_Before (N, Root_Type (Etype (Match)));
+                  end if;
+
                   Rewrite (Match, New_Occurrence_Of
                     (Class_Wide_Equivalent_Type
                       (Etype (Match)), Sloc (Match)));
+
                   Analyze (Match);
                end if;
 

-- { dg-do compile }
-- { dg-options "-gnatX0" }

package Mutably_Tagged1 is

  generic
    type T is private;
  package G is
  end G;

  type Rec is tagged null record with Size'Class => 128;

  package My_G is new G (Rec'Class);

end Mutably_Tagged1;