[Ada] Hang on generic declaration with ambiguous formal package

[Pierre-Marie de Rodat]

This patch fixes the compiler whereby malformed code featuring a generic
package declaration with a formal package may result in an infinite loop
when the name of the formal package is hidden by the formal parameter
itself.

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

gcc/ada/

        * sem_ch12.adb (Analyze_Associations): Add check for errors on
        the generic formal before continuing with instantiation.
diff --git a/gcc/ada/sem_ch12.adb b/gcc/ada/sem_ch12.adb
--- a/gcc/ada/sem_ch12.adb
+++ b/gcc/ada/sem_ch12.adb
@@ -1948,10 +1948,19 @@ package body Sem_Ch12 is
                   end if;
 
                when N_Formal_Package_Declaration =>
-                  Match :=
-                    Matching_Actual
-                      (Defining_Identifier (Formal),
-                       Defining_Identifier (Original_Node (Analyzed_Formal)));
+                  --  The name of the formal package may be hidden by the
+                  --  formal parameter itself.
+
+                  if Error_Posted (Analyzed_Formal) then
+                     Abandon_Instantiation (Instantiation_Node);
+
+                  else
+                     Match :=
+                       Matching_Actual
+                         (Defining_Identifier (Formal),
+                          Defining_Identifier
+                            (Original_Node (Analyzed_Formal)));
+                  end if;
 
                   if No (Match) then
                      if Partial_Parameterization then