[Ada] Spurious constraint error on conversion of access types

[Ada] Spurious constraint error on conversion of access types

[Pierre-Marie de Rodat]

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This patch fixes an error in the compiler whereby a spurious constraint
error is raised at runtime on type conversions between
access-to-discriminanted types when the object being converted is null.

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

gcc/ada/

	* checks.adb (Make_Discriminant_Constraint_Check): Add check for
	null when the type being converted is an access type.

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diff --git a/gcc/ada/checks.adb b/gcc/ada/checks.adb
--- a/gcc/ada/checks.adb
+++ b/gcc/ada/checks.adb
@@ -3658,6 +3658,20 @@ package body Checks is
          Cond := Build_Discriminant_Checks (Expr, Expr_Type);
          Set_Discriminant_Constraint (Expr_Type, Old_Constraints);
 
+         --  Conversion between access types requires that we check for null
+         --  before checking discriminants.
+
+         if Is_Access_Type (Etype (Expr)) then
+            Cond := Make_And_Then (Loc,
+                      Left_Opnd  =>
+                        Make_Op_Ne (Loc,
+                          Left_Opnd  =>
+                            Duplicate_Subexpr_No_Checks
+                              (Expr, Name_Req => True),
+                          Right_Opnd => Make_Null (Loc)),
+                      Right_Opnd => Cond);
+         end if;
+
          Insert_Action (N,
            Make_Raise_Constraint_Error (Loc,
              Condition => Cond,