Reject slicing a packed array

In Ada mode, gdb rejects slicing a packed array.  However, with
-fgnat-encodings=minimal, gdb will instead print incorrect results.
This patch changes gdb to also reject slicing a packed array in this
mode.

FWIW I believe that this rejection is a gdb limitation.  Removing it
looked complicated, though, and meanwhile my main goal for the time
being is to bring the DWARF encodings up to par with Gnat encodings.

gdb/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_is_any_packed_array_type): New function.
	(ada_evaluate_subexp) <case TERNOP_SLICE>: Use it.

gdb/testsuite/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mod_from_name.exp: Test printing slice.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-11-04  Tom Tromey  <tromey@adacore.com>
+
+	* ada-lang.c (ada_is_any_packed_array_type): New function.
+	(ada_evaluate_subexp) <case TERNOP_SLICE>: Use it.
+
 2020-11-04  Tom Tromey  <tromey@adacore.com>
 
 	* dwarf2/read.c (recognize_bound_expression)

gdb/ada-lang.c

@@ -2012,6 +2012,17 @@ ada_is_unconstrained_packed_array_type (struct type *type)
   return 0;
 }
 
+/* Return true if TYPE is a (Gnat-encoded) constrained packed array
+   type, or if it is an ordinary (non-Gnat-encoded) packed array.  */
+
+static bool
+ada_is_any_packed_array_type (struct type *type)
+{
+  return (ada_is_constrained_packed_array_type (type)
+	  || (type->code () == TYPE_CODE_ARRAY
+	      && TYPE_FIELD_BITSIZE (type, 0) % 8 != 0));
+}
+
 /* Given that TYPE encodes a packed array type (constrained or unconstrained),
    return the size of its elements in bits.  */
 
@@ -10609,7 +10620,7 @@ ada_evaluate_subexp (struct type *expect_type, struct expression *exp,
 	  TYPE_TARGET_TYPE (value_type (array)) =
 	    ada_aligned_type (TYPE_TARGET_TYPE (value_type (array)));
 
-	if (ada_is_constrained_packed_array_type (value_type (array)))
+	if (ada_is_any_packed_array_type (value_type (array)))
 	  error (_("cannot slice a packed array"));
 
 	/* If this is a reference to an array or an array lvalue,

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-11-04  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.ada/mod_from_name.exp: Test printing slice.
+
 2020-11-04  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.ada/O2_float_param.exp: Test different -fgnat-encodings

gdb/testsuite/gdb.ada/mod_from_name.exp

@@ -40,4 +40,5 @@ foreach_with_prefix scenario {all minimal} {
     }
     gdb_test "print xp" \
 	"= \\(y => \\(-1, -2, -3, -4, -5, -6, -7, -8, -9, -10\\)\\)"
+    gdb_test "print slice" "cannot slice a packed array"
 }