Fix artificial overflow during GENERIC folding

The Ada compiler gives a bogus warning:
storage_offset1.ads:16:52: warning: Constraint_Error will be raised at run
time [enabled by default]

Ironically enough, this occurs because of an intermediate conversion to an
unsigned type which is supposed to hide overflows but is counter-productive
for constants because TREE_OVERFLOW is always set for them, so it ends up
setting a bogus TREE_OVERFLOW when converting back to the original type.

The fix simply redirects INTEGER_CSTs to the other, direct path without the
intermediate conversion to the unsigned type.

gcc/
	* match.pd ((T)P - (T)(P + A) -> -(T) A): Avoid artificial overflow
	on constants.

gcc/testsuite/
	* gnat.dg/specs/storage_offset1.ads: New test.

1 files changed, 18 insertions, 0 deletions

diff --git a/gcc/testsuite/gnat.dg/specs/storage_offset1.ads b/gcc/testsuite/gnat.dg/specs/storage_offset1.ads
new file mode 100644
index 0000000..f0f7a61
--- /dev/null
+++ b/gcc/testsuite/gnat.dg/specs/storage_offset1.ads
@@ -0,0 +1,18 @@
+-- { dg-do compile }
+
+with System.Storage_Elements; use System.Storage_Elements;
+with System;
+
+package Storage_Offset1 is
+
+  type Rec is record
+    I1, I2 : Integer;
+  end record;
+
+  type Ptr is access all Rec;
+
+  R : Ptr := new Rec;
+
+  Offset : constant Storage_Offset := R.I1'Address - R.I2'Address;
+
+end Storage_Offset1;