don't demote expressions just because const_eval fails

this might introduce subtle bugs to code generation

src/librustc_passes/consts.rs

@@ -498,7 +498,6 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
                 Err(ConstEvalErr { kind: Math(ConstMathErr::Overflow(Op::Shl)), ..}) |
                 Err(ConstEvalErr { kind: IndexOpFeatureGated, ..}) => {},
                 Err(msg) => {
-                    self.qualif = self.qualif | ConstQualif::NOT_CONST;
                     self.tcx.sess.add_lint(CONST_ERR, ex.id,
                                            msg.span,
                                            msg.description().into_owned())

src/test/compile-fail/const-err.rs

@@ -8,8 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// these errors are not actually "const_err", they occur in trans/consts
+// and are unconditional warnings that can't be denied or allowed
+
 #![feature(rustc_attrs)]
 #![allow(exceeding_bitshifts)]
+#![allow(const_err)]
 
 fn black_box<T>(_: T) {
     unimplemented!()
@@ -21,6 +25,7 @@ fn main() {
     //~^ WARN attempted to negate with overflow
     let b = 200u8 + 200u8 + 200u8;
     //~^ WARN attempted to add with overflow
+    //~| WARN attempted to add with overflow
     let c = 200u8 * 4;
     //~^ WARN attempted to multiply with overflow
     let d = 42u8 - (42u8 + 1);

src/test/compile-fail/const-err2.rs

@@ -0,0 +1,34 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_attrs)]
+#![allow(exceeding_bitshifts)]
+#![deny(const_err)]
+
+fn black_box<T>(_: T) {
+    unimplemented!()
+}
+
+fn main() {
+    let a = -std::i8::MIN;
+    //~^ ERROR attempted to negate with overflow
+    let b = 200u8 + 200u8 + 200u8;
+    //~^ ERROR attempted to add with overflow
+    //~| ERROR attempted to add with overflow
+    let c = 200u8 * 4;
+    //~^ ERROR attempted to multiply with overflow
+    let d = 42u8 - (42u8 + 1);
+    //~^ ERROR attempted to subtract with overflow
+    let _e = [5u8][1];
+    black_box(a);
+    black_box(b);
+    black_box(c);
+    black_box(d);
+}