From 948f8090ae66ea9207473bf97d5c0dfaee4244ed Mon Sep 17 00:00:00 2001
From: Tim Chevalier <chevalier@alum.wellesley.edu>
Date: Tue, 2 Aug 2011 19:02:38 -0700
Subject: [PATCH] Handle _|_ - typed discriminants in alts correctly

Stop me, won't you, if you've heard this one before?

Closes #794
---
 src/comp/middle/trans_alt.rs      | 13 ++++++++++++-
 src/test/run-fail/alt-disc-bot.rs |  4 ++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 src/test/run-fail/alt-disc-bot.rs

diff --git a/src/comp/middle/trans_alt.rs b/src/comp/middle/trans_alt.rs
index 024f29224559..4a1734d3bc11 100644
--- a/src/comp/middle/trans_alt.rs
+++ b/src/comp/middle/trans_alt.rs
@@ -425,6 +425,18 @@ fn trans_alt(cx: &@block_ctxt, expr: &@ast::expr, arms: &ast::arm[],
              id: ast::node_id, output: &trans::out_method) -> result {
     let bodies = ~[];
     let match: match = ~[];
+    let er = trans::trans_expr(cx, expr);
+    if (ty::type_is_bot(bcx_tcx(cx), ty::expr_ty(bcx_tcx(cx), expr))) {
+        // No need to generate code for alt,
+        // since the disc diverges.
+        if (!cx.build.is_terminated()) {
+            ret rslt(cx, cx.build.Unreachable());
+        }
+        else {
+            ret rslt(cx, C_nil());
+        }
+    }
+
     for a: ast::arm  in arms {
         let body = new_scope_block_ctxt(cx, "case_body");
         bodies += ~[body];
@@ -445,7 +457,6 @@ fn trans_alt(cx: &@block_ctxt, expr: &@ast::expr, arms: &ast::arm[],
     }
 
     let exit_map = ~[];
-    let er = trans::trans_expr(cx, expr);
     let t = trans::node_id_type(cx.fcx.lcx.ccx, expr.id);
     let v = trans::spill_if_immediate(er.bcx, er.val, t);
     compile_submatch(er.bcx, match, ~[v],
diff --git a/src/test/run-fail/alt-disc-bot.rs b/src/test/run-fail/alt-disc-bot.rs
new file mode 100644
index 000000000000..8b4d30ef6ac4
--- /dev/null
+++ b/src/test/run-fail/alt-disc-bot.rs
@@ -0,0 +1,4 @@
+// error-pattern:quux
+fn f() -> ! { fail "quux" }
+fn g() -> int { alt f() { true { 1 } false { 0 } }; }
+fn main() { g(); }