From cf9200b81594071d6378df2294da1c737c780c05 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <superjoe30@gmail.com>
Date: Mon, 17 Sep 2018 18:13:38 -0400
Subject: [PATCH] dereferencing a *u0 is comptime-known to be 0

---
 src/ir.cpp          | 7 +++++++
 test/cases/eval.zig | 6 ++++++
 2 files changed, 13 insertions(+)

diff --git a/src/ir.cpp b/src/ir.cpp
index ce044aa0f3..f4b727412e 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -11187,6 +11187,13 @@ static IrInstruction *ir_get_deref(IrAnalyze *ira, IrInstruction *source_instruc
                 }
             }
         }
+        // dereferencing a *u0 is comptime known to be 0
+        if (child_type->id == ZigTypeIdInt && child_type->data.integral.bit_count == 0) {
+            IrInstruction *result = ir_create_const(&ira->new_irb, source_instruction->scope,
+                source_instruction->source_node, child_type);
+            init_const_unsigned_negative(&result->value, child_type, 0, false);
+            return result;
+        }
         // TODO if the instruction is a const ref instruction we can skip it
         IrInstruction *load_ptr_instruction = ir_build_load_ptr(&ira->new_irb, source_instruction->scope,
                 source_instruction->source_node, ptr);
diff --git a/test/cases/eval.zig b/test/cases/eval.zig
index 7e9c9a739d..7f8ad8ed07 100644
--- a/test/cases/eval.zig
+++ b/test/cases/eval.zig
@@ -682,3 +682,9 @@ test "refer to the type of a generic function" {
 }
 
 fn doNothingWithType(comptime T: type) void {}
+
+test "zero extend from u0 to u1" {
+    var zero_u0: u0 = 0;
+    var zero_u1: u1 = zero_u0;
+    assert(zero_u1 == 0);
+}