Sema: allow dereferencing ill-defined pointers to zero-bit types at comptime

It doesn't matter if a pointer to a zero-bit (i.e. OPV) type is
undefined or runtime-known; we still know the result of the dereference
at comptime. Code may use this, for instance, when allocating zero-bit
types: `@as(*void, undefined)` is entirely reasonable to use at runtime,
since we know the pointer will never be accessed, thus it should be
valid at comptime too.

1 files changed, 8 insertions, 0 deletions

diff --git a/test/behavior/comptime_memory.zig b/test/behavior/comptime_memory.zig
index 71d177395b..98e6e65725 100644
--- a/test/behavior/comptime_memory.zig
+++ b/test/behavior/comptime_memory.zig
@@ -420,3 +420,11 @@ test "mutate entire slice at comptime" {
         buf[1..3].* = x;
     }
 }
+
+test "dereference undefined pointer to zero-bit type" {
+    const p0: *void = undefined;
+    try testing.expectEqual({}, p0.*);
+
+    const p1: *[0]u32 = undefined;
+    try testing.expect(p1.*.len == 0);
+}