compiler: un-implement ziglang#19634

This commit reverts the handling of partially-undefined values in
bitcasting to transform these bits into an arbitrary arithmetic value,
like happens on `master` today.

As @andrewrk rightly points out, ziglang#19634 has unfortunate consequences
for the standard library, and likely requires more thought. To avoid
a major breaking change, it has been decided to revert this design
decision for now, and make a more informed decision further down the
line.

Author: mlugg

lib/std/net.zig

@@ -278,9 +278,6 @@ pub const Ip4Address = extern struct {
             },
         };
         const out_ptr = mem.asBytes(&result.sa.addr);
-        if (@inComptime()) {
-            @memset(out_ptr, 0); // TODO: #19634
-        }
 
         var x: u8 = 0;
         var index: u8 = 0;
@@ -392,9 +389,6 @@ pub const Ip6Address = extern struct {
                 .addr = undefined,
             },
         };
-        if (@inComptime()) {
-            @memset(std.mem.asBytes(&result.sa.addr), 0); // TODO: #19634
-        }
         var ip_slice: *[16]u8 = result.sa.addr[0..];
 
         var tail: [16]u8 = undefined;
@@ -513,9 +507,6 @@ pub const Ip6Address = extern struct {
                 .addr = undefined,
             },
         };
-        if (@inComptime()) {
-            @memset(std.mem.asBytes(&result.sa.addr), 0); // TODO: #19634
-        }
         var ip_slice: *[16]u8 = result.sa.addr[0..];
 
         var tail: [16]u8 = undefined;

lib/std/packed_int_array.zig

@@ -214,21 +214,13 @@ pub fn PackedIntArrayEndian(comptime Int: type, comptime endian: Endian, comptim
         /// or, more likely, an array literal.
         pub fn init(ints: [int_count]Int) Self {
             var self: Self = undefined;
-            if (@inComptime()) {
-                // TODO: #19634
-                @memset(&self.bytes, 0xAA);
-            }
             for (ints, 0..) |int, i| self.set(i, int);
             return self;
         }
 
         /// Initialize all entries of a packed array to the same value.
         pub fn initAllTo(int: Int) Self {
             var self: Self = undefined;
-            if (@inComptime()) {
-                // TODO: #19634
-                @memset(&self.bytes, 0xAA);
-            }
             self.setAll(int);
             return self;
         }

src/Sema/bitcast.zig

@@ -681,12 +681,24 @@ const PackValueBits = struct {
         const vals, const bit_offset = pack.prepareBits(want_ty.bitSize(zcu));
 
         for (vals) |val| {
-            if (Value.fromInterned(val).isUndef(zcu)) {
-                // The value contains undef bits, so is considered entirely undef.
-                return zcu.undefValue(want_ty);
-            }
+            if (!Value.fromInterned(val).isUndef(zcu)) break;
+        } else {
+            // All bits of the value are `undefined`.
+            return zcu.undefValue(want_ty);
         }
 
+        // TODO: we need to decide how to handle partially-undef values here.
+        // Currently, a value with some undefined bits becomes `0xAA` so that we
+        // preserve the well-defined bits, because we can't currently represent
+        // a partially-undefined primitive (e.g. an int with some undef bits).
+        // In future, we probably want to take one of these two routes:
+        // * Define that if any bits are `undefined`, the entire value is `undefined`.
+        //   This is a major breaking change, and probably a footgun.
+        // * Introduce tracking for partially-undef values at comptime.
+        //   This would complicate a lot of operations in Sema, such as basic
+        //   arithmetic.
+        // This design complexity is tracked by #19634.
+
         ptr_cast: {
             if (vals.len != 1) break :ptr_cast;
             const val = Value.fromInterned(vals[0]);