AstGen: do not forward result pointers through @as

The `coerce_result_ptr` instruction is highly problematic and leads to
unintentional memory reinterpretation in some cases. It is more correct
to simply not forward result pointers through this builtin.

`coerce_result_ptr` is still used for struct and array initializations,
where it can still cause issues. Eliminating this usage will be a future
change.

Resolves: #16991

1 files changed, 18 insertions, 0 deletions

diff --git a/test/behavior/cast.zig b/test/behavior/cast.zig
index 334259661d..34d18f6717 100644
--- a/test/behavior/cast.zig
+++ b/test/behavior/cast.zig
@@ -2502,3 +2502,21 @@ test "numeric coercions with undefined" {
     to = 42.0;
     try expectEqual(@as(f32, 42.0), to);
 }
+
+test "@as does not corrupt values with incompatible representations" {
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    const x: f32 = @as(f16, blk: {
+        if (false) {
+            // Trick the compiler into trying to use a result pointer if it can!
+            break :blk .{undefined};
+        }
+        break :blk 1.23;
+    });
+    try std.testing.expectApproxEqAbs(@as(f32, 1.23), x, 0.001);
+}