Commit e584558bd8

@@ -17931,23 +17931,14 @@ fn zirAlignCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!A
     const ptr = try sema.resolveInst(extra.rhs);
     const ptr_ty = sema.typeOf(ptr);
 
-    // TODO in addition to pointers, this instruction is supposed to work for
-    // pointer-like optionals and slices.
     try sema.checkPtrOperand(block, ptr_src, ptr_ty);
 
-    // TODO compile error if the result pointer is comptime known and would have an
-    // alignment that disagrees with the Decl's alignment.
-
-    const ptr_info = ptr_ty.ptrInfo().data;
-    const dest_ty = try Type.ptr(sema.arena, sema.mod, .{
-        .pointee_type = ptr_info.pointee_type,
-        .@"align" = dest_align,
-        .@"addrspace" = ptr_info.@"addrspace",
-        .mutable = ptr_info.mutable,
-        .@"allowzero" = ptr_info.@"allowzero",
-        .@"volatile" = ptr_info.@"volatile",
-        .size = ptr_info.size,
-    });
+    var ptr_info = ptr_ty.ptrInfo().data;
+    ptr_info.@"align" = dest_align;
+    var dest_ty = try Type.ptr(sema.arena, sema.mod, ptr_info);
+    if (ptr_ty.zigTypeTag() == .Optional) {
+        dest_ty = try Type.Tag.optional.create(sema.arena, dest_ty);
+    }
 
     if (try sema.resolveDefinedValue(block, ptr_src, ptr)) |val| {
         if (try val.getUnsignedIntAdvanced(sema.mod.getTarget(), null)) |addr| {
@@ -17960,7 +17951,7 @@ fn zirAlignCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!A
 
     try sema.requireRuntimeBlock(block, inst_data.src(), ptr_src);
     if (block.wantSafety() and dest_align > 1 and
-        try sema.typeHasRuntimeBits(block, sema.src, dest_ty.elemType2()))
+        try sema.typeHasRuntimeBits(block, sema.src, ptr_info.pointee_type))
     {
         const val_payload = try sema.arena.create(Value.Payload.U64);
         val_payload.* = .{
@@ -17985,7 +17976,7 @@ fn zirAlignCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!A
         } else is_aligned;
         try sema.addSafetyCheck(block, ok, .incorrect_alignment);
     }
-    return sema.coerceCompatiblePtrs(block, dest_ty, ptr, ptr_src);
+    return sema.bitCast(block, dest_ty, ptr, ptr_src);
 }
 
 fn zirBitCount(

@@ -556,3 +556,11 @@ test "comptime alloc alignment" {
     var bytes2_addr = @ptrToInt(&bytes2);
     try expect(bytes2_addr & 0xff == 0);
 }
+
+test "@alignCast null" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+
+    var ptr: ?*anyopaque = null;
+    const aligned: ?*anyopaque = @alignCast(@alignOf(?*anyopaque), ptr);
+    try expect(aligned == null);
+}