fix implicit cast bitcast result to error union by returning

src/ir.cpp

@@ -15207,6 +15207,9 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
                     parent_ptr_type->data.pointer.is_const, parent_ptr_type->data.pointer.is_volatile, PtrLenSingle,
                     parent_ptr_align, 0, 0, parent_ptr_type->data.pointer.allow_zero);
 
+            if (value->value.special == ConstValSpecialRuntime) {
+                parent_result_loc->value.special = ConstValSpecialRuntime;
+            }
             result_loc->written = true;
             result_loc->resolved_loc = ir_analyze_ptr_cast(ira, suspend_source_instr, parent_result_loc,
                     ptr_type, result_bit_cast->base.source_instruction, false);

test/stage1/behavior/bitcast.zig

@@ -112,3 +112,16 @@ test "bitcast packed struct to integer and back" {
     S.doTheTest();
     comptime S.doTheTest();
 }
+
+test "implicit cast to error union by returning" {
+    const S = struct {
+        fn entry() void {
+            expect((func(-1) catch unreachable) == maxInt(u64));
+        }
+        pub fn func(sz: i64) anyerror!u64 {
+            return @bitCast(u64, sz);
+        }
+    };
+    S.entry();
+    comptime S.entry();
+}