Assertion computing source location span of a non-initial multi-prong

[jacobly0]

comptime {
    switch (@as(enum { x }, .x)) {
        else => {},
        .a, .b => {},
    }
}

$ zig-debug build-obj bug.zig
thread 135248 panic: reached unreachable code
src/Zcu.zig:1701:32: 0x18d5f22 in span (zig)
                        } else unreachable;
                               ^
src/Compilation.zig:3357:42: 0x18e6a7d in addModuleErrorMsg (zig)
    const err_span = try err_src_loc.span(gpa);
                                         ^
src/Compilation.zig:3161:34: 0x18ee64c in getAllErrorsAlloc (zig)
            try addModuleErrorMsg(zcu, &bundle, error_msg.*);
                                 ^
src/Compilation.zig:3288:44: 0x1a375f2 in totalErrorCount (zig)
    var errors = try comp.getAllErrorsAlloc();
                                           ^
src/Compilation.zig:3284:28: 0x18c2ed4 in anyErrors (zig)
    return (totalErrorCount(comp) catch return true) != 0;
                           ^
src/Compilation.zig:2286:18: 0x18b9d6f in update (zig)
    if (anyErrors(comp)) {
                 ^
src/main.zig:4453:20: 0x18f30d7 in updateModule (zig)
    try comp.update(prog_node);
                   ^
src/main.zig:3643:21: 0x1959a21 in buildOutputType (zig)
        updateModule(comp, color, root_prog_node) catch |err| switch (err) {
                    ^
src/main.zig:270:31: 0x17e83e7 in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .{ .build = .Obj });
                              ^
src/main.zig:205:20: 0x17e52e5 in main (zig)
    return mainArgs(gpa, arena, args);
                   ^
lib/std/start.zig:656:37: 0x17e4dee in main (zig)
            const result = root.main() catch |err| {
                                    ^
???:?:?: 0x7fe92b5fc3ed in ??? (libc.so.6)
Unwind information for `libc.so.6:0x7fe92b5fc3ed` was not available, trace may be incomplete

Aborted (core dumped)
$ zig-release build-obj bug.zig
bug.zig:1:1: error: no field named 'a' in enum 'bug.comptime__enum_156'
comptime {
^~~~~~~~
bug.zig:2:17: note: enum declared here
    switch (@as(enum { x }, .x)) {
                ^~~~~~~~~~
referenced by:
    root: lib/std/start.zig:3:22
    comptime: lib/std/start.zig:27:9
    2 reference(s) hidden; use '-freference-trace=4' to see all references