Unreachable code when preprocessor invokes parser with undefined division argument

[ehaas]

(clang errors with error: division by zero in preprocessor expression for this snippet)

#if 1/q
#endif

thread 18069770 panic: reached unreachable code
/Users/ehaas/source/zig/build/lib/zig/std/debug.zig:226:14: 0x104f172f8 in std.debug.assert (arocc)
    if (!ok) unreachable; // assertion failure
             ^
/Users/ehaas/source/arocc/src/Parser.zig:3631:15: 0x104fe0607 in Parser.Result.saveValue (arocc)
        assert(!p.in_macro);
              ^
/Users/ehaas/source/arocc/src/Parser.zig:3625:24: 0x1050419ae in Parser.Result.shouldEval (arocc)
        try a.saveValue(p);
                       ^
/Users/ehaas/source/arocc/src/Parser.zig:3362:32: 0x105009235 in Parser.Result.adjustTypes (arocc)
            return a.shouldEval(b, p);
                               ^
/Users/ehaas/source/arocc/src/Parser.zig:4238:32: 0x105078204 in Parser.mulExpr (arocc)
        if (try lhs.adjustTypes(percent.?, &rhs, p, if (tag == .mod_expr) .integer else .arithmetic)) {
                               ^
/Users/ehaas/source/arocc/src/Parser.zig:4196:28: 0x105077734 in Parser.addExpr (arocc)
    var lhs = try p.mulExpr();
                           ^
/Users/ehaas/source/arocc/src/Parser.zig:4164:28: 0x105076cc4 in Parser.shiftExpr (arocc)
    var lhs = try p.addExpr();
                           ^
/Users/ehaas/source/arocc/src/Parser.zig:4135:30: 0x105076374 in Parser.compExpr (arocc)
    var lhs = try p.shiftExpr();
                             ^
/Users/ehaas/source/arocc/src/Parser.zig:4110:29: 0x1050758f4 in Parser.eqExpr (arocc)
    var lhs = try p.compExpr();
                            ^
/Users/ehaas/source/arocc/src/Parser.zig:4090:27: 0x1050739d4 in Parser.andExpr (arocc)
    var lhs = try p.eqExpr();
                          ^
/Users/ehaas/source/arocc/src/Parser.zig:4070:28: 0x10506c7c4 in Parser.xorExpr (arocc)
    var lhs = try p.andExpr();
                           ^
/Users/ehaas/source/arocc/src/Parser.zig:4050:28: 0x10505ff34 in Parser.orExpr (arocc)
    var lhs = try p.xorExpr();
                           ^
/Users/ehaas/source/arocc/src/Parser.zig:4028:27: 0x105040744 in Parser.landExpr (arocc)
    var lhs = try p.orExpr();
                          ^
/Users/ehaas/source/arocc/src/Parser.zig:4006:29: 0x1050085b4 in Parser.lorExpr (arocc)
    var lhs = try p.landExpr();
                            ^
/Users/ehaas/source/arocc/src/Parser.zig:3968:29: 0x104fd6dd4 in Parser.condExpr (arocc)
    var cond = try p.lorExpr();
                            ^
/Users/ehaas/source/arocc/src/Parser.zig:3199:27: 0x104f7dc4e in Parser.macroExpr (arocc)
    const res = p.condExpr() catch |e| switch (e) {
                          ^
/Users/ehaas/source/arocc/src/Preprocessor.zig:465:28: 0x104f4e192 in Preprocessor.expr (arocc)
    return parser.macroExpr();
                           ^
/Users/ehaas/source/arocc/src/Preprocessor.zig:181:40: 0x104f428b9 in Preprocessor.preprocess (arocc)
                        if (try pp.expr(&tokenizer)) {
                                       ^
/Users/ehaas/source/arocc/src/main.zig:237:22: 0x104f27818 in processSource (arocc)
    try pp.preprocess(source);
                     ^
/Users/ehaas/source/arocc/src/main.zig:223:22: 0x104f1c151 in handleArgs (arocc)
        processSource(comp, source, builtin, user_macros) catch |e| switch (e) {
                     ^
/Users/ehaas/source/arocc/src/main.zig:41:15: 0x104f192dd in main (arocc)
    handleArgs(&comp, args) catch |err| switch (err) {
              ^
/Users/ehaas/source/zig/build/lib/zig/std/start.zig:524:29: 0x104f96b0c in std.start.callMain (arocc)
            return root.main();
                            ^
/Users/ehaas/source/zig/build/lib/zig/std/start.zig:469:12: 0x104f1d257 in std.start.callMainWithArgs (arocc)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/Users/ehaas/source/zig/build/lib/zig/std/start.zig:434:12: 0x104f1d195 in std.start.main (arocc)
    return @call(.{ .modifier = .always_inline }, callMainWithArgs, .{ @intCast(usize, c_argc), c_argv, envp });
           ^
???:?:?: 0x7fff204ecf3c in ??? (???)
???:?:?: 0x1 in ??? (???)
[1]    71375 abort      ~/source/arocc/zig-out/bin/arocc test.c