Branch miscompiled as if it were comptime known

[adlerd]

I tried running this program

const std = @import("std");
pub fn main() anyerror!void{
    var args = std.process.args();
    _ = args.skip();
    const flag = args.skip();
    return std.fmt.format(std.io.getStdOut().writer(), "{any}\n{s}\n", .{
        flag,
        if (flag) @as([]const u8, "YES") else "NO",
    });
}

zig version: 0.9.0-dev.874+4334dbb9a

When running the program with no arguments, the expected output is:

false
NO

The observed output is:

false
YES

This example was minimized from another example; the general problem seems to be format arguments initialized by a branch between two constant strings, when the branch is not comptime known.

[rohlem]

Probably a manifestation/duplicate of the nasty #3882 / #4491 / #5230 / #7097 .
The stage1 compiler is alarmingly buggy with conditional control flow filling tuples.
The current workaround is to write the values to variables first, and use those in the tuple.

const answer = if (flag) @as([]const u8, "YES") else "NO";
return std.fmt.format(std.io.getStdOut().writer(), "{any}\n{s}\n", .{
    flag,
    answer,
});

Not great. Would probably be better for stage1 to hard-error if we could reliably detect these scenarios.