Switch branch of noreturn union field with mutable capture is incorrectly semantically analyzed

[ifreund]

Zig Version

0.11.0-dev.168+b2ffe113d

Steps to Reproduce and Observed Behavior

This test should pass, but it fails due to the @compileError() being triggered.

test "mutable capture of noreturn union field" {
    const U = union(enum) {
        a: u32,
        b: noreturn,
    };
    var a = U{ .a = 1 };
    switch (a) {
        .a => {},
        .b => |*val| {
            _ = val;
            @compileError("Shouldn't be triggered");
        },
    }
}

Expected Behavior

The test should pass. Note that we have test coverage in place for immutable captures and the given test passes if |val| is used instead of |*val|.

[nektro]

should the compile error instead then be that a .b case was added since the compiler can know it should be unreachable in this case? related #12863

[ifreund]

No, there should be no compile error. See the existing noreturn union field behavior tests.

This example doesn't demonstrate the motivating use-case for this behavior, namely "disabling" a tagged union field based on a comptime-known value. For example

const View = union(enum) {
   xdg_toplevel: XdgToplevel,
   xwayland_view: if (build_options.xwayland) XwaylandView else noreturn,
}