improve compile error when using comptime concatenation and operand has runtime known length

[nektro]

Zig Version

0.10.0-dev.2444+83beed09e

Steps to Reproduce

const std = @import("std");

test {
    const a = foo() ++ "bb";
    try std.testing.expect(a.len == 5);
}

fn foo() []const u8 {
    return "aaa";
}

Expected Behavior

All 1 tests passed.

Actual Behavior

test.zig:4:18: error: unable to resolve comptime value
    const a = foo() ++ "bb";
                 ^

only affects stage2

[nektro]

moving a to decl scope makes this work, so this only affects consts in block scope

[Vexu]

Not a bug, stage2 implements #7147 so the operands aren't implicitly conptime but slice concatenation still requires the length to be known.

[andrewrk]

Yep this is working as designed.

[andrewrk]

Re-opening for a better compile error. It should say something like, "array concatenation operand requires comptime-known length".