stage2: comptime-known value isn't comptime-known through generic function

[mitchellh]

Zig Version

0.10.0-dev.1335+c64279b15

Steps to Reproduce

The test program is below: Note:

• The switch return type is required for the function sig.
• The switch return value is required for the function body.
• The function call is required
• If you wrap the whole test in a comptime {} block it works.

const std = @import("std");
const Type = std.builtin.Type;

test "Tuple" {
    const fields_list = fields(@TypeOf(.{}));
    if (fields_list.len != 0)
        @compileError("Argument count mismatch");
}

pub fn fields(comptime T: type) switch (@typeInfo(T)) {
    .Struct => []const Type.StructField,
    else => unreachable,
} {
    return switch (@typeInfo(T)) {
        .Struct => |info| info.fields,
        else => unreachable,
    };
}

Expected Behavior

Should not have any errors (stage1 works), since fields.len should be comptime known.

The reason: this is how std.meta tests Tuple and ArgsTuple, so this is exercised in the stdlib.

Actual Behavior

Triggers @compileError