This works correctly:

import std.algorithm;
void main() {
    auto array = [40, 20, 10, 30];
    array = remove!"a <= 20"(array); // OK
    assert(array == [40, 30]);
}



This doesn't compile:

import std.algorithm;
void main() {
    auto array = [40, 20, 10, 30];
    auto p = (int x) => x <= 20;
    array = remove!p(array); // Error.
    assert(array == [40, 30]);
}



DMD 2.061alpha gives:

test.d(5): Error: variable p cannot be read at compile time


While a module-level function:

import std.algorithm;
bool p(int x) { return x <= 20; }
void main() {
    auto array = [40, 20, 10, 30];
    array = remove!p(array);
    assert(array == [40, 30]);
}


DMD gives:

...\dmd2\src\phobos\std\algorithm.d(6843): Error: not a property p
...\dmd2\src\phobos\std\algorithm.d(6949): Error: not a property p