import std.range;

void main() {
    string a = "foo";
    string b = "bar";
    auto myChain = chain(a, b);
}

C:\dmd2\windows\bin\..\..\src\phobos\std\range.d(937): Error: this._input._field_field_0[index] isn't mutable
C:\dmd2\windows\bin\..\..\src\phobos\std\range.d(937): Error: this._input._field_field_1[index] isn't mutable

The fix is to test for immutability in the static if statement on line 930 of std.range:

static if (allSameType && is(typeof(_input.field[0][0] = ElementType.init)))