Given the program:

=========================
import std.stdio;
import std.ascii;
import std.algorithm;
import std.string;

void main() {
//stdin.byChunk(1024).joiner().map!(a =>
toUpper(a)).copy(stdout.lockingTextWriter());

stdin.byLine(KeepTerminator.yes).joiner().map!(a =>
toUpper(a)).copy(stdout.lockingTextWriter());

//stdin.byLine(KeepTerminator.yes).joiner().copy(stdout.lockingTextWriter());

//stdin.byLine(KeepTerminator.yes).copy(stdout.lockingTextWriter());
}
==============================
Compile & run with:

foo <foo.d

trying one of the 4 versions. Versions 1 and 4 work, 2 and 3 fail
horribly.

The output is all scrambled, like this:

IMPORT STD.ASCII;
IMPORT STD.ALGORITIMPORT STD.STRING;
M;

MPORT STD.STRING;
VVOID MAIN() {
STDIN.BYLINE(KEEPTERMINATOR.YES).JOINER().MAP!(A =>
TOUPPER(A)).COPY(STDOUT.LOCKINGTEXT //STDIN.B
YLINE(KEEPTERMINATOR.YES).JOINER().COPY(STDOUT.LOCKINGTEXTWRITER());
CKINGTEXTWRITER());
//STDIN.BYLINE(KEEPTERMINATOR.YES).COPY(STDOUT.LOCKINGTEXTWRITER());
ITER());
}
//STDIN.BYLINE(KEEPTERMINATOR.YES).COPY(STDOUT.LOCKINGTEXTWRITER());
} 

Analysis from Andrei:

Go to algorithm.d line 2370, when joiner() is defined. Then go down to method prepare(). That method calls _items.front.empty, i.e. it assumes _items.front "works" but at the same time saves _current. At the moment _items.front is called, _current gets overwritten.
The code should be changed to not assume that _items.front is independent from _current.