Why is returning a reference to a function local value not a compile error?

Question

The following code invokes undefined behaviour.

int& foo()
{
  int bar = 1234;
  return bar;
}

g++ issues a warning:

Answer 1

To expand on the earlier answers, the ISO C++ standard does not capture the distinction between warnings and errors to begin with; it simply uses the term 'diagnostic' when referring to what a compiler must emit upon seeing an ill-formed program. Quoting N3337, 1.4, paragraphs 1 and 2:

The set of diagnosable rules consists of all syntactic and semantic rules in this International Standard except for those rules containing an explicit notation that “no diagnostic is required” or which are described as resulting in “undefined behavior.”

Although this International Standard states only requirements on C++ implementations, those requirements are often easier to understand if they are phrased as requirements on programs, parts of programs, or execution of programs. Such requirements have the following meaning:

• If a program contains no violations of the rules in this International Standard, a conforming implementation shall, within its resource limits, accept and correctly execute that program.

• If a program contains a violation of any diagnosable rule or an occurrence of a construct described in this Standard as “conditionally-supported” when the implementation does not support that construct, a conforming implementation shall issue at least one diagnostic message.

• If a program contains a violation of a rule for which no diagnostic is required, this International Standard places no requirement on implementations with respect to that program.