Re: lambdas and non-captured, unevaluated operands

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Sat, 24 Apr 2010 18:05:18 CST
On 23 Apr., 15:16, Paul Bibbings <> wrote:

From the above, my reading is that g5 is OK since the presence of i does
not, in this context, constitute a 'use'.


Thus, from [basic.def.odr],

   "An expression is potentially evaluated unless it is an unevaluated
   operand (Clause 5) or a subexpression thereof. A variable or
   non-overloaded function whose name appears as a potentially-evaluated
   expression is used ..."

and, of course, this is the case for the operand to sizeof in the
example above:

[expr.sizeof] ?5.3.3/1:

   "The sizeof operator yields the number of bytes in the object
   representation of its operand. The operand is either an expression,
   which is an unevaluated operand (Clause 5), or a parenthesized

What appears to be missing in [expr.prim.lambda] (unless it is me that
is missing something) is some statement indicating how it happens - in
the absence of explicit or implicit capture - that i is nevertheless
`available' in the context of the compound-statement of the
lambda-expression in g5, above; in effect, by what mechanism it is
available outside of being captured.

What you are missing is [expr.prim.lambda]/7:

"The lambda-expression's compound-statement yields the function-body
of the function call operator, but for purposes of name lookup (3.4),
[..], the compound-statement is considered in the context of the

So this defines the name lookup of any entity named in the
lambda-expression. It means that the name 'i' within g5 is
resolved in the same way as this name would be resolved
at the textual position where the lambda expression occurred,

    void f2() {
       int i = 1;
       // ...
       void g5(int = sizeof i);

This is also well-formed, because i is not used, since it appears
in an unevaluated operand (I'm aware that several compilers don't
get that right).

I am wanting to get a better sense of this since gcc-4.5.0 does not
accept the above code, failing it with:

   11:53:03 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/nano/gcc_bugs $cat _5_1_2_13.cpp
   // file: _5_1_2_13.cpp

   void f2() {
      int i = 1;
      // ...
      void g5(int = ([]{ return sizeof i; })());

   11:53:06 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/nano/gcc_bugs $i686-pc-cygwin-gcc-4.5.0 -Wall
      -Wno-unused -std=c++0x -pedantic -c _5_1_2_13.cpp
   _5_1_2_13.cpp: In lambda function:
   _5_1_2_13.cpp:6:37: error: local variable ?i? may not appear in this
   _5_1_2_13.cpp:6:40: error: return-statement with a value, in function
      returning 'void'
   _5_1_2_13.cpp: In function ?void f2()?:
   _5_1_2_13.cpp:6:44: error: default argument for ?int <anonymous>? has
      type ?void?

This is a compiler bug.

HTH & Greetings from Bremen,

Daniel Kr?gler

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