int n = {{0}};

[Keith Thompson]

Citations are from N1570 (C11 draft). I don't think there are any
relevant differences in other editions or drafts of the standard.

Quick summary: Why is
int n = {{0}};
undefined behavior rather than a constraint violation?

6.7.9 (Initialization) includes a list of constraints, starting with "No
initializer shall attempt to provide a value for an object not contained
within the entity being initialized."

6.7.9p11 says:

The initializer for a scalar shall be a single expression,
optionally enclosed in braces. The initial value of the object is
that of the expression (after conversion); the same type constraints
and conversions as for simple assignment apply, taking the type of
the scalar to be the unqualified version of its declared type.

(I presume that "optionally enclosed in braces" is not intended to permit
more than one level of braces.)

Both
int n = 0;
and
int n = {0};
are clearly valid, with identical semantics. But this:
int n = {{0}};
violates that requirement. Since the requirement is under Semantics,
not Constraints, code that violates it has undefined behavior. Starting
with C99, this is mentioned in Annex J.

My question: Why is this undefined behavior rather than a constraint
violation?

I suggest that moving that paragraph from Semantics to Constraints would
be an improvement. Extra braces on a scalar initializer are easy to
detect, and requiring a diagnostic should not be a significant burden,
and would promote consistency. (Alternatively, the standard could have
explicitly allowed arbitrarily nested braces.)

With the current requirements, a compiler could quietly generate code to
set n to 42, though I doubt that anyone would do that.

(I note that gcc issues a non-fatal warning with "-std=c17
-pedantic-errors", while clang treats it as a fatal error.)

[Richard Damon]

Post by Keith Thompson
Citations are from N1570 (C11 draft). I don't think there are any
relevant differences in other editions or drafts of the standard.
Quick summary: Why is
int n = {{0}};
undefined behavior rather than a constraint violation?
6.7.9 (Initialization) includes a list of constraints, starting with "No
initializer shall attempt to provide a value for an object not contained
within the entity being initialized."
The initializer for a scalar shall be a single expression,
optionally enclosed in braces. The initial value of the object is
that of the expression (after conversion); the same type constraints
and conversions as for simple assignment apply, taking the type of
the scalar to be the unqualified version of its declared type.
(I presume that "optionally enclosed in braces" is not intended to permit
more than one level of braces.)
Both
int n = 0;
and
int n = {0};
int n = {{0}};
violates that requirement. Since the requirement is under Semantics,
not Constraints, code that violates it has undefined behavior. Starting
with C99, this is mentioned in Annex J.
My question: Why is this undefined behavior rather than a constraint
violation?
I suggest that moving that paragraph from Semantics to Constraints would
be an improvement. Extra braces on a scalar initializer are easy to
detect, and requiring a diagnostic should not be a significant burden,
and would promote consistency. (Alternatively, the standard could have
explicitly allowed arbitrarily nested braces.)
With the current requirements, a compiler could quietly generate code to
set n to 42, though I doubt that anyone would do that.
(I note that gcc issues a non-fatal warning with "-std=c17
-pedantic-errors", while clang treats it as a fatal error.)

I wonder a bit if this is bowing to conflicting existing
implementations. So were giving a fatal error for the extra braces,
while some specifically permitted them, and have a measurable code base
that uses that permision.

[Tim Rentsch]

Post by Keith Thompson
Citations are from N1570 (C11 draft). I don't think there are any
relevant differences in other editions or drafts of the standard.
Quick summary: Why is
int n = {{0}};
undefined behavior rather than a constraint violation?

Because there is no practical benefit to changing what the
standard says now.

Post by Keith Thompson
6.7.9 (Initialization) includes a list of constraints, starting
with "No initializer shall attempt to provide a value for an
object not contained within the entity being initialized."
The initializer for a scalar shall be a single expression,
optionally enclosed in braces. The initial value of the
object is that of the expression (after conversion); the same
type constraints and conversions as for simple assignment
apply, taking the type of the scalar to be the unqualified
version of its declared type.
(I presume that "optionally enclosed in braces" is not intended to
permit more than one level of braces.)
Both
int n = 0;
and
int n = {0};
int n = {{0}};
violates that requirement. Since the requirement is under
Semantics, not Constraints, code that violates it has undefined
behavior. Starting with C99, this is mentioned in Annex J.
My question: Why is this undefined behavior rather than a
constraint violation?

There's no practical benefit to changing it. The current rule
has been in place since the original ANSI C standard, more than
30 years ago. Whatever the motivation may have been at that
time, there is no incentive to change the rules now; compilers
already give diagnostics in such cases, even though they don't
have to. Changing the rules would mean throwing away a degree
of freedom with no significant benefit.

Post by Keith Thompson
I suggest that moving that paragraph from Semantics to Constraints
would be an improvement. Extra braces on a scalar initializer are
easy to detect,

All work is easy if it's someone else who is doing it. Certainly
it would be easier just to leave things as is.

Post by Keith Thompson
and requiring a diagnostic should not be a significant burden,

This statement assumes other people have the same reaction that
you do. Not everyone does.

Post by Keith Thompson
and would promote consistency. [...]

There is already consistency. There is no evidence to suggest
that changing the standard would make a difference at all to what
code people write. Most people are driven by what warnings or
errors they get, and compilers already give these messages.

Post by Keith Thompson
With the current requirements, a compiler could quietly generate
code to set n to 42, though I doubt that anyone would do that.

If such forms were constraint violations, compilers could give a
warning (which they already do), and then generate code to set n
to 42. In practical terms there is no difference from the
current situation.

Post by Keith Thompson
(I note that gcc issues a non-fatal warning with "-std=c17
-pedantic-errors", while clang treats it as a fatal error.)

If anyone absolutely wants a diagnostic, they can simply compile
with gcc or clang.

Note that clang has a compiler option to turn off the diagnostic
for such cases. Presumably that option is there because some
people want to use it. Making double braces a constraint violation
would interfere with what they want to do. What is suggested as an
improvement looks like all downside and no upside.

[Keith Thompson]

Post by Tim Rentsch

Post by Keith Thompson
Citations are from N1570 (C11 draft). I don't think there are any
relevant differences in other editions or drafts of the standard.
Quick summary: Why is
int n = {{0}};
undefined behavior rather than a constraint violation?

Because there is no practical benefit to changing what the
standard says now.

Do you have an opinion about whether it should have been specified as a
constraint violation rather than undefined behavior in the first place?

[...]

Post by Tim Rentsch
There's no practical benefit to changing it. The current rule
has been in place since the original ANSI C standard, more than
30 years ago. Whatever the motivation may have been at that
time, there is no incentive to change the rules now; compilers
already give diagnostics in such cases, even though they don't
have to. Changing the rules would mean throwing away a degree
of freedom with no significant benefit.

It would remove a degree of freedom for implementers, one that few if
any currently take advantage of. Meaningless degrees of freedom for
implementers do not help users.

[...]

[Tim Rentsch]

Post by Keith Thompson

Post by Tim Rentsch

Post by Keith Thompson
Citations are from N1570 (C11 draft). I don't think there are any
relevant differences in other editions or drafts of the standard.
Quick summary: Why is
int n = {{0}};
undefined behavior rather than a constraint violation?

Because there is no practical benefit to changing what the
standard says now.

Do you have an opinion about whether it should have been specified as a
constraint violation rather than undefined behavior in the first place?

I don't have any pertinent information on which to base such an
opinion. It's possible there was existing code that prompted
what decision was made at the time. It's also possible that what
was done is just the result of an oversight. I'm inclined to think
that there was some reasoning that led to what was actually done
(ie, that there is undefined behavior but no constraint violation),
but that's just a general impression, not based on any specific
facts. If there was such reasoning, then without more information
I would tend to favor supporting it.

I guess I could add that there are other constructions in C that
are undefined behavior, and are identifiable at compile time, but
which are not constraint violations, and I think that's the right
decision for at least some of them.

Post by Keith Thompson

Post by Tim Rentsch
There's no practical benefit to changing it. The current rule
has been in place since the original ANSI C standard, more than
30 years ago. Whatever the motivation may have been at that
time, there is no incentive to change the rules now; compilers
already give diagnostics in such cases, even though they don't
have to. Changing the rules would mean throwing away a degree
of freedom with no significant benefit.

It would remove a degree of freedom for implementers, one that
few if any currently take advantage of.

As I mentioned in my earlier post, the clang compiler does take
advantage of this particular degree of freedom, with an option to
turn off diagnostics corresponding to such cases.

Post by Keith Thompson
Meaningless degrees of freedom for implementers do not help
users.

I infer from the existence of the clang option that some people
think the degree of freedom here is both meaningful and useful.