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CWG3221 [diff.stat] Add entry for C and C++ defining the scope of for loop identifiers differently #984
Description
Activity
That's not actually correct: [basic.scope.block] says that each iteration statement introduces a block scope, but the substatement also introduces a block scope. So there are two different scopes, but you're not allowed to declare an
iin both of them because we have a special rule prohibiting it.I don't know the exact rationale, and a vague "catch some bugs" seems unconvincing to me. I've suggested in the past that we abolish all special rules of this kind.
That's not actually correct: [basic.scope.block] says that each iteration statement introduces a block scope, but the substatement also introduces a block scope. So there are two different scopes, but you're not allowed to declare an
iin both of them because we have a special rule prohibiting it.Perhaps a better way to describe it would be:
Affected subclause: [stmt.for]
Change: Names declared inside of the init-statement of a for loop cannot be hidden by the same names declared directly in the statement of the same for loop.
[Example 1:for(int i=0;;){ int i=1; // valid C, invalid in C++ return i; }
- end example]
Rationale: <to be decided>
Effect on original feature: Change to semantics of well-defined feature.
Difficulty of converting: Syntactic transformation. A second compound statement will allow reusing the name.
How widely used: Seldom.Removing the special-case rule is probably an EWG matter.
- changed the title
[-][diff.stat] Add entry for C and C++ defining the scope of for loop identifiers differently[/-][+]CWG3221 [diff.stat] Add entry for C and C++ defining the scope of for loop identifiers differently[/+]on Aug 23, 2026 I believe the possessive form should be C++'s instead of C++', at least that is what the note in [basic.pre]/1 uses.
Full name of submitter (unless configured in github; will be published with the issue): Jay Ghiron
Reference (section label): [diff.stat]
Issue description:
See C DR 466:
In C2Y the same also applies to if and switch.
Partial suggested resolution:
Create a new paragraph at the end of [diff.stat]: