[temp.param] Delete outdated wording at p15 #5204

ChuanqiXu9 · 2022-01-14T02:24:22Z

The wording at [temp.param]/15 says:

A template-parameter shall not be given default arguments by two
different declarations if one is reachable from the other.

But it is conflicted with [basic.def.odr]/13:

There can be more than one definition of a
...
default template argument
...
in a program provided that each definition appears in a different
translation unit and the definitions satisfy the [same-meaning
criteria of the ODR].

[temp.param] should be deleted otherwise we couldn't modularize a real
project.

Here is an reduced example:

// templ.h
template<class T = int> class X{}
// foo.cppm
module;
#include "templ.h"
export module foo;
// decls to make 'X' not discard.
//bar.cpp
import foo;
#include "templ.h"

The example above here should be able to be accepted.

The wording at [temp.param]/15 says: > A template-parameter shall not be given default arguments by two > different declarations if one is reachable from the other. But it is conflicted with [basic.def.odr]/13: > There can be more than one definition of a > ... > default template argument > ... > in a program provided that each definition appears in a different > translation unit and the definitions satisfy the [same-meaning > criteria of the ODR]. [temp.param] should be deleted otherwise we couldn't modularize a real project.

tkoeppe · 2022-01-14T21:33:09Z

I'd welcome some core feedback on whether this is a) correct and b) really editorial.

zygoloid · 2022-01-14T22:54:42Z

P1811R0 says this:

Note that this change should apply to all constructs that are permitted to be repeated across translation units but not within a single translation unit, including redeclarations of default arguments and default template arguments in addition to redefinitions of classes, functions, variables, and enumerations.

... which is clear about the intent. The wording there interacts with P1766R1 which made default template arguments be under the control of the ODR, but seemingly failed to strike the (essentially redundant but slightly wrong after P1766R1, and entirely wrong after P1811R0) text in [temp.param]/15 that is the subject of this issue.

Given the history here, and that those two papers both clearly intended to override this pre-existing wording, this seems OK to handle editorially to me.

jensmaurer

Agreed, let's fix this editorially.

tkoeppe · 2022-01-15T00:22:25Z

OK, thanks everyone, in particular @ChuanqiXu9 for raising this!

…gument across modules See cplusplus/draft#5204 for a detailed background. Simply, the test redundant-template-default-arg.cpp attached to this patch should be accepted instead of being complained about the redefinition. Reviewed By: urnathan, rsmith, ChuanqiXu Differential Revision: https://reviews.llvm.org/D118034

jensmaurer approved these changes Jan 14, 2022

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jensmaurer assigned tkoeppe Jan 14, 2022

tkoeppe merged commit b345505 into cplusplus:main Jan 15, 2022

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[temp.param] Delete outdated wording at p15 #5204

[temp.param] Delete outdated wording at p15 #5204

ChuanqiXu9 commented Jan 14, 2022

tkoeppe commented Jan 14, 2022

zygoloid commented Jan 14, 2022 •

edited

jensmaurer left a comment

tkoeppe commented Jan 15, 2022

[temp.param] Delete outdated wording at p15 #5204

[temp.param] Delete outdated wording at p15 #5204

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ChuanqiXu9 commented Jan 14, 2022

tkoeppe commented Jan 14, 2022

zygoloid commented Jan 14, 2022 • edited

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tkoeppe commented Jan 15, 2022

zygoloid commented Jan 14, 2022 •

edited