Re: [PATCH] c++: Suppress error when cv-qualified reference is introduced by typedef [PR101783]

[Jason Merrill <jason@redhat.com>]

On 8/28/21 07:54, nick huang via Gcc-patches wrote:
> Reference with cv-qualifiers should be ignored instead of causing an error
> because standard accepts cv-qualified references introduced by typedef which
> is ignored.
> Therefore, the fix prevents GCC from reporting error by not setting variable
> "bad_quals" in case the reference is introduced by typedef. Still the
> cv-qualifier is silently ignored.
> Here I quote spec (https://timsong-cpp.github.io/cppwp/dcl.ref#1):
> "Cv-qualified references are ill-formed except when the cv-qualifiers
> are introduced through the use of a typedef-name ([dcl.typedef],
> [temp.param]) or decltype-specifier ([dcl.type.decltype]),
> in which case the cv-qualifiers are ignored."
> 
> PR c++/101783
> 
> gcc/cp/ChangeLog:
> 
> 2021-08-27  qingzhe huang  <nickhuang99@hotmail.com>
> 
> 	* tree.c (cp_build_qualified_type_real):

The git commit verifier rejects this commit message with

Checking 1fa0fbcdd15adf936ab4fae584f841beb35da1bb: FAILED ERR: missing 
description of a change:
"	* tree.c (cp_build_qualified_type_real):"

(your initial patch had a description here, you just need to copy it over)

ERR: PR 101783 in subject but not in changelog:
"c++: Suppress error when cv-qualified reference is introduced by 
typedef [PR101783]"

(the PR number needs to have a Tab before it)

In Jonathan's earlier reply he asked how you tested the patch; this 
message still doesn't say anything about that.

https://gcc.gnu.org/contribute.html#testing

What is the legal status of your contributions?

https://gcc.gnu.org/contribute.html#legal

Existing code tries to handle this with the tf_ignore_bad_quals, but the 
unnecessary use of typename gets past the code that tries to set the 
flag.  But your approach is nice and straightforward, so let's go ahead 
with it.

> gcc/testsuite/ChangeLog:
> 
> 2021-08-27  qingzhe huang  <nickhuang99@hotmail.com>
> 
> 	* g++.dg/parse/pr101783.C: New test.
> 
> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
> index 8840932dba2..7aa4318a574 100644
> --- a/gcc/cp/tree.c
> +++ b/gcc/cp/tree.c
> @@ -1356,12 +1356,22 @@ cp_build_qualified_type_real (tree type,
>     /* A reference or method type shall not be cv-qualified.
>        [dcl.ref], [dcl.fct].  This used to be an error, but as of DR 295
>        (in CD1) we always ignore extra cv-quals on functions.  */
> +
> +  /* PR 101783

Let's cite where this comes from in the standard ([dcl.ref]/1), and not 
the PR number.

> +     Cv-qualified references are ill-formed except when the cv-qualifiers
> +     are introduced through the use of a typedef-name ([dcl.typedef],
> +     [temp.param]) or decltype-specifier ([dcl.type.decltype]),
> +     in which case the cv-qualifiers are ignored.
> +   */
>     if (type_quals & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE)
>         && (TYPE_REF_P (type)
>   	  || FUNC_OR_METHOD_TYPE_P (type)))
>       {
> -      if (TYPE_REF_P (type))
> +      // do NOT set bad_quals when non-method reference is introduced by typedef.
> +      if (TYPE_REF_P (type)
> +	  && (!typedef_variant_p (type) || FUNC_OR_METHOD_TYPE_P (type)))
>   	bad_quals |= type_quals & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE);
> +      // non-method reference introduced by typedef is also dropped silently

These two // comments seem redundant with the quote from the standard 
above, let's drop them.

>         type_quals &= ~(TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE);
>       }
>   
> diff --git a/gcc/testsuite/g++.dg/parse/pr101783.C b/gcc/testsuite/g++.dg/parse/pr101783.C
> new file mode 100644
> index 00000000000..4e0a435dd0b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/pr101783.C
> @@ -0,0 +1,5 @@
> +template<class T> struct A{
> +        typedef T& Type;
> +};
> +template<class T> void f(const typename A<T>::Type){}
> +template <> void f<int>(const typename A<int>::Type){}
> 
> 
>