c++: fix cases of core1001/1322 by not dropping cv-qualifier of function parameter of type of typename or decltype[PR101402,PR102033,PR102034,PR102039,PR102044]

These bugs are considered duplicate cases of PR51851 which has been suspended 
since 2012, an issue known as "core1001/1322". Considering this background, 
it deserves a long comment to explain.

Many people believed the root cause of this family of bugs is related with 
the nature of how and when the array type is converted to pointer type during 
function signature is calculated. This is true, but we may need to go into details 
to understand the exact reason.

There is a pattern for these bugs(PR101402,PR102033,PR102034,PR102039). In the 
template function declaration, the function parameter is consisted of a "const" 
followed by a typename-type which is actually an array type. According to 
standard, function signature is calculated by dropping so-called 
"top-level-cv-qualifier". As a result, the templater specialization complains 
no matching to declaration can be found because specialization has const and 
template function declaration doesn't have const which is dropped as mentioned.
Obviously the template function declaration should NOT drop the const. But why?
Let's review the procedure of standard first.
(https://timsong-cpp.github.io/cppwp/dcl.fct#5.sentence-3)

"After determining the type of each parameter, any parameter of type “array of T” 
or of function type T is adjusted to be “pointer to T”. After producing the list 
of parameter types, any top-level cv-qualifiers modifying a parameter type are 
deleted when forming the function type."

Please note the action of deleting top-level cv-qualifiers happens at last stage 
after array type is converted to pointer type. More importantly, there are two 
conditions:
a) Each type must be able to be determined.
b) The cv-qualifier must be top-level.
Let's analysis if these two conditions can be met one by one.
1) Keyword "typename" indicates inside template it involves dependent name
 (https://timsong-cpp.github.io/cppwp/n4659/temp.res#2) for which the name lookup 
can be postponed until template instantiation. Clearly the type of dependent 
name cannot be determined without name lookup. Then we can NOT proceed to next 
step until concrete template argument type is determined during specialization. 
2) After “array of T” is converted to “pointer to T”, the cv-qualifiers are no 
longer top-level! Unfortunately in standard there is no definition 
of "top-level". Mr. Dan Saks's articals (https://www.dansaks.com/articles.shtml) 
are tremendous help! Especially this wonderful paper (https://www.dansaks.com/articles/2000-02%20Top-Level%20cv-Qualifiers%20in%20Function%20Parameters.pdf)  
discusses this topic in details. In one short sentence, the "const" before 
array type is NOT top-level-cv-qualifier and should NOT be dropped.

So, understanding the root cause makes the fix very clear: Let's NOT drop 
cv-qualifier for typename-type inside template. Leave this task for template
substitution later when template specialization locks template argument types.

Similarly inside template, "decltype" may also include dependent name and 
the best strategy for parser is to preserve all original declaration and 
postpone the task till template substitution.

Here is an interesting observation to share. Originally my fix is trying to 
use function "resolve_typename_type" to see if the "typename-type" is indeed
an array type so as to decide whether the const should be dropped. It works 
for cases of PR101402,PR102033(with a small fix of function), but cannot 
succeed on cases of PR102034,PR102039. Especially PR102039 is impossible 
because it depends on template argument. This helps me realize that parser 
should not do any work if it cannot be 100% successful. All can wait.

At last I want to acknowledge other efforts to tackle this core 1001/1322 from 
PR92010 which is an irreplaceable different approach from this fix by doing 
rebuilding template function signature during template substitution stage. 
After all, this fix can only deal with dependent type started with "typename"
or "decltype" which is not the case of pr92010.
 

gcc/cp/ChangeLog:

2021-08-30  qingzhe huang  <nickhuang99@hotmail.com>

	* decl.c (grokparms):

gcc/testsuite/ChangeLog:

2021-08-30  qingzhe huang  <nickhuang99@hotmail.com>

	* g++.dg/parse/pr101402.C: New test.
	* g++.dg/parse/pr102033.C: New test.
	* g++.dg/parse/pr102034.C: New test.
	* g++.dg/parse/pr102039.C: New test.
	* g++.dg/parse/pr102044.C: New test.

On 8/31/21 09:55, nick huang via Gcc-patches wrote:
> These bugs are considered duplicate cases of PR51851 which has been suspended
> since 2012, an issue known as "core1001/1322". Considering this background,
> it deserves a long comment to explain.
> 
> Many people believed the root cause of this family of bugs is related with
> the nature of how and when the array type is converted to pointer type during
> function signature is calculated. This is true, but we may need to go into details
> to understand the exact reason.
> 
> There is a pattern for these bugs(PR101402,PR102033,PR102034,PR102039). In the
> template function declaration, the function parameter is consisted of a "const"
> followed by a typename-type which is actually an array type. According to
> standard, function signature is calculated by dropping so-called
> "top-level-cv-qualifier". As a result, the templater specialization complains
> no matching to declaration can be found because specialization has const and
> template function declaration doesn't have const which is dropped as mentioned.
> Obviously the template function declaration should NOT drop the const. But why?
> Let's review the procedure of standard first.
> (https://timsong-cpp.github.io/cppwp/dcl.fct#5.sentence-3)
> 
> "After determining the type of each parameter, any parameter of type “array of T”
> or of function type T is adjusted to be “pointer to T”. After producing the list
> of parameter types, any top-level cv-qualifiers modifying a parameter type are
> deleted when forming the function type."
> 
> Please note the action of deleting top-level cv-qualifiers happens at last stage
> after array type is converted to pointer type. More importantly, there are two
> conditions:
> a) Each type must be able to be determined.
> b) The cv-qualifier must be top-level.
> Let's analysis if these two conditions can be met one by one.
> 1) Keyword "typename" indicates inside template it involves dependent name
>   (https://timsong-cpp.github.io/cppwp/n4659/temp.res#2) for which the name lookup
> can be postponed until template instantiation. Clearly the type of dependent
> name cannot be determined without name lookup. Then we can NOT proceed to next
> step until concrete template argument type is determined during specialization.
> 2) After “array of T” is converted to “pointer to T”, the cv-qualifiers are no
> longer top-level! Unfortunately in standard there is no definition
> of "top-level". Mr. Dan Saks's articals (https://www.dansaks.com/articles.shtml)
> are tremendous help! Especially this wonderful paper (https://www.dansaks.com/articles/2000-02%20Top-Level%20cv-Qualifiers%20in%20Function%20Parameters.pdf)
> discusses this topic in details. In one short sentence, the "const" before
> array type is NOT top-level-cv-qualifier and should NOT be dropped.
> 
> So, understanding the root cause makes the fix very clear: Let's NOT drop
> cv-qualifier for typename-type inside template. Leave this task for template
> substitution later when template specialization locks template argument types.
> 
> Similarly inside template, "decltype" may also include dependent name and
> the best strategy for parser is to preserve all original declaration and
> postpone the task till template substitution.
> 
> Here is an interesting observation to share. Originally my fix is trying to
> use function "resolve_typename_type" to see if the "typename-type" is indeed
> an array type so as to decide whether the const should be dropped. It works
> for cases of PR101402,PR102033(with a small fix of function), but cannot
> succeed on cases of PR102034,PR102039. Especially PR102039 is impossible
> because it depends on template argument. This helps me realize that parser
> should not do any work if it cannot be 100% successful. All can wait.
> 
> At last I want to acknowledge other efforts to tackle this core 1001/1322 from
> PR92010 which is an irreplaceable different approach from this fix by doing
> rebuilding template function signature during template substitution stage.
> After all, this fix can only deal with dependent type started with "typename"
> or "decltype" which is not the case of pr92010.

Unfortunately, your patch breaks

template <class T>
struct A
{
   void f(T);
};

template <class T>
void A<T>::f(const T)
{ }

which is certainly questionable code, but is currently also accepted by 
clang and EDG compilers.

Why doesn't the PR92010 fix address these testcases as well?

> gcc/cp/ChangeLog:
> 
> 2021-08-30  qingzhe huang  <nickhuang99@hotmail.com>
> 
> 	* decl.c (grokparms):
> 
> gcc/testsuite/ChangeLog:
> 
> 2021-08-30  qingzhe huang  <nickhuang99@hotmail.com>
> 
> 	* g++.dg/parse/pr101402.C: New test.
> 	* g++.dg/parse/pr102033.C: New test.
> 	* g++.dg/parse/pr102034.C: New test.
> 	* g++.dg/parse/pr102039.C: New test.
> 	* g++.dg/parse/pr102044.C: New test.
> 
> 
> diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
> index e0c603aaab6..940c43ce707 100644
> --- a/gcc/cp/decl.c
> +++ b/gcc/cp/decl.c
> @@ -14384,7 +14384,16 @@ grokparms (tree parmlist, tree *parms)
>   
>   	  /* Top-level qualifiers on the parameters are
>   	     ignored for function types.  */
> -	  type = cp_build_qualified_type (type, 0);
> +
> +	  int type_quals = 0;
> +	  /* Inside template declaration, typename and decltype indicating
> +	     dependent name and cv-qualifier are preserved until
> +	     template instantiation.
> +	     PR101402/PR102033/PR102034/PR102039/PR102044 */
> +	  if (processing_template_decl
> +		&& (TREE_CODE (type) == TYPENAME_TYPE || TREE_CODE (type) == DECLTYPE_TYPE))
> +		  type_quals = CP_TYPE_CONST_P(type);
> +	  type = cp_build_qualified_type (type, type_quals);
>   	  if (TREE_CODE (type) == METHOD_TYPE)
>   	    {
>   	      error ("parameter %qD invalidly declared method type", decl);
> diff --git a/gcc/testsuite/g++.dg/parse/pr101402.C b/gcc/testsuite/g++.dg/parse/pr101402.C
> new file mode 100644
> index 00000000000..58d9c4f8542
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/pr101402.C
> @@ -0,0 +1,5 @@
> +template<class T> struct A {
> +     typedef T arr[3];
> +};
> +template<class T> void f(const typename A<T>::arr) { } // #1
> +template void f<int>(const A<int>::arr);
> diff --git a/gcc/testsuite/g++.dg/parse/pr102033.C b/gcc/testsuite/g++.dg/parse/pr102033.C
> new file mode 100644
> index 00000000000..0d5cc17620f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/pr102033.C
> @@ -0,0 +1,34 @@
> +/* {dg-do compile } */
> +/* {dg-options "-std=c++11" } */
> +
> +namespace test1
> +{
> +template<class TA>
> +struct A{
> +        template<class TB>
> +        using Type=TB[3];
> +};
> +template<class TA, class TB>
> +void f(const typename A<TA>::template Type<TB>){}
> +template <>
> +void f<int, char>(const typename A<int>::template Type<char>){}
> +}
> +namespace test2
> +{
> +template<class TA>
> +struct A{
> +        template<class TB>
> +        struct B{
> +                using TB_Alias=TB;
> +                template<class TC=TB_Alias>
> +                struct C{
> +                        typedef TC Arr3[3];
> +                };
> +        };
> +};
> +template<class TA, class TB>
> +void f(const typename A<TA>::template B<TB>::template C<>::Arr3){}
> +template <>
> +void f<int, char>(const typename A<int>::template B<char>::template C<>::Arr3){}
> +}
> +
> diff --git a/gcc/testsuite/g++.dg/parse/pr102034.C b/gcc/testsuite/g++.dg/parse/pr102034.C
> new file mode 100644
> index 00000000000..37fdce52912
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/pr102034.C
> @@ -0,0 +1,13 @@
> +/*{dg-do compile} */
> +template<class TA>
> +struct A{
> +        template<class TB>
> +        struct B{
> +        typedef TB Arr3[3];
> +        };
> +};
> +template<class TA, class TB>
> +void f(const typename A<TA>::template B<TB>::Arr3){}
> +template <>
> +void f<int, char>(const typename A<int>::B<char>::Arr3){}
> +
> diff --git a/gcc/testsuite/g++.dg/parse/pr102039.C b/gcc/testsuite/g++.dg/parse/pr102039.C
> new file mode 100644
> index 00000000000..25d3e77fd74
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/pr102039.C
> @@ -0,0 +1,24 @@
> +namespace test1
> +{
> +struct A{
> +        typedef int Arr3[3];
> +};
> +
> +template<class T>
> +void f(const typename T::Arr3){}
> +
> +template<>
> +void f<A>(const int[3]){}
> +}
> +
> +namespace test2
> +{
> +struct A{
> +        typedef int Arr3[3];
> +};
> +template<class T>
> +void f(const typename T::Arr3){}
> +template<>
> +void f<A>(const int*){}
> +}
> +
> diff --git a/gcc/testsuite/g++.dg/parse/pr102044.C b/gcc/testsuite/g++.dg/parse/pr102044.C
> new file mode 100644
> index 00000000000..bef6a920b47
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/pr102044.C
> @@ -0,0 +1,32 @@
> +/* {dg-do compile } */
> +/* {dg-options "-std=c++11" } */
> +namespace test1
> +{
> +template<unsigned int N, class T>
> +void f(const T[N]){}
> +
> +template<unsigned int N, class T>
> +using fPtr=decltype(f<N,T>)*;
> +
> +template<unsigned int N, class T>
> +fPtr<N,T> af[N]={&f<N,T>};
> +
> +template<unsigned int N, class T>
> +void g(const decltype(af<N,T>)){}
> +
> +template<>
> +void g<1,int>(const fPtr<1,int>[1]){}
> +}
> +
> +namespace test2
> +{
> +template <class T>
> +struct A{
> +T arr3[3];
> +};
> +template <class T>
> +void f(const decltype(A<T>::arr3)){}
> +template <>
> +void f<int>(const int[3]){}
> +}
> +
> 
>