[cfe-dev] Help with integral template argument suffix and cast printing

[David Rector via cfe-dev]

You’re not making a mistake, the template argument was not constructed with the proper type sugar, which means there’s nothing you can do to distinguish deduced from non-deduced template arguments as is.

I think the following will solve it; change line https://github.com/llvm/llvm-project/blob/master/clang/lib/Sema/SemaTemplate.cpp#L6823 <https://github.com/llvm/llvm-project/blob/master/clang/lib/Sema/SemaTemplate.cpp#L6823> to the following:

QualType CanonParamType = Context.getCanonicalType(ParamType);

// FIXME: this renders CanonParamType non-canonical, but…why do we need 
// a canonical type in the first place to construct template args?  
// Seems to just lose type sugar info prematurely.
if (Param->getType()->getAs<AutoType>())
  CanonParamType = Context.getAutoType(CanonParamType, AutoTypeKeyword::Auto,
                                       false, false);

See if that gets your thing to work (both T->getAs<AutoType>() and T->getAs<BuiltinType>() should return non-null for deduced builtins, allowing you to T->getAs<AutoType>() use distinguish deduced from non-deduced), then see if it breaks any others tests (it shouldn’t, because nothing should depend on template arguments being constructed with a canonical type at that point — I think we should be able to get rid of CanonParamType and use ParamType in its place in that function).

Good luck,

Dave


> On Aug 7, 2020, at 8:30 AM, Pratyush Das <reikdas at gmail.com> wrote:
> 
> This is where we get the Integral type from the TemplateArgument - https://github.com/llvm/llvm-project/blob/master/clang/lib/AST/TemplateBase.cpp#L55 <https://github.com/llvm/llvm-project/blob/master/clang/lib/AST/TemplateBase.cpp#L55>. 
> 
> I tried - 
> bool flag = false;
> if (auto *autoT = T->getAs<AutoType>()) {
>  flag = true;
> }
> 
> But flag is not true for the example - 
> template<auto N> struct S {};
> template<> struct S<1> { using type = int };
> S<1L>::type t;
> 
> Am I making a mistake in getting the AutoType?
> 
> On Wed, 22 Jul 2020 at 20:23, David Rector <davrecthreads at gmail.com <mailto:davrecthreads at gmail.com>> wrote:
> The issue may be that the proper sugar isn’t being stored in the integral type when the TemplateArgument is created, so that there is no way to distinguish a non-deduced BuiltInType from a deduced one.
> 
> The type of the NonTypeTemplateParmDecl N in 
> 
> template<auto N> struct S {}` is an AutoType — so far so good.
> 
> But the type of the integral TemplateArgument '1' in S<1>, though, seems to be a BuiltInType — no sugar atop it, nothing to distinguish it from the situation where N had a BuiltInType instead of an AutoType.  
> 
> If I understand DeducedTypes correctly, when they are substituted, they should remain as sugar atop the substitution (someone correct me if I’m wrong), and that does not seem to happen here.
> 
> If others agree this is the issue, I would imagine you will have to dig around to figure out where the template argument is being created, and wrap the integral’s type in an AutoType.  Then testing if getAs<DeducedType>() before testing getAs<BuiltInType>() should tell you when your BuiltInType was deduced.  
> 
> That’s my last best guess anyway.  Good luck,
> 
> Dave
> 
>> On Jul 22, 2020, at 3:53 AM, Pratyush Das <reikdas at gmail.com <mailto:reikdas at gmail.com>> wrote:
>> 
>> That change does not work :( 
>> 
>> Thanks though! 
>> 
>> On Tue, 21 Jul 2020 at 23:30, David Rector <davrecthreads at gmail.com <mailto:davrecthreads at gmail.com>> wrote:
>> 
>> I would try replacing
>> 
>>  else if (auto *DT = T->getContainedDeducedType())
>> 
>> with
>> 
>>   else if (auto *DT = T->getAs<DeducedType>())
>> 
>> I think that is what you want anyway, since getContainedDeducedType seems to look not only through type sugar but through pointee types, array element types, etc.
>> 
>> To be sure, your example still should have worked using getContainedDeducedType but I think GetContainedDeducedTypeVisitor may have a problem in its implementation: it doesn’t handle all the possible sugar types.   In this case, you probably have a SubstTemplateTypeParmType, and I don’t see a VisitSubstTemplateTypeParmType implementation in there anywhere, so it is probably returning nullptr instead of desugaring and continuing to search.  That’s my best guess anyway from my perusal.
>> 
>> If this change works, it is probably another reason to replace stuff like GetContainedDeducedTypeVisitor with a more advanced getAs<T>(), with an extra template param that would allow you to look through e.g. pointee types, element types, function return types etc. when desired.
>> 
>> If that doesn’t work though, disregard.  Good luck,
>> 
>> Dave
>> 
>>> On Jul 21, 2020, at 9:06 AM, Pratyush Das via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>> 
>>> Hi, 
>>> 
>>> I am a little stuck with https://reviews.llvm.org/D77598 <https://reviews.llvm.org/D77598> and would really appreciate any help.
>>> 
>>> I am trying to get the proper suffixes in the error messages for the following two examples - 
>>> 
>>> template<auto N> struct S {};
>>> template<> struct S<1> { using type = int };
>>> S<1L>::type t;
>>> 
>>> which should give the error message with a suffix - 
>>> error: no type named 'type' in 'S<1L>'; did you mean 'S<1>::type'?
>>> 
>>> and
>>> 
>>> template <unsigned long long N> struct enable_if_unsigned_long_long {};
>>> template <> struct enable_if_unsigned_long_long<1> { typedef int type; }; 
>>> void test_unsigned_long_long() { enable_if_unsigned_long_long<2>::type i; } 
>>> 
>>> which should give no suffix in the error message - 
>>> error: no type named 'type' in 'enable_if_unsigned_int<2>'; did you mean 'enable_if_unsigned_int<1>::type'?
>>> 
>>> I am trying to find the correct combination that would enable suffix in the first case, but not in the second case. I have been tinkering with checks for DeducedType and DependentType, but always either the suffixes are disabled for both the cases, or enabled for both the cases. 
>>> 
>>> Can anyone please provide any insight on how to proceed?
>>> 
>>> Thanks!
>>> 
>>> -- 
>>> Pratyush Das(Reik)
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev <https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>> 
>> 
>> 
>> -- 
>> Pratyush Das(Reik)
> 
> 
> 
> -- 
> Pratyush Das(Reik)

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