Re: [PATCH] c++: Fix ICE in tsubst_default_argument [PR92010]

[Jason Merrill <jason@redhat.com>]

On 3/30/20 3:58 PM, Patrick Palka wrote:
> On Thu, 26 Mar 2020, Jason Merrill wrote:
> 
>> On 3/22/20 9:21 PM, Patrick Palka wrote:
>>> This patch relaxes an assertion in tsubst_default_argument that exposes a
>>> latent
>>> bug in how we substitute an array type into a cv-qualified wildcard function
>>> parameter type.  Concretely, the latent bug is that given the function
>>> template
>>>
>>>     template<typename T> void foo(const T t);
>>>
>>> one would expect the type of foo<int[]> to be void(const int*), but we
>>> (seemingly prematurely) strip function parameter types of their top-level
>>> cv-qualifiers when building the function's TYPE_ARG_TYPES, and instead end
>>> up
>>> obtaining void(int*) as the type of foo<int[]> after substitution and
>>> decaying.
>>>
>>> We still however correctly substitute into and decay the formal parameter
>>> type,
>>> obtaining const int* as the type of t after substitution.  But this then
>>> leads
>>> to us tripping over the assert in tsubst_default_argument that verifies the
>>> formal parameter type and the function type are consistent.
>>>
>>> Assuming it's too late at this stage to fix the substitution bug, we can
>>> still
>>> relax the assertion like so.  Tested on x86_64-pc-linux-gnu, does this look
>>> OK?
>>
>> This is core issues 1001/1322, which have not been resolved.  Clang does the
>> substitution the way you suggest; EDG rejects the testcase because the two
>> substitutions produce different results.  I think it would make sense to
>> follow the EDG behavior until this issue is actually resolved.
> 
> Here is what I have so far towards that end.  When substituting into the
> PARM_DECLs of a function decl, we now additionally check if the
> aforementioned Core issues are relevant and issue a (fatal) diagnostic
> if so.  This patch checks this in tsubst_decl <case PARM_DECL> rather
> than in tsubst_function_decl for efficiency reasons, so that we don't
> have to perform another traversal over the DECL_ARGUMENTS /
> TYPE_ARG_TYPES just to implement this check.

Hmm, this seems like writing more complicated code for a very marginal 
optimization; how many function templates have so many parameters that 
walking over them once to compare types will have any effect on compile 
time?

> Is something like this what you have in mind?
> 
> -- >8 --
> 
> Subject: [PATCH] c++: Reject some instantiations that depend on resolution of
>   Core issues 1001/1322
> 
> gcc/cp/ChangeLog:
> 
> 	Core issues 1001 and 1322
> 	PR c++/92010
> 	* pt.c (tsubst_decl) <case PARM_DECL>: Detect and reject the case where
> 	the function type depends on whether we strip top-level qualifiers from
> 	the type of T before or after substitution.
> 
> gcc/testsuite/ChangeLog:
> 
> 	Core issues 1001 and 1322
> 	PR c++/92010
> 	* g++.dg/template/array33.C: New test.
> 	* g++.dg/template/array34.C: New test.
> ---
>   gcc/cp/pt.c                             | 70 ++++++++++++++++++++++++-
>   gcc/testsuite/g++.dg/template/array33.C | 39 ++++++++++++++
>   gcc/testsuite/g++.dg/template/array34.C | 63 ++++++++++++++++++++++
>   3 files changed, 171 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/template/array33.C
>   create mode 100644 gcc/testsuite/g++.dg/template/array34.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 8564eb11df4..fd99053df36 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -14072,9 +14072,77 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
>                 /* We're dealing with a normal parameter.  */
>                 type = tsubst (TREE_TYPE (t), args, complain, in_decl);
>   
> +	    const int type_quals = cp_type_quals (type);
> +
> +	    /* Determine whether the function type after substitution into the
> +	       type of the function parameter T depends on the resolution of
> +	       Core issues 1001/1322, which have not been resolved.  In
> +	       particular, the following detects the case where the resulting
> +	       function type depends on whether we strip top-level qualifiers
> +	       from the type of T before or after substitution.
> +
> +	       This can happen only when the dependent type of T is a
> +	       cv-qualified wildcard type, and substitution yields a
> +	       (cv-qualified) array type before array-to-pointer conversion.  */
> +	    tree unqual_expanded_types = NULL_TREE;
> +	    if (TREE_CODE (type) == ARRAY_TYPE
> +		&& (type_quals & (TYPE_QUAL_CONST|TYPE_QUAL_VOLATILE))
> +		&& DECL_FUNCTION_SCOPE_P (t)
> +		&& !DECL_TEMPLATE_PARM_P (t))
> +	      {
> +		tree type_pattern = (PACK_EXPANSION_P (TREE_TYPE (t))
> +				     ? PACK_EXPANSION_PATTERN (TREE_TYPE (t))
> +				     : TREE_TYPE (t));
> +		if (WILDCARD_TYPE_P (type_pattern)
> +		    && cv_qualified_p (type_pattern))
> +		  {
> +		    /* Substitute into the corresponding wildcard type that is
> +		       stripped of its own top-level cv-qualifiers.  */
> +		    tree unqual_type;
> +		    if (PACK_EXPANSION_P (TREE_TYPE (t)))
> +		      {
> +			if (unqual_expanded_types == NULL_TREE)
> +			  {
> +			    tree unqual_pack_expansion
> +			      = copy_node (TREE_TYPE (t));
> +			    PACK_EXPANSION_PATTERN (unqual_pack_expansion)
> +			      = cv_unqualified (type_pattern);
> +			    unqual_expanded_types
> +			      = tsubst_pack_expansion (unqual_pack_expansion,
> +						       args, tf_none, in_decl);
> +			  }
> +			unqual_type = TREE_VEC_ELT (unqual_expanded_types, i);
> +		      }
> +		    else
> +		      {
> +			tree unqual_type_pattern = cv_unqualified (type_pattern);
> +			unqual_type = tsubst (unqual_type_pattern, args,
> +					      tf_none, in_decl);
> +		      }
> +		    /* Check if the top-level cv-qualifiers on the wildcard type
> +		       make a difference in the resulting type.  */
> +		    int unqual_type_quals = cp_type_quals (unqual_type);
> +		    if (type_quals & ~unqual_type_quals
> +			& (TYPE_QUAL_CONST|TYPE_QUAL_VOLATILE))
> +		      {
> +			sorry ("the type of function %qD after substitution "
> +			       "depends on the resolution of Core issues "
> +			       "1001 and 1332",
> +			       DECL_NAME (DECL_CONTEXT (t)));
> +			if (PACK_EXPANSION_P (TREE_TYPE (t)))
> +			  inform (input_location, "when substituting into the "
> +				  "function parameter pack %q#D", t);
> +			else
> +			  inform (input_location, "when substituting into the "
> +				  "function parameter %q#D", t);
> +			type = error_mark_node;
> +		      }
> +		  }
> +	      }
> +
>               type = type_decays_to (type);
>               TREE_TYPE (r) = type;
> -            cp_apply_type_quals_to_decl (cp_type_quals (type), r);
> +	    cp_apply_type_quals_to_decl (type_quals, r);
>   
>               if (DECL_INITIAL (r))
>                 {
> diff --git a/gcc/testsuite/g++.dg/template/array33.C b/gcc/testsuite/g++.dg/template/array33.C
> new file mode 100644
> index 00000000000..20b005bc865
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/array33.C
> @@ -0,0 +1,39 @@
> +// Reject some instantiations whose result depend on the resolution of //
> +// Core issues 1001 and 1332, which are not yet resolved.
> +// { dg-do compile }
> +// { dg-additional-options "-Wno-volatile" }
> +
> +template<typename T>
> +void foo0(T t = 0);			// { dg-bogus "" }
> +
> +template<typename T>
> +void foo1(const T = 0);			// { dg-message "sorry, unimplemented" }
> +
> +template<typename T>
> +void foo2(volatile T t = 0);		// { dg-message "sorry, unimplemented" }
> +
> +template<typename T>
> +void foo3(const volatile T t = 0);	// { dg-message "sorry, unimplemented" }
> +
> +int main()
> +{
> +  foo0<char[]>();			// { dg-bogus "" }
> +  foo0<const char[]>();			// { dg-bogus "" }
> +  foo0<volatile char[]>();		// { dg-bogus "" }
> +  foo0<const volatile char[]>();	// { dg-bogus "" }
> +
> +  foo1<char[]>();			// { dg-message "required from here" }
> +  foo1<const char[]>();			// { dg-bogus "" }
> +  foo1<volatile char[]>();		// { dg-message "required from here" }
> +  foo1<const volatile char[]>();	// { dg-bogus "" }
> +
> +  foo2<char[]>();			// { dg-message "required from here" }
> +  foo2<const char[]>();			// { dg-message "required from here" }
> +  foo2<volatile char[]>();		// { dg-bogus "" }
> +  foo2<const volatile char[]>();	// { dg-bogus "" }
> +
> +  foo3<char[]>();			// { dg-message "required from here" }
> +  foo3<const char[]>();			// { dg-message "required from here" }
> +  foo3<volatile char[]>();		// { dg-message "required from here" }
> +  foo3<const volatile char[]>();	// { dg-bogus "" }
> +}
> diff --git a/gcc/testsuite/g++.dg/template/array34.C b/gcc/testsuite/g++.dg/template/array34.C
> new file mode 100644
> index 00000000000..316ea2f6407
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/array34.C
> @@ -0,0 +1,63 @@
> +// Reject some instantiations whose result depend on the resolution of
> +// Core issues 1001 and 1332, which are not yet resolved.
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-Wno-volatile" }
> +
> +template<typename... Ts>
> +void foo0(Ts... t);			// { dg-bogus "" }
> +
> +template<typename... Ts>
> +void foo1(const Ts... t);		// { dg-message "sorry, unimplemented" }
> +
> +template<typename... Ts>
> +void foo2(volatile Ts... t);		// { dg-message "sorry, unimplemented" }
> +
> +template<typename... Ts>
> +void foo3(const volatile Ts... t);	// { dg-message "sorry, unimplemented" }
> +
> +template<typename... Ts>
> +void bar0()
> +{
> +  foo0<int, Ts..., int>(0, 0, 0);
> +}
> +
> +template<typename... Ts>
> +void bar1()
> +{
> +  foo1<int, Ts..., int>(0, 0, 0);
> +}
> +
> +template<typename... Ts>
> +void bar2()
> +{
> +  foo2<int, Ts..., int>(0, 0, 0);
> +}
> +
> +template<typename... Ts>
> +void bar3()
> +{
> +  foo3<int, Ts..., int>(0, 0, 0);
> +}
> +
> +int main()
> +{
> +  bar0<char[]>();			// { dg-bogus "" }
> +  bar0<const char[]>();			// { dg-bogus "" }
> +  bar0<volatile char[]>();		// { dg-bogus "" }
> +  bar0<const volatile char[]>();	// { dg-bogus "" }
> +
> +  bar1<char[]>();			// { dg-message "required from here" }
> +  bar1<const char[]>();			// { dg-bogus "" }
> +  bar1<volatile char[]>();		// { dg-message "required from here" }
> +  bar1<const volatile char[]>();	// { dg-bogus "" }
> +
> +  bar2<char[]>();			// { dg-message "required from here" }
> +  bar2<const char[]>();			// { dg-message "required from here" }
> +  bar2<volatile char[]>();		// { dg-bogus "" }
> +  bar2<const volatile char[]>();	// { dg-bogus "" }
> +
> +  bar3<char[]>();			// { dg-message "required from here" }
> +  bar3<const char[]>();			// { dg-message "required from here" }
> +  bar3<volatile char[]>();		// { dg-message "required from here" }
> +  bar3<const volatile char[]>();	// { dg-bogus "" }
> +}
>