[PATCH] c++: normalization of non-templated return-type-req [PR100946]

Patrick Palka ppalka@redhat.com
Thu Jun 10 20:17:24 GMT 2021


On Thu, 10 Jun 2021, Jason Merrill wrote:

> On 6/9/21 3:34 PM, Patrick Palka wrote:
> > Here the satisfaction cache is conflating the satisfaction value of the
> > two return-type-requirements because the corresponding constrained
> > 'auto's have level 2, but they capture an empty current_template_parms.
> > This ultimately causes the satisfaction cache to think the type
> > constraint doesn't depend on the deduced type of the expression.
> > 
> > When normalizing the constraints on an 'auto', the assumption made by
> > normalize_placeholder_type_constraints is that the level of the 'auto'
> > is one greater than the captured current_template_parms, an assumption
> > which is not holding here.  To fix this, this patch adds a dummy level
> > to current_template_parms to match processing_template_decl when parsing
> > a non-templated return-type-requirement.  This patch also makes us
> > verify this assumption upon creation of a constrained 'auto'.
> > 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk/11?
> > 
> > 	PR c++/100946
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* parser.c (cp_parser_compound_requirement): When parsing a
> > 	non-templated return-type-requirement, add a dummy level
> > 	to current_template_parms.
> > 	* pt.c (make_constrained_placeholder_type): Verify the depth
> > 	of current_template_parms is consistent with the level of
> > 	the 'auto'.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp2a/concepts-return-req3.C: New test.
> > ---
> >   gcc/cp/parser.c                                   | 12 ++++++++++++
> >   gcc/cp/pt.c                                       |  8 ++++++++
> >   gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C |  6 ++++++
> >   3 files changed, 26 insertions(+)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C
> > 
> > diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> > index d59a829d0b9..8278a5608ae 100644
> > --- a/gcc/cp/parser.c
> > +++ b/gcc/cp/parser.c
> > @@ -29181,6 +29181,18 @@ cp_parser_compound_requirement (cp_parser *parser)
> >         cp_lexer_consume_token (parser->lexer);
> >         cp_token *tok = cp_lexer_peek_token (parser->lexer);
> >   +      auto ctp = make_temp_override (current_template_parms);
> > +      if (!current_template_parms)
> > +	{
> > +	  /* We're parsing a return-type-requirement within a non-templated
> > +	     requires-expression.  Update current_template_parms to agree with
> > +	     processing_template_decl so that the normalization context that's
> > +	     captured by the corresponding constrained 'auto' is sensible.  */
> > +	  gcc_checking_assert (processing_template_decl == 1);
> > +	  current_template_parms
> > +	    = build_tree_list (size_int (1), make_tree_vec (0));
> > +	}
> 
> How about handling this in normalization, rather than in the parser?

That works nicely too, how does the below look?  Tested
x86_64-pc-linux-gnu.

> 
> If you set current_template_parms here, when is it cleared?

The 'ctp' temp_override would take care of that.

-- >8 --

Subject: [PATCH] c++: normalization of non-templated return-type-req
 [PR100946]

Here the satisfaction cache is conflating the satisfaction value of the
two return-type-requirements because the corresponding constrained
'auto's have level 2, but they capture an empty current_template_parms.
This ultimately causes the satisfaction cache to think the type
constraint doesn't depend on the deduced type of the expression.

When normalizing the constraints on an 'auto', the assumption made by
normalize_placeholder_type_constraints is that the level of the 'auto'
is one greater than the depth of the captured current_template_parms, an
assumption which is not holding here.  This patch relaxes makes
normalize_placeholder_type_constraints adjust the normalization context
appropriately in this situation.

	PR c++/100946

gcc/cp/ChangeLog:

	* constraint.cc (normalize_placeholder_type_constraints): When
	normalizing a non-templated return-type-requirement, add a dummy
	level to initial_parms.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-return-req3.C: New test.
---
 gcc/cp/constraint.cc                              | 9 +++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C | 6 ++++++
 2 files changed, 15 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 03ce8eb9ff2..74b16d27101 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3065,6 +3065,15 @@ normalize_placeholder_type_constraints (tree t, bool diag)
      scope for this placeholder type; use them as the initial template
      parameters for normalization.  */
   tree initial_parms = TREE_PURPOSE (ci);
+
+  if (!initial_parms && TEMPLATE_TYPE_LEVEL (t) == 2)
+    /* This is a return-type-requirement of a non-templated requires-expression,
+       which are parsed under processing_template_decl == 1 and empty
+       current_template_parms; hence the 'auto' has level 2 and initial_parms
+       is empty.  Fix up initial_parms to be consistent with the value of
+       processing_template_decl whence the 'auto' was created.  */
+    initial_parms = build_tree_list (size_int (1), make_tree_vec (0));
+
   /* The 'auto' itself is used as the first argument in its own constraints,
      and its level is one greater than its template depth.  So in order to
      capture all used template parameters, we need to add an extra level of
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C
new file mode 100644
index 00000000000..a546c6457be
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C
@@ -0,0 +1,6 @@
+// PR c++/100946
+// { dg-do compile { target c++20 } }
+
+template<class T> concept C = __is_same(T, int);
+static_assert(requires { { 0 } -> C; });
+static_assert(requires { { true } -> C; }); // { dg-error "failed" }
-- 
2.32.0.rc2



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