[gcc r11-6842] c++: Fix excessive instantiation inside decltype [PR71879]

[gcc r11-6842] c++: Fix excessive instantiation inside decltype [PR71879]

[Patrick Palka]

https://gcc.gnu.org/g:f645da0e4ab9438dfd0c047c710c7ec6a7d6d8f3

commit r11-6842-gf645da0e4ab9438dfd0c047c710c7ec6a7d6d8f3
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jan 21 14:04:55 2021 -0500

    c++: Fix excessive instantiation inside decltype [PR71879]
    
    Here after resolving the address of a template-id inside decltype, we
    end up instantiating the chosen specialization (from the call to
    mark_used in resolve_nondeduced_context), even though only its type is
    needed.
    
    This patch sets cp_unevaluated_operand throughout finish_decltype_type,
    so that in particular it's set during the call to
    resolve_nondeduced_context within.
    
    gcc/cp/ChangeLog:
    
            PR c++/71879
            * semantics.c (finish_decltype_type): Set up a cp_unevaluated
            sentinel at the start of the function.  Remove a now-redundant
            manual adjustment of cp_unevaluated_operand.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/71879
            * g++.dg/cpp0x/decltype-71879.C: New test.

Diff:
---
 gcc/cp/semantics.c                          | 5 +++--
 gcc/testsuite/g++.dg/cpp0x/decltype-71879.C | 5 +++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 244fc70d02d..067095276af 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -10080,6 +10080,9 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
       return error_mark_node;
     }
 
+  /* decltype is an unevaluated context.  */
+  cp_unevaluated u;
+
   /* Depending on the resolution of DR 1172, we may later need to distinguish
      instantiation-dependent but not type-dependent expressions so that, say,
      A<decltype(sizeof(T))>::U doesn't require 'typename'.  */
@@ -10095,9 +10098,7 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
     }
   else if (processing_template_decl)
     {
-      ++cp_unevaluated_operand;
       expr = instantiate_non_dependent_expr_sfinae (expr, complain);
-      --cp_unevaluated_operand;
       if (expr == error_mark_node)
 	return error_mark_node;
     }
diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C b/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
new file mode 100644
index 00000000000..9da4d40ca70
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
@@ -0,0 +1,5 @@
+// PR c++/71879
+// { dg-do compile { target c++11 } }
+
+template <class T> void f(T x) { x.fail(); }
+using R = decltype(&f<int>);