[gcc r13-4761] c++: constantness of non-dependent NTTP argument [PR107437]

[gcc r13-4761] c++: constantness of non-dependent NTTP argument [PR107437]

[Patrick Palka]

https://gcc.gnu.org/g:982629bea416df976686467f235e09cb1a5531cc

commit r13-4761-g982629bea416df976686467f235e09cb1a5531cc
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Dec 17 11:24:44 2022 -0500

    c++: constantness of non-dependent NTTP argument [PR107437]
    
    Here we're rejecting the use of the lambda capture of 't' (of empty
    type) as a template argument ultimately because convert_nontype_argument
    checks constantness using is_constant_expression, which returns false
    for lambda captures since want_rval=false.  But in this case I believe
    an lvalue-to-rvalue conversion of the argument is implied, so we should
    be using is_rvalue_constant_expression instead (which would return true
    here).
    
    However, it doesn't seem necessary to consider constantness at all
    when deciding whether to instantiate a non-dependent argument in
    convert_nontype_argument.  So this patch gets rid of the problematic
    constantness test altogether, which incidentally also fixes the similar
    dg-ice'd testcase from PR87765.  This is in line with a similar
    change we made to finish_decltype_type in r12-7564-gec0f53a3a542e7.
    
            PR c++/107437
            PR c++/87765
    
    gcc/cp/ChangeLog:
    
            * pt.cc (convert_nontype_argument): Relax is_nondep_const_expr
            test to !inst_dep_expr_p.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/lambda-generic-107437.C: New test.
            * g++.dg/cpp1z/constexpr-lambda26.C: Remove dg-ice.

Diff:
---
 gcc/cp/pt.cc                                       |  2 +-
 gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C | 21 +++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C    |  1 -
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index bc566ab702b..2516cca590e 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7318,7 +7318,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
       && has_value_dependent_address (expr))
     /* If we want the address and it's value-dependent, don't fold.  */;
   else if (processing_template_decl
-	   && is_nondependent_constant_expression (expr))
+	   && !instantiation_dependent_expression_p (expr))
     non_dep = true;
   if (error_operand_p (expr))
     return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C
new file mode 100644
index 00000000000..f9b4e0187e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C
@@ -0,0 +1,21 @@
+// PR c++/107437
+// { dg-do compile { target c++14 } }
+
+struct integral_constant {
+  constexpr operator int() const { return 42; }
+};
+
+template<int N>
+struct A {
+  static constexpr int value = N;
+};
+
+template<class T>
+void f(T t) {
+  [=](auto) {
+    A<t> a; // { dg-bogus "constant" }
+    return a.value;
+  }(0);
+}
+
+template void f(integral_constant);
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C
index 0cdb400d21c..e66cd1dee64 100644
--- a/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C
@@ -1,7 +1,6 @@
 // PR c++/87765
 // { dg-do compile { target c++17 } }
 // { dg-additional-options "-fchecking" }
-// { dg-ice "cxx_eval_constant_expression" }
 
 template <int N>
 using foo = int;