[gcc r13-2633] c++: template-id arguments are evaluated [PR101906]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:c3ba0eaaa223f7b8208d279e3f39ff134912f9e9

commit r13-2633-gc3ba0eaaa223f7b8208d279e3f39ff134912f9e9
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Jun 7 14:19:53 2022 -0400

    c++: template-id arguments are evaluated [PR101906]
    
    Here we're neglecting to clear cp_unevaluated_operand when substituting
    into the arguments of the alias template-id 'skip<(T(), 0), T>' with T=A,
    which means cp_unevaluated_operand remains set during mark_used for
    A::A() and so we don't synthesize it.  Later constant evaluation for
    the substituted template argument '(A(), 0)' (from coerce_template_parms)
    fails with "'constexpr A::A()' used before its definition" since it was
    never synthesized.
    
    This doesn't happen with a class template because tsubst_aggr_type
    clears cp_unevaluated_operand during substitution thereof.  But since
    template arguments are generally manifestly constant-evaluated, which in
    turn are evaluated even in an unevaluated operand, we should be clearing
    cp_unevaluated_operand more broadly whenever substituting into any set
    of template arguments.  To that end this patch makes us clear it during
    tsubst_template_args.
    
            PR c++/101906
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_template_args): Set cp_evaluated here.
            (tsubst_aggr_type): Not here.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/template/evaluated1.C: New test.
            * g++.dg/template/evaluated1a.C: New test.
            * g++.dg/template/evaluated1b.C: New test.
            * g++.dg/template/evaluated1c.C: New test.

Diff:
---
 gcc/cp/pt.cc                                |  6 +++---
 gcc/testsuite/g++.dg/template/evaluated1.C  | 17 +++++++++++++++++
 gcc/testsuite/g++.dg/template/evaluated1a.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/template/evaluated1b.C | 17 +++++++++++++++++
 gcc/testsuite/g++.dg/template/evaluated1c.C | 17 +++++++++++++++++
 5 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 31e3e391098..4c6b343ab6e 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -13616,6 +13616,9 @@ tsubst_template_args (tree t, tree args, tsubst_flags_t complain, tree in_decl)
   if (t == error_mark_node)
     return error_mark_node;
 
+  /* In "sizeof(X<I>)" we need to evaluate "I".  */
+  cp_evaluated ev;
+
   const int len = TREE_VEC_LENGTH (t);
   tree *elts = XALLOCAVEC (tree, len);
   int expanded_len_adjust = 0;
@@ -13888,9 +13891,6 @@ tsubst_aggr_type (tree t,
 	  tree argvec;
 	  tree r;
 
-	  /* In "sizeof(X<I>)" we need to evaluate "I".  */
-	  cp_evaluated ev;
-
 	  /* Figure out what arguments are appropriate for the
 	     type we are trying to find.  For example, given:
 
diff --git a/gcc/testsuite/g++.dg/template/evaluated1.C b/gcc/testsuite/g++.dg/template/evaluated1.C
new file mode 100644
index 00000000000..41845c65acb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/evaluated1.C
@@ -0,0 +1,17 @@
+// PR c++/101906
+// Verify the template arguments of an alias template-id are evaluated even
+// in an unevaluated context.
+// { dg-do compile { target c++11 } }
+
+template<int, class T> using skip = T;
+
+template<class T>
+constexpr unsigned sizeof_() {
+  return sizeof(skip<(T(), 0), T>);
+}
+
+struct A {
+  int m = -1;
+};
+
+static_assert(sizeof_<A>() == sizeof(A), "");
diff --git a/gcc/testsuite/g++.dg/template/evaluated1a.C b/gcc/testsuite/g++.dg/template/evaluated1a.C
new file mode 100644
index 00000000000..78286871004
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/evaluated1a.C
@@ -0,0 +1,16 @@
+// PR c++/101906
+// Like unevaluated1.C, but where the unevaluated context is a
+// constraint instead of sizeof.
+// { dg-do compile { target c++20 } }
+
+template<int> using voidify = void;
+
+template<class T>
+concept constant_value_initializable
+  = requires { typename voidify<(T(), 0)>; };
+
+struct A {
+  int m = -1;
+};
+
+static_assert(constant_value_initializable<A>);
diff --git a/gcc/testsuite/g++.dg/template/evaluated1b.C b/gcc/testsuite/g++.dg/template/evaluated1b.C
new file mode 100644
index 00000000000..7994065ac86
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/evaluated1b.C
@@ -0,0 +1,17 @@
+// PR c++/101906
+// Like unevaluated1.C, but using a function template instead of an
+// alias template.
+// { dg-do compile { target c++14 } }
+
+template<int, class T> T skip();
+
+template<class T>
+constexpr unsigned sizeof_() {
+  return sizeof(skip<(T(), 0), T>());
+}
+
+struct A {
+  int m = -1;
+};
+
+static_assert(sizeof_<A>() == sizeof(A), "");
diff --git a/gcc/testsuite/g++.dg/template/evaluated1c.C b/gcc/testsuite/g++.dg/template/evaluated1c.C
new file mode 100644
index 00000000000..15c55821c01
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/evaluated1c.C
@@ -0,0 +1,17 @@
+// PR c++/101906
+// Like unevaluated1b.C, but using a variable template instead of a
+// function template.
+// { dg-do compile { target c++14 } }
+
+template<int, class T> T skip;
+
+template<class T>
+constexpr unsigned sizeof_() {
+  return sizeof(skip<(T(), 0), T>);
+}
+
+struct A {
+  int m = -1;
+};
+
+static_assert(sizeof_<A>() == sizeof(A), "");