[gcc(refs/vendors/redhat/heads/gcc-10-branch)] c++: premature requires-expression folding [PR95020]

[Jakub Jelinek]

https://gcc.gnu.org/g:0971201a1e5e1e5df7de666ea1c4b2605a51cc76

commit 0971201a1e5e1e5df7de666ea1c4b2605a51cc76
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Jun 2 08:52:21 2020 -0400

    c++: premature requires-expression folding [PR95020]
    
    In the testcase below we're prematurely folding away the
    requires-expression to 'true' after substituting in the function's
    template arguments, but before substituting in the lambda's deduced
    template arguments.
    
    This patch removes the uses_template_parms check when deciding in
    tsubst_requires_expr whether to keep around a new requires-expression.
    Regardless of whether the template arguments are dependent, there still
    might be more template parameters to later substitute in (as in the
    below testcase) and even if not, tsubst_expr doesn't perform full
    semantic processing unless !processing_template_decl, so we should still
    wait until then to fold away the requires-expression.
    
    gcc/cp/ChangeLog:
    
            PR c++/95020
            * constraint.cc (tsubst_requires_expr): Produce a new
            requires-expression when processing_template_decl, even if
            template arguments are not dependent.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/95020
            * g++.dg/cpp2a/concepts-lambda7.C: New test.
    
    (cherry picked from commit 7e52f8b1e03776575b92574252d9b6bbed9f1af4)

Diff:
---
 gcc/cp/constraint.cc                          |  4 +---
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 18190c820dd..65c047a2f62 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2173,9 +2173,7 @@ tsubst_requires_expr (tree t, tree args,
   if (reqs == error_mark_node)
     return boolean_false_node;
 
-  /* In certain cases, produce a new requires-expression.
-     Otherwise the value of the expression is true.  */
-  if (processing_template_decl && uses_template_parms (args))
+  if (processing_template_decl)
     return finish_requires_expr (cp_expr_location (t), parms, reqs);
 
   return boolean_true_node;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C
new file mode 100644
index 00000000000..50746b777a3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C
@@ -0,0 +1,14 @@
+// PR c++/95020
+// { dg-do compile { target c++2a } }
+
+template<typename>
+void foo() {
+  auto t = [](auto v) {
+    static_assert(requires { *v; }); // { dg-error "static assertion failed" }
+  };
+  t(0);
+}
+
+void bar() {
+  foo<void>();
+}