[PATCH] c++: premature requires-expression folding [PR95020]

[Patrick Palka]

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 happens because during the first tsubst_requires_expr,
processing_template_decl is 1 but 'args' is just {void} and therefore
non-dependent, so we end up folding away the requires-expression to
boolean_true_node before we could substitute in the lambda's template
arguments and determine that '*v' is ill-formed.

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
testcase below -- and even if not, tsubst_expr doesn't perform full
semantic processing unless !processing_template_decl, so it seems we
should wait until then to fold away the requires-expression.

Passes 'make check-c++', does this look OK to commit after a full
bootstrap/regtest?

gcc/cp/ChangeLog:

	PR c++/95020
	* constraint.c (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++/cpp2a/concepts-lambda7.C: New test.
---
 gcc/cp/constraint.cc                          |  4 +---
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 4ad17f3b7d8..8ee347cae60 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>();
+}
-- 
2.26.2.561.g07d8ea56f2