[PATCH] c++: Lambda in friend of constrained class [PR94645]

[Patrick Palka]

In the testcase below, when grokfndecl processes the operator() decl for the
lambda inside the friend function foo, processing_template_decl is rightly 1,
but template_class_depth on the lambda's closure type incorrectly returns 0
instead of 1.

Since processing_template_decl > template_class_depth, this makes grokfndecl
think that the operator() has its own set of template arguments, and so we
attach the innermost set of constraints -- those belonging to struct l -- to the
operator() decl.  We then get confused when checking constraints_satisfied_p on
the operator() because it doesn't have template information and yet has
constraints associated with it.

This patch fixes template_class_depth to return the correct template nesting
level in cases like these, so that when it hits a friend function it walks into
the DECL_FRIEND_CONTEXT of the friend rather than into the CP_DECL_CONTEXT.

Bootstrapped and regtested on x86_64-pc-linux-gnu, and also tested on cmcstl2,
range-v3, and fmt libraries.

gcc/cp/ChangeLog:

	PR c++/94645
	* pt.c (template_class_depth): Walk into the DECL_FRIEND_CONTEXT of a
	friend declaration rather than into its CP_DECL_CONTEXT.

gcc/testsuite/ChangeLog:

	PR c++/94645
	* g++.dg/cpp2a/concepts-lambda6.C: New test.
---
 gcc/cp/pt.c                                   |  7 ++++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda6.C | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda6.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 0e3e7b2039a..08a35c41007 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -390,7 +390,12 @@ template_class_depth (tree type)
 	++depth;
 
       if (DECL_P (type))
-	type = CP_DECL_CONTEXT (type);
+	{
+	  if (tree fctx = DECL_FRIEND_CONTEXT (type))
+	    type = fctx;
+	  else
+	    type = CP_DECL_CONTEXT (type);
+	}
       else if (LAMBDA_TYPE_P (type) && LAMBDA_TYPE_EXTRA_SCOPE (type))
 	type = LAMBDA_TYPE_EXTRA_SCOPE (type);
       else
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda6.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda6.C
new file mode 100644
index 00000000000..244421e64ec
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda6.C
@@ -0,0 +1,19 @@
+// PR c++/94632
+// { dg-do compile { target concepts } }
+
+struct unordered_map {
+  int cend() const noexcept;
+};
+
+template <typename a> concept HasMapInterface = requires(a t) { t.cend(); };
+
+template <typename Mapper> requires HasMapInterface<decltype(Mapper::map())>
+struct l {
+  friend void foo(l opt) { ([]() {})(); }
+};
+
+struct p {
+  static unordered_map map();
+};
+
+void g(l<p> *y) { foo(*y); }
-- 
2.26.2.266.ge870325ee8