c++: lambda this capture and requires [PR120123]

We shouldn't need to be within the lambda body to look through it to the
enclosing non-static member function.

This change is a small subset of r16-970.

	PR c++/120123

gcc/cp/ChangeLog:

	* lambda.cc (nonlambda_method_basetype): Look through lambdas
	even when current_class_ref is null.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda24.C: New test.

2 files changed, 14 insertions, 4 deletions

diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
index b2e0ecd..352e1b9 100644
--- a/gcc/cp/lambda.cc
+++ b/gcc/cp/lambda.cc
@@ -1033,12 +1033,9 @@ current_nonlambda_function (void)
 tree
 nonlambda_method_basetype (void)
 {
-  if (!current_class_ref)
-    return NULL_TREE;
-
   tree type = current_class_type;
   if (!type || !LAMBDA_TYPE_P (type))
-    return type;
+    return current_class_ref ? type : NULL_TREE;
 
   while (true)
     {
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda24.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda24.C
new file mode 100644
index 0000000..28f56ca
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda24.C
@@ -0,0 +1,13 @@
+// PR c++/120123
+// { dg-do compile { target c++20 } }
+
+struct H {
+    void member(int) {}
+    void call() {
+        [this]() {
+            [this](const auto& v)
+                requires requires { /*this->*/member(v); }
+            { return member(v); }(0);
+        };
+    }
+};