c++: lambda in DMI in class template [PR95870]

Here enclosing_instantiation_of was failing to find a match because otctx is
struct S<T> and current_function_decl is S<int>::S(), so the latter has more
function contexts, and we end up trying to compare S() to NULL_TREE.

After spending a bit of time working on establishing the correspondence in
this case (class <=> constructor), it occurred to me that we could just use
DECL_SOURCE_LOCATION, which is unique for lambdas, since they cannot be
redeclared.  Since we're so close to release, for now I'm only doing this
for the case that was failing before.

gcc/cp/ChangeLog:

	PR c++/95870
	* pt.c (enclosing_instantiation_of): Compare DECL_SOURCE_LOCATION if
	there is no enclosing non-lambda function.

gcc/testsuite/ChangeLog:

	PR c++/95870
	* g++.dg/cpp0x/lambda/lambda-nsdmi10.C: New test.

gcc/cp/pt.c

@@ -14371,6 +14371,19 @@ enclosing_instantiation_of (tree otctx)
 		  || instantiated_lambda_fn_p (tctx));
        tctx = decl_function_context (tctx))
     ++lambda_count;
+
+  if (!tctx)
+    {
+      /* Match using DECL_SOURCE_LOCATION, which is unique for all lambdas.
+
+	 For GCC 11 the above condition limits this to the previously failing
+	 case where all enclosing functions are lambdas (95870).  FIXME.  */
+      for (tree ofn = fn; ofn; ofn = decl_function_context (ofn))
+	if (DECL_SOURCE_LOCATION (ofn) == DECL_SOURCE_LOCATION (otctx))
+	  return ofn;
+      gcc_unreachable ();
+    }
+
   for (; fn; fn = decl_function_context (fn))
     {
       tree ofn = fn;

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nsdmi10.C

@@ -0,0 +1,12 @@
+// PR c++/95870
+// { dg-do compile { target c++11 } }
+
+template <typename> struct S {
+  S();
+  int b = []() -> int { enum E {}; return 1; }();
+};
+struct C : S<int> {
+  C();
+};
+template <typename T> S<T>::S() = default;
+C::C() {}