c++: generic lambda, local class, __func__ [PR108242]

Here we are trying to do name lookup in a deferred instantiation of t() and
failing to find __func__.  tsubst_expr already tries to instantiate members
of local classes, but was failing with the partial instantiation of generic
lambdas.

	PR c++/108242

gcc/cp/ChangeLog:

	* pt.cc (tsubst_expr) [TAG_DEFN]: Handle partial instantiation.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-generic-func2.C: New test.

gcc/cp/pt.cc

@@ -19341,7 +19341,10 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case TAG_DEFN:
       tmp = tsubst (TREE_TYPE (t), args, complain, NULL_TREE);
-      if (CLASS_TYPE_P (tmp))
+      if (dependent_type_p (tmp))
+	/* This is a partial instantiation, try again when full.  */
+	add_stmt (build_min (TAG_DEFN, tmp));
+      else if (CLASS_TYPE_P (tmp))
 	{
 	  /* Local classes are not independent templates; they are
 	     instantiated along with their containing function.  And this

gcc/testsuite/g++.dg/cpp1y/lambda-generic-func2.C

@@ -0,0 +1,18 @@
+// PR c++/108242
+// { dg-do compile { target c++14 } }
+
+template<int F>
+void my_fun()
+{
+    [&](auto) {
+        static constexpr char const* fun_name = __func__;
+        struct t
+        {
+            t() { fun_name; };
+        } t1;
+    }(12);
+}
+
+int main() {
+    my_fun<1>();
+}