Add visitor test plugin

tests/plugins/lit.local.cfg

@@ -11,6 +11,7 @@ if (config.plugins_supported):
                              '--d-version=IN_LLVM',
                              '-J' + config.ldc2_source_dir + '/dmd/res',
                              '--shared',
+                             '--defaultlib=',
                            ]
     if (platform.system() == 'Darwin'):
         plugin_compile_flags.append('-L-Wl,-undefined,dynamic_lookup')

tests/plugins/visitor_example.d

@@ -0,0 +1,51 @@
+// REQUIRES: Plugins
+
+// RUN: split-file %s %t --leading-lines
+// RUN: %ldc -g %t/plugin.d %plugin_compile_flags -of=%t/plugin%so
+// RUN: %ldc -wi -c -o- --plugin=%t/plugin%so %t/testcase.d 2>&1 | FileCheck %t/testcase.d
+
+//--- plugin.d
+import dmd.dmodule;
+import dmd.errors;
+import dmd.location;
+import dmd.visitor;
+import dmd.declaration;
+import dmd.dsymbol;
+
+extern(C++) class MyVisitor : SemanticTimeTransitiveVisitor {
+    alias visit = SemanticTimeTransitiveVisitor.visit;
+
+    override void visit(VarDeclaration vd) {
+        if (vd.aliasTuple)
+            warning(vd.loc, "It works!");
+    }
+}
+
+extern(C) void runSemanticAnalysis(Module m) {
+    scope v = new MyVisitor();
+    if (!m.members)
+        return;
+    m.members.foreachDsymbol((s) {
+        s.accept(v);
+    });
+}
+
+//--- testcase.d
+alias AliasSeq(TList...) = TList;
+
+int i = 0;
+struct A {
+    ~this() {
+        i *= 2;
+    }
+}
+
+void main() {
+    {
+        // CHECK: testcase.d([[@LINE+1]]): Warning:
+        AliasSeq!(A, A) params;
+        i = 1;
+    }
+
+    assert(i == 4);
+}