Add support for simple return with only a BeforeTemplate

core/src/main/java/com/google/errorprone/refaster/RefasterScanner.java

@@ -101,7 +101,7 @@ public Void visitVariable(VariableTree node, Context context) {
 
   @Override
   public Void scan(Tree tree, Context context) {
-    if (tree == null) {
+    if (tree == null || tree instanceof JCTree.JCPackageDecl) {
       return null;
     }
     JCCompilationUnit compilationUnit = context.get(JCCompilationUnit.class);

core/src/main/java/com/google/errorprone/refaster/UFreeIdent.java

@@ -23,6 +23,8 @@
 import com.sun.source.tree.Tree;
 import com.sun.source.util.TreeScanner;
 import com.sun.tools.javac.code.Symbol;
+import com.sun.tools.javac.code.Symbol.TypeSymbol;
+import com.sun.tools.javac.tree.JCTree;
 import com.sun.tools.javac.tree.JCTree.JCExpression;
 import com.sun.tools.javac.util.Names;
 import javax.annotation.Nullable;
@@ -99,7 +101,15 @@ public Boolean visitIdentifier(IdentifierTree ident, Void v) {
       if (!isGood) {
         return Choice.none();
       } else if (currentBinding == null) {
+        Symbol currentExprSymbol = ASTHelpers.getSymbol(expression);
+        if (currentExprSymbol instanceof TypeSymbol
+            || (currentExprSymbol instanceof Symbol.MethodSymbol
+                && target instanceof JCTree.JCIdent)) {
+          // The `JCExpression` by itself does not represent a valid Java expression.
+          return Choice.none();
+        }
         unifier.putBinding(key(), expression);
+
         return Choice.of(unifier);
       } else if (currentBinding.toString().equals(expression.toString())) {
         // TODO(lowasser): try checking types here in a way that doesn't reject

core/src/test/java/com/google/errorprone/refaster/TemplateIntegrationTest.java

@@ -365,4 +365,9 @@ public void staticImportClassToken() throws IOException {
   public void suppressWarnings() throws IOException {
     runTest("SuppressWarningsTemplate");
   }
+
+  @Test
+  public void onlyBeforeTemplate() throws IOException {
+    runTest("OnlyBeforeSimpleReturnTemplate");
+  }
 }

core/src/test/java/com/google/errorprone/refaster/testdata/input/OnlyBeforeSimpleReturnTemplateExample.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2021 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.refaster.testdata;
+
+/** Test data for {@code OnlyBeforeSimpleReturnTemplate}. */
+public class OnlyBeforeSimpleReturnTemplateExample {
+  public String foo(String s) {
+    return s;
+  }
+}

core/src/test/java/com/google/errorprone/refaster/testdata/output/OnlyBeforeSimpleReturnTemplateExample.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2021 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.refaster.testdata;
+
+/** Test data for {@code OnlyBeforeSimpleReturnTemplate}. */
+public class OnlyBeforeSimpleReturnTemplateExample {
+  public String foo(String s) {
+    return /* match found */ s;
+  }
+}

core/src/test/java/com/google/errorprone/refaster/testdata/template/OnlyBeforeSimpleReturnTemplate.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2021 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.refaster.testdata.template;
+
+import com.google.errorprone.refaster.annotation.BeforeTemplate;
+
+/**
+ * Template for handling the case where only the {@link BeforeTemplate} is specified with a simple return
+ * statement.
+ */
+public class OnlyBeforeSimpleReturnTemplate {
+  @BeforeTemplate
+  String before(String s) {
+    return s;
+  }
+}