[ggj][ast] feat: enable postfix operator for ExprStatement (#282)

* feat: enable postfix operator for ExprStatement

* fix: merge master

* fix: merge master

Author: miraleung

src/main/java/com/google/api/generator/engine/ast/ExprStatement.java

@@ -53,9 +53,11 @@ public ExprStatement build() {
                 || (expr instanceof ReferenceConstructorExpr)
                 || (expr instanceof AssignmentExpr)
                 || (expr instanceof ThrowExpr)
-                || (expr instanceof ReturnExpr),
+                || (expr instanceof ReturnExpr)
+                || (expr instanceof UnaryOperationExpr
+                    && ((UnaryOperationExpr) expr).isPostfixIncrement()),
             "Expression statements must be either a method invocation, assignment, throw, "
-                + "this/super constructor, or return expression");
+                + "this/super constructor, return, or unary post-fix operation expression");
       }
       return exprStatement;
     }

src/main/java/com/google/api/generator/engine/ast/UnaryOperationExpr.java

@@ -30,6 +30,10 @@ public void accept(AstNodeVisitor visitor) {
     visitor.visit(this);
   }
 
+  public boolean isPostfixIncrement() {
+    return operatorKind().equals(OperatorKind.UNARY_POST_INCREMENT);
+  }
+
   public static UnaryOperationExpr postfixIncrementWithExpr(Expr expr) {
     return builder()
         .setExpr(expr)

src/test/java/com/google/api/generator/engine/ast/ExprStatementTest.java

@@ -71,6 +71,14 @@ public void validExprStatement_return() {
         ReturnExpr.withExpr(ValueExpr.withValue(StringObjectValue.withValue("asdf"))));
   }
 
+  @Test
+  public void validExprStatement_unaryOperation() {
+    assertValidExprStatement(
+        UnaryOperationExpr.postfixIncrementWithExpr(
+            VariableExpr.withVariable(
+                Variable.builder().setType(TypeNode.INT).setName("i").build())));
+  }
+
   @Test
   public void invalidExprStatement_variable() {
     Variable variable = Variable.builder().setType(TypeNode.INT).setName("libraryClient").build();
@@ -85,6 +93,15 @@ public void invalidExprStatement_value() {
     assertInvalidExprStatement(valueExpr);
   }
 
+  @Test
+  public void invalidExprStatement_logicalNotUnaryOperator() {
+    Expr logicalNotExpr =
+        UnaryOperationExpr.logicalNotWithExpr(
+            VariableExpr.withVariable(
+                Variable.builder().setType(TypeNode.BOOLEAN).setName("foo").build()));
+    assertInvalidExprStatement(logicalNotExpr);
+  }
+
   private static void assertInvalidExprStatement(Expr expr) {
     assertThrows(
         IllegalStateException.class,