Added appropriate throw block for error and added test for async impl…

…ementation

src/lpython/semantics/python_ast_to_asr.cpp

@@ -4051,15 +4051,16 @@ class SymbolTableVisitor : public CommonVisitor<SymbolTableVisitor> {
     // Implement visit_Global for Symbol Table visitor.
     void visit_Global(const AST::Global_t &/*x*/) {}
 
-    void visit_AsyncFunctionDef(const AST::AsyncFunctionDef_t &/*x*/){
+    void visit_AsyncFunctionDef(const AST::AsyncFunctionDef_t &x){
         try
         {
             // to be implemented
         }
         catch(const std::exception& e)
         {
             std::cerr << e.what() << '\n';
-        }        
+        }
+        throw SemanticError("The `async` keyword is currently not supported", x.base.base.loc);
     }
 
     void visit_FunctionDef(const AST::FunctionDef_t &x) {

tests/errors/test_async.py

@@ -0,0 +1,4 @@
+async def test_async():
+    print("done")
+
+test_async()

tests/reference/asr-test_async-361297c.json

@@ -0,0 +1,13 @@
+{
+    "basename": "asr-test_async-361297c",
+    "cmd": "lpython --show-asr --no-color {infile} -o {outfile}",
+    "infile": "tests/errors/test_async.py",
+    "infile_hash": "f4d737246effd50f1798a81f07042ad15a045e275448fe0226334f03",
+    "outfile": null,
+    "outfile_hash": null,
+    "stdout": null,
+    "stdout_hash": null,
+    "stderr": "asr-test_async-361297c.stderr",
+    "stderr_hash": "abf614594f89a7a6d93d469d512e31de5adc64feef866957de80cd03",
+    "returncode": 2
+}

tests/reference/asr-test_async-361297c.stderr

@@ -0,0 +1,9 @@
+semantic error: The `async` keyword is currently not supported
+ --> tests/errors/test_async.py:1:1 - 2:17
+  |
+1 |    async def test_async():
+  |    ^^^^^^^^^^^^^^^^^^^^^^^...
+...
+  |
+2 |        print("done")
+  | ...^^^^^^^^^^^^^^^^^ 

tests/tests.toml

@@ -784,6 +784,10 @@ ast_new = true
 
 # tests/errors
 
+[[test]]
+filename = "errors/test_async.py"
+asr = true
+
 [[test]]
 filename = "errors/test_str_indexing.py"
 asr = true