Only emit the error for Structs for now

[certik]

No description provided.

[certik]

In Main, I get:

Internal Compiler Error: Unhandled exception
Traceback (most recent call last):
  File "/Users/ondrej/repos/lpython/src/bin/lpython.cpp", line 1767
    return emit_c(arg_file, runtime_library_dir, lpython_pass_manager,
  File "/Users/ondrej/repos/lpython/src/bin/lpython.cpp", line 333
    r1 = LCompilers::LPython::python_ast_to_asr(al, lm, nullptr, *ast, diagnostics, compiler_options, true, "", infile);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 7670
    auto res2 = body_visitor(al, lm, *ast_m, diagnostics, unit, main_module, module_name,
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 7621
    BodyVisitor b(al, lm, unit, diagnostics, main_module, module_name, ast_overload, allow_implicit_casting);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 4724
    visit_stmt(*x.m_body[i]);
  File "/Users/ondrej/repos/lpython/src/lpython/python_ast.h", line 1883
    void visit_stmt(const stmt_t &b) { visit_stmt_t(b, self()); }
  File "/Users/ondrej/repos/lpython/src/lpython/python_ast.h", line 1751
    case stmtType::FunctionDef: { v.visit_FunctionDef((const FunctionDef_t &)x); return; }
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 4837
    handle_fn(x, *f);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 4813
    transform_stmts(body, x.n_body, x.m_body);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 4684
    this->visit_stmt(*m_body[i]);
  File "/Users/ondrej/repos/lpython/src/lpython/python_ast.h", line 1883
    void visit_stmt(const stmt_t &b) { visit_stmt_t(b, self()); }
  File "/Users/ondrej/repos/lpython/src/lpython/python_ast.h", line 1758
    case stmtType::AnnAssign: { v.visit_AnnAssign((const AnnAssign_t &)x); return; }
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 5560
    transform_stmts(body, x.n_body, x.m_body);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 4684
    this->visit_stmt(*m_body[i]);
  File "/Users/ondrej/repos/lpython/src/lpython/python_ast.h", line 1883
    void visit_stmt(const stmt_t &b) { visit_stmt_t(b, self()); }
  File "/Users/ondrej/repos/lpython/src/lpython/python_ast.h", line 1757
    case stmtType::AugAssign: { v.visit_AugAssign((const AugAssign_t &)x); return; }
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 4963
    visit_AnnAssignUtil(x, var_name, init_expr);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 2853
    pptr_shape, x.base.base.loc);
  File "/Users/ondrej/repos/lpython/src/lpython/semantics/python_ast_to_asr.cpp", line 2738
    ASR::StructType_t *st = ASR::down_cast<ASR::StructType_t>(ASR::down_cast<ASR::Struct_t>(asr_alloc_type)->m_derived_type);
  File "/Users/ondrej/repos/lpython/src/libasr/asr.h", line 41
    LCOMPILERS_ASSERT(is_a<T>(*f));
AssertFailed: is_a<T>(*f)

This is now fixed.