Bug: Trying to access items from data-structure contained within `Const` fails

[kmr-srbh]

Overview

Accessing values from a data-structure contained within Const fails with different errors for different data-structures.

Dictionary

from lpython import i32, dict, Const

d: Const[dict[str, i32]] = {"a": 1, "b": 2, "c": 3}
print(d["a"])

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
semantic error: Type mismatch in index, expected a single integer or slice
 --> ./examples/example.py:4:9
  |
4 | print(d["a"])
  |         ^^^ type mismatch (found: 'str', expected: 'i32' or slice)

Note: Please report unclear or confusing messages as bugs at
https://github.com/lcompilers/lpython/issues.

List

from lpython import i32, list, Const

l: Const[list[i32]] = [1, 2, 3, 4, 5]
print(l[0])

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
ASR verify pass error: ASR verify: The variable in ArrayItem must be an array, not a scalar
Internal Compiler Error: Unhandled exception
Traceback (most recent call last):
  Binary file "/home/saurabh-kumar/Projects/System/lpython/src/bin/lpython", in _start()
  File "./csu/../csu/libc-start.c", line 360, in __libc_start_main_impl()
  File "./csu/../sysdeps/x86/libc-start.c", line 58, in __libc_start_call_main()
  File "/home/saurabh-kumar/Projects/System/lpython/src/bin/lpython.cpp", line 1873, in main()
    err = compile_python_to_object_file(arg_file, tmp_o, runtime_library_dir,
  File "/home/saurabh-kumar/Projects/System/lpython/src/bin/lpython.cpp", line 787, in compile_python_to_object_file()
    !(arg_c && compiler_options.po.disable_main), "__main__", infile);
LCompilersException: Verify failed

Tuple

from lpython import i32, tuple, Const

t: Const[tuple[i32, i32, i32, i32, i32]] = (1, 2, 3, 4, 5)
print(t[0])

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
Internal Compiler Error: Unhandled exception
.
.
.
LCompilersException: Not implemented 8

String

from lpython import i32, str, Const

s: Const[str] = "Hello, LPython!"
print(s[0])

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
ASR verify pass error: ASR verify: The variable in ArrayItem must be an array, not a scalar
Internal Compiler Error: Unhandled exception
Traceback (most recent call last):
  Binary file "/home/saurabh-kumar/Projects/System/lpython/src/bin/lpython", in _start()
  File "./csu/../csu/libc-start.c", line 360, in __libc_start_main_impl()
  File "./csu/../sysdeps/x86/libc-start.c", line 58, in __libc_start_call_main()
  File "/home/saurabh-kumar/Projects/System/lpython/src/bin/lpython.cpp", line 1873, in main()
    err = compile_python_to_object_file(arg_file, tmp_o, runtime_library_dir,
  File "/home/saurabh-kumar/Projects/System/lpython/src/bin/lpython.cpp", line 787, in compile_python_to_object_file()
    !(arg_c && compiler_options.po.disable_main), "__main__", infile);
LCompilersException: Verify failed

Fix

Check for Const type when handling subscript indices and do required processing for mutable types like dict and list. For types which are already immutable, like str and tuple, throw an error stating that Const is not required.