Array.astype not supported with string literals

[njriasan]

Array.astype not working with literal strings

Providing a literal string like "float32" doesn't work with array.astype. Here is a reproducer from master:

import numba
import numpy as np

@numba.njit
def ex():
      data = np.arange(1000)
      return data.astype("float32")
  
print(ex())

Which produces the output:

Traceback (most recent call last):
  File "test.py", line 9, in <module>
    print(ex())
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 421, in _compile_for_args
    error_rewrite(e, 'typing')
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 360, in error_rewrite
    raise e
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 373, in _compile_for_args
    return_val = self.compile(tuple(argtypes))
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 911, in compile
    cres = self._compiler.compile(args, return_type)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 84, in compile
    raise retval
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 94, in _compile_cached
    retval = self._compile_core(args, return_type)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/dispatcher.py", line 107, in _compile_core
    cres = compiler.compile_extra(self.targetdescr.typing_context,
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler.py", line 675, in compile_extra
    return pipeline.compile_extra(func)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler.py", line 419, in compile_extra
    return self._compile_bytecode()
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler.py", line 483, in _compile_bytecode
    return self._compile_core()
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler.py", line 462, in _compile_core
    raise e
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler.py", line 453, in _compile_core
    pm.run(self.state)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler_machinery.py", line 343, in run
    raise patched_exception
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler_machinery.py", line 334, in run
    self._runPass(idx, pass_inst, state)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler_lock.py", line 35, in _acquire_compile_lock
    return func(*args, **kwargs)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler_machinery.py", line 289, in _runPass
    mutated |= check(pss.run_pass, internal_state)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/compiler_machinery.py", line 262, in check
    mangled = func(compiler_state)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/typed_passes.py", line 105, in run_pass
    typemap, return_type, calltypes, errs = type_inference_stage(
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/typed_passes.py", line 83, in type_inference_stage
    errs = infer.propagate(raise_errors=raise_errors)
  File "/Users/nicholasriasanovsky/Documents/bodo/nick-numba/numba/core/typeinfer.py", line 1074, in propagate
    raise errors[0]
numba.core.errors.TypingError: Failed in nopython mode pipeline (step: nopython frontend)
Invalid use of BoundFunction(array.astype for array(int64, 1d, C)) with parameters (Literal[str](float32))

During: resolving callee type: BoundFunction(array.astype for array(int64, 1d, C))
During: typing of call at test.py (7)


File "test.py", line 7:
def ex():
    <source elided>
    data = np.arange(1000)
    return data.astype("float32")

• I have tried using the latest released version of Numba (most recent is
  visible in the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included a self contained code sample to reproduce the problem.
  i.e. it's possible to run as 'python bug.py'.

[njriasan]

This seems like a simple fix and I will submit a PR with the change.