Why is not every variable initialized to its data type specific default value upon declaration in Bash?

Question

When I execute the following code in Bash version "GNU bash, version 4.1.10(4)-release (i686-pc-cygwin)", I get:

declare a
declare -p a
# Output: -bash: declare: a: not found
declare -i b
declare -p b
# Output: -bash: declare: b: not found
declare -a c
declare -p c
# Output: declare -a c='()'
declare -A d
declare -p d
# Output: declare -A d='()'

Arguably I think that the above variables should either be initialized in all cases or in none of them. That would seem more consistent rather than initializing only arrays upon declaration.

Answer 1

I presume one could always safely and uniformly declare and initialize variables as follows:

declare a=""
declare -p a
# Output: declare -- a=""
declare -i b=0
declare -p b
# Output: declare -i b="0"
declare -a c=()
declare -p c
# Output: declare -a c='()'
declare -A d=()
declare -p d
# Output: declare -A d='()'

Given that there seems to be differing behavior accross different releases of the Bash shell.

When one doesn't provide an explicit initialization value while declaring a variable the result might not what one expects as demonstrated in the following example with local variables:

function foobar {
  declare a
  declare -i b
  declare -a c
  declare -A d
  declare -p a b c d
  a=a
  b=42
  c+=(c)
  d+=([d]=42)
  declare -p a b c d
}
foobar
# Output:
# declare -- a=""
# declare -i b=""
# declare -a c='()'
# declare -A d='()'
# Output:
# declare -- a="a"
# declare -i b="42"
# declare -a c='([0]="c")'
# declare -A d='([d]="42" )'
declare -p a b c d
# Output:
# bash: declare: a: not found
# bash: declare: b: not found
# bash: declare: c: not found
# bash: declare: d: not found

In the case of local variables and late initialization everything works as expected. Particularly note that the first declare -p a b c d inside the foobar function reports all variables as being initialized to their data type specific default values. Compare that to the global variable case where the a and b variables were reported as -bash: declare: a: not found and -bash: declare: b: not found, respectively.