Re: [bug #49093] ifdef checks for non-empty value, not definition

[Philip Guenther]

On Wed, Sep 14, 2016 at 3:13 PM, Paul D. Smith <address@hidden> wrote:
...
> Probably an example like this would help make the doc more clear.

There *is* an example like that in the doc in at least version 4.2.1!

     Note that `ifdef' only tests whether a variable has a value.  It
     does not expand the variable to see if that value is nonempty.
     Consequently, tests using `ifdef' return true for all definitions
     except those like `foo ='.  To test for an empty value, use
     `ifeq ($(foo),)'.  For example,

          bar =
          foo = $(bar)
          ifdef foo
          frobozz = yes
          else
          frobozz = no
          endif

     sets `frobozz' to `yes', while:

          foo =
          ifdef foo
          frobozz = yes
          else
          frobozz = no
          endif

     sets `frobozz' to `no'.