Proper usage of multiple rules for the same targets

[Tiphaine Turpin]

Hello,


I have discovered an issue while using an additional rule with just 
prerequisites, as suggested in section "Multiple Rules for One Target" of the 
documentation, when the "main" rule is a pattern rule. The problem it that this 
changes the behavior in case of missing source files in a non intuitive way.


Here is a minimal example Makefile:


%.t: %.x
        echo using implicit rule

foo.t : additional_prerequisite


If I run 'make foo.t', then

- if foo.x and additional_prerequisite both exist, then the pattern rule is 
executed as expected, and outputs 'using implicit rule'

- if foo.x is missing then the build succeeds with "Nothing to be done for 
'foo.t'."


However, what I would actually want is to get an error, as the one I get if I 
remove the additional prerequisite rule: "No rule to make target 'foo.t'."


I found the explanation to this in the documentation: if foo.x is missing then 
the pattern rule is not "enabled" because a pattern rule only matches when "all 
prerequisites in that rule either exist or can be built" as mentioned in 
sections "How Patterns Match" and "Using Implicit Rules". The latter 
additionally says:

'A file “can be made” if it is mentioned explicitly in the makefile as a target 
or a prerequisite, or if an implicit rule can be recursively found for how to 
make it.'

In my case, foo.x clearly can't be made, so the pattern rule just does not 
apply and the additional rule is interpreted as "making" foo.t by doing nothing.


So, a simple solution is to make sure that foo.x exists or can be made, for 
example with another pattern rule:

%.x:
        $(error no $@)

Could anyone suggest another approach ? Is it better to avoid additional rules 
for this purpose, adding prerequisites to the pattern rule instead ? Is there a 
way to define a rule as "dependency-only", so that it can't be used to actually 
make its target using the implicit "no-op" reciepe ?


Thanks,

Tiphaine Turpin