[bug #19236] Imported variable with trailing backslash messes up make's line parsing in nested evaluations

[Paul D. Smith]

Update of bug #19236 (project make):

                  Status:                    None => Not A Bug              
             Open/Closed:                    Open => Closed                 

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Follow-up Comment #1:

I agree this is unpleasant; however I don't see anything make can do about
it.  Eval works by first expanding its argument(s), then evaluating the
result.  In your example, after the expansion of $(Y) make sees:

ifeq (1,1)
B := X\
endif

and there's no possible way that the eval can know that this backslash was
originally contained in the variable A, rather than being written directly by
you.

The only solution to this is for YOU to be more cautious about when you allow
variables to expand.  If you defer the expansion of A so that it's done by the
eval itself instead of being done up front before eval is invoked, then you'll
get the behavior you want; change the definition of Y as follows:

define Y
ifeq (1,1)
B := $$(A)
endif
endef

By using $$(A), it won't expand during the expansion of Y, and eval will see
this:

ifeq (1,1)
B := $(A)
endif

with no backslash... THEN when eval evaluates this IT will expand A and set B
to the right value (X\).  For your "real life" example this would translate
to:

define $(COMP)_CUSTOM_WX_TARGET
ifneq ($(filter bcb%,$(TOOLSET)),)
$(1): export PATH := $(BCCDIR)bin;$$(PATH)
endif
endef

Although, as a general rule, I find it best to defer expansion of ALL
variables other than those that require it (typically as part of a call), so
the above might be more safely written:

define $(COMP)_CUSTOM_WX_TARGET
ifneq ($$(filter bcb%,$$(TOOLSET)),)
$(1): export PATH := $$(BCCDIR)bin;$$(PATH)
endif
endef

I'm closing this for now but if anyone has any ideas on how to make this work
better please add a note or bring it up on the mailing lists.

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