[SV 48643] Update the implicit rule search docs

* NEWS: Announce the change.
* doc/make.texi (Implicit Rule Search): Update the definition of
"ought to exist" and add the extra step for compatibility mode.

2 files changed, 17 insertions, 2 deletions

diff --git a/NEWS b/NEWS
index 9d8ff2b3..f0a9f75a 100644
--- a/NEWS
+++ b/NEWS
@@ -78,6 +78,14 @@ https://sv.gnu.org/bugs/index.php?group=make&report_id=111&fix_release_id=109&se
   made as a side-effect of some other target's recipe are now noticed as
   expected.
 
+* GNU make has sometimes chosen unexpected, and sub-optimal, chains of
+  implicit rules due to the definition of "ought to exist" in the implicit
+  rule search algorithm, which considered any prerequisite mentioned in the
+  makefile as "ought to exist".  This algorithm has been modified to prefer
+  prerequisites mentioned explicitly in the target being built and only if
+  that results in no matching rule, will GNU make consider prerequisites
+  mentioned in other targets as "ought to exist".
+
 * GNU Make can now be built for MS-Windows using the Tiny C tcc compiler.
   Port provided by Christian Jullien <eligis@orange.fr>
 
diff --git a/doc/make.texi b/doc/make.texi
index 5eea2a94..f97f15ff 100644
--- a/doc/make.texi
+++ b/doc/make.texi
@@ -27,7 +27,7 @@ of @cite{The GNU Make Manual}, for GNU @code{make} version @value{VERSION}.
 Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995,
 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007,
 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019,
-2020 Free Software Foundation, Inc.
+2020, 2021 Free Software Foundation, Inc.
 
 @quotation
 Permission is granted to copy, distribute and/or modify this document
@@ -10964,7 +10964,7 @@ the front of each prerequisite name.@refill
 @item
 Test whether all the prerequisites exist or ought to exist.  (If a
 file name is mentioned in the makefile as a target or as an explicit
-prerequisite, then we say it ought to exist.)
+prerequisite of target T, then we say it ought to exist.)
 
 If all prerequisites exist or ought to exist, or there are no prerequisites,
 then this rule applies.
@@ -10995,6 +10995,13 @@ made by implicit rules, then this rule applies.
 @end enumerate
 
 @item
+If no pattern rule has been found then try step 5 and step 6 again with a
+modified definition of ``ought to exist'': if a filename is mentioned as a
+target or as an explicit prerequisite of @emph{any} target, then it ought to
+exist.  This check is only present for backward-compatibility with older
+versions of GNU make: we don't recommend relying on it.
+
+@item
 If no implicit rule applies, the rule for @code{.DEFAULT}, if any,
 applies.  In that case, give @var{t} the same recipe that
 @code{.DEFAULT} has.  Otherwise, there is no recipe for @var{t}.