tests/make: extend and explain test for the ':_' modifier

usr.bin/make/unit-tests/varmod-remember.mk

@@ -1,36 +1,64 @@
-# $NetBSD: varmod-remember.mk,v 1.7 2023/02/09 07:34:15 sjg Exp $
+# $NetBSD: varmod-remember.mk,v 1.8 2023/02/09 08:22:10 rillig Exp $
 #
-# Tests for the :_ modifier, which saves the current variable value
+# Tests for the :_ modifier, which saves the current expression value
 # in the _ variable or another, to be used later again.
 
-# this should result in "1=A 2=B 3=C"
-ABC= ${A B C:L:_:range:@i@$i=${_:[$i]}@}
 
-# we compare this with a repeat later
-x:= ${ABC}
+# The ':_' modifier is typically used in situations where the value of an
+# expression is needed at the same time as a sequence of numbers.  In these
+# cases, the value of the expression is saved in the temporary variable '_',
+# from where it is taken later in the same expression.
+ABC=	${A B C:L:_:range:@i@$i=${_:[$i]}@}
+DEF=	${D E F:L:_:range:@i@$i=${_:[$i]}@}
+GHI=	${G H I:L:_:range:@i@$i=${_:[$i]}@}
 
-.if ${1 2 3:L:_:@var@${_}@} != "1 2 3 1 2 3 1 2 3"
+ABC.global:=	${ABC}		# is evaluated in the global scope
+.if ${ABC.global} != "1=A 2=B 3=C"
 .  error
 .endif
 
+.if ${DEF} != "1=D 2=E 3=F"	# is evaluated in the command line scope
+.  error
+.endif
+
+# Before var.c 1.1040 from 2023-02-09, the temporary variable '_' was placed
+# in the scope of the current evaluation, which meant that after the first
+# ':_' modifier had been evaluated in command line scope, all further
+# evaluations in global scope could not overwrite the variable '_' anymore,
+# as the command line scope takes precedence over the global scope.
+# The expression ${GHI} therefore evaluated to '1=D 2=E 3=F', reusing the
+# value of '_' from the previous evaluation in command line scope.
+GHI.global:=	${GHI}		# is evaluated in the global scope
+.if ${GHI.global} != "1=G 2=H 3=I"
+.  error
+.endif
+
+.MAKEFLAGS: -d0
+
+
 # In the parameterized form, having the variable name on the right side of
-# the = assignment operator is confusing.  In almost all other situations
-# the variable name is on the left-hand side of the = operator.  Luckily
-# this modifier is only rarely needed.
+# the = assignment operator looks confusing.  In almost all other situations,
+# the variable name is on the left-hand side of the = operator, therefore
+# '_=SAVED' looks like it would copy 'SAVED' to '_'.  Luckily, this modifier
+# is only rarely needed.
 .if ${1 2 3:L:@var@${var:_=SAVED:}@} != "1 2 3"
 .  error
 .elif ${SAVED} != "3"
 .  error
 .endif
 
-# The ':_' modifier takes a variable name as optional argument.  This variable
-# name can refer to other variables, though this was rather an implementation
-# oversight than an intended feature.  The variable name stops at the first
-# '}' or ')' and thus cannot use the usual form ${VARNAME} of long variable
-# names.
+
+# The ':_' modifier takes a variable name as optional argument.  Before var.c
+# 1.867 from 2021-03-14, this variable name could refer to other variables,
+# such as in 'VAR.$p'.  It was not possible to refer to 'VAR.${param}' though,
+# as that form caused a parse error.  The cause for the parse error in
+# '${...:_=VAR.${param}}' is that the variable name is parsed in an ad-hoc
+# manner, stopping at the first ':', ')' or '}', without taking any nested
+# expressions into account.  Due to this inconsistency that short expressions
+# are possible but long expressions aren't, the name of the temporary variable
+# is no longer expanded.
 #
-# Because of all these edge-casey conditions, this "feature" has been removed
-# in var.c 1.867 from 2021-03-14.
+# TODO: Warn about the unusual variable name '$S'.
 S=	INDIRECT_VARNAME
 .if ${value:L:@var@${var:_=$S}@} != "value"
 .  error
@@ -38,10 +66,15 @@ S=	INDIRECT_VARNAME
 .  error
 .endif
 
-# we *should* get the same result as for $x above
-X:= ${ABC}
-.if $X != $x
-.  error
+
+# When a variable using ':_' refers to another variable that also uses ':_',
+# the value of the temporary variable '_' from the inner expression leaks into
+# the evaluation of the outer expression.  If the expressions were evaluated
+# independently, the last word of the result would be outer_='outer' instead.
+INNER=	${inner:L:_:@i@$i inner_='$_'@}
+OUTER=	${outer:L:_:@o@$o ${INNER} outer_='$_'@}
+.if ${OUTER} != "outer inner inner_='inner' outer_='inner'"
 .endif
 
+
 all: