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===================================================================
RCS file: /ftp/cvs/cvsroot/src/usr.bin/make/unit-tests/var-op-expand.mk,v
rcsdiff: /ftp/cvs/cvsroot/src/usr.bin/make/unit-tests/var-op-expand.mk,v: warning: Unknown phrases like `commitid ...;' are present.
retrieving revision 1.2
retrieving revision 1.3
diff -u -p -r1.2 -r1.3
--- src/usr.bin/make/unit-tests/var-op-expand.mk	2020/08/16 14:25:16	1.2
+++ src/usr.bin/make/unit-tests/var-op-expand.mk	2020/11/08 13:46:15	1.3
@@ -1,9 +1,27 @@
-# $NetBSD: var-op-expand.mk,v 1.2 2020/08/16 14:25:16 rillig Exp $
+# $NetBSD: var-op-expand.mk,v 1.3 2020/11/08 13:46:15 rillig Exp $
 #
 # Tests for the := variable assignment operator, which expands its
 # right-hand side.
 
 # TODO: Implementation
 
+# XXX: edge case: When a variable name refers to an undefined variable, the
+# behavior differs between the '=' and the ':=' assignment operators.
+# This bug exists since at least 1993-03-21.
+#
+# The '=' operator expands the undefined variable to an empty string, thus
+# assigning to VAR_ASSIGN_.  In the name of variables to be set, it should
+# really be forbidden to refer to undefined variables.
+#
+# The ':=' operator expands the variable name twice.  In one of these
+# expansions, the undefined variable expression is preserved (controlled by
+# preserveUndefined in VarAssign_EvalSubst), in the other expansion it expands
+# to an empty string.  This way, 2 variables are created using a single
+# variable assignment.  It's magic. :-/
+.MAKEFLAGS: -dv
+VAR_ASSIGN_${UNDEF}=	undef value
+VAR_SUBST_${UNDEF}:=	undef value
+.MAKEFLAGS: -d0
+
 all:
 	@:;