[osh-language] Fix "${undef-\z}"

VSub_ArgDQ should have the same backslash behavior as DQ.

Reported by Crestwave on issue #695.

frontend/lexer_def.py

@@ -68,6 +68,12 @@ def R(pat, tok_type):
   C('\\\n', Id.Ignored_LineCont),
 ]
 
+# Only 4 characters are backslash escaped inside "".
+# https://www.gnu.org/software/bash/manual/bash.html#Double-Quotes
+_DQ_BACKSLASH = [
+  R(r'\\[$`"\\]', Id.Lit_EscapedChar)
+]
+
 VAR_NAME_RE = r'[a-zA-Z_][a-zA-Z0-9_]*'
 
 # All Kind.VSub
@@ -398,10 +404,7 @@ def IsKeyword(name):
   R(r'[^\0]', Id.Lit_Other),  # everything else is literal
 ]
 
-LEXER_DEF[lex_mode_e.DQ] = [
-  # Only 4 characters are backslash escaped inside "".
-  # https://www.gnu.org/software/bash/manual/bash.html#Double-Quotes
-  R(r'\\[$`"\\]', Id.Lit_EscapedChar),
+LEXER_DEF[lex_mode_e.DQ] = _DQ_BACKSLASH + [
   C('\\\n', Id.Ignored_LineCont),
 ] + _LEFT_SUBS + _VARS + [
   R(r'[^$`"\0\\]+', Id.Lit_Chars),  # matches a line at most
@@ -410,7 +413,7 @@ def IsKeyword(name):
   R(r'[^\0]', Id.Lit_Other),  # e.g. "$"
 ]
 
-_VS_ARG_COMMON = _BACKSLASH + [
+_VS_ARG_COMMON = [
   C('/', Id.Lit_Slash),  # for patsub (not Id.VOp2_Slash)
   C('#', Id.Lit_Pound),  # for patsub prefix (not Id.VOp1_Pound)
   C('%', Id.Lit_Percent),  # for patsdub suffix (not Id.VOp1_Percent)
@@ -419,14 +422,16 @@ def IsKeyword(name):
 
 # Kind.{LIT,IGNORED,VS,LEFT,RIGHT,Eof}
 LEXER_DEF[lex_mode_e.VSub_ArgUnquoted] = \
-  _VS_ARG_COMMON + _LEFT_SUBS + _LEFT_UNQUOTED + _VARS + [
+  _BACKSLASH + _VS_ARG_COMMON + _LEFT_SUBS + _LEFT_UNQUOTED + _VARS + [
   # NOTE: added < and > so it doesn't eat <()
   R(r'[^$`/}"\'\0\\#%<>]+', Id.Lit_Chars),
   R(r'[^\0]', Id.Lit_Other),  # e.g. "$", must be last
 ]
 
 # Kind.{LIT,IGNORED,VS,LEFT,RIGHT,Eof}
-LEXER_DEF[lex_mode_e.VSub_ArgDQ] = _VS_ARG_COMMON + _LEFT_SUBS + _VARS + [
+LEXER_DEF[lex_mode_e.VSub_ArgDQ] = \
+  _DQ_BACKSLASH +  _VS_ARG_COMMON + _LEFT_SUBS + _VARS + [
+
   R(r'[^$`/}"\0\\#%]+', Id.Lit_Chars),  # matches a line at most
 
   # Weird wart: even in double quoted state, double quotes are allowed

spec/var-op-test.test.sh

@@ -148,3 +148,36 @@ echo ${#arr[@]}
 ## N-I dash stdout-json: ""
 ## N-I mksh status: 1
 ## N-I mksh stdout-json: ""
+
+#### "\z" as arg
+echo "${undef-\$}"
+echo "${undef-\(}"
+echo "${undef-\z}"
+echo "${undef-\"}"
+echo "${undef-\`}"
+echo "${undef-\\}"
+## STDOUT:
+$
+\(
+\z
+"
+`
+\
+## END
+## BUG yash STDOUT:
+$
+(
+z
+"
+`
+\
+## END
+
+#### "\e" as arg
+echo "${undef-\e}"
+## STDOUT:
+\e
+## END
+## BUG zsh/mksh stdout-repr: '\x1b\n'
+## BUG yash stdout: e
+