[gcc r13-2508] libcpp: Named universal character escapes and delimited escape sequence tweaks

[gcc r13-2508] libcpp: Named universal character escapes and delimited escape sequence tweaks

[Jakub Jelinek]

https://gcc.gnu.org/g:572f5e1bc68e131b25cd2d5ba231e932f5038904

commit r13-2508-g572f5e1bc68e131b25cd2d5ba231e932f5038904
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Sep 7 08:44:38 2022 +0200

    libcpp: Named universal character escapes and delimited escape sequence tweaks
    
    On Tue, Aug 30, 2022 at 09:10:37PM +0000, Joseph Myers wrote:
    > I'm seeing build failures of glibc for powerpc64, as illustrated by the
    > following C code:
    >
    > #if 0
    > \NARG
    > #endif
    >
    > (the actual sysdeps/powerpc/powerpc64/sysdep.h code is inside #ifdef
    > __ASSEMBLER__).
    >
    > This shows some problems with this feature - and with delimited escape
    > sequences - as it affects C.  It's fine to accept it as an extension
    > inside string and character literals, because \N or \u{...} would be
    > invalid in the absence of the feature (i.e. the syntax for such literals
    > fails to match, meaning that the rule about undefined behavior for a
    > single ' or " as a pp-token applies).  But outside string and character
    > literals, the usual lexing rules apply, the \ is a pp-token on its own and
    > the code is valid at the preprocessing level, and with expansion of macros
    > appearing before or after the \ (e.g. u defined as a macro in the \u{...}
    > case) it may be valid code at the language level as well.  I don't know
    > what older C++ versions say about this, but for C this means e.g.
    >
    > #define z(x) 0
    > #define a z(
    > int x = a\NARG);
    >
    > needs to be accepted as expanding to "int x = 0;", not interpreted as
    > using the \N feature in an identifier and produce an error.
    
    The following patch changes this, so that:
    1) outside of string/character literals, \N without following { is never
       treated as an error nor warning, it is silently treated as \ separate
       token followed by whatever is after it
    2) \u{123} and \N{LATIN SMALL LETTER A WITH ACUTE} are not handled as
       extension at all outside of string/character literals in the strict
       standard modes (-std=c*) except for -std=c++{23,2b}, only in the
       -std=gnu* modes, because it changes behavior on valid sources, e.g.
       #define z(x) 0
       #define a z(
       int x = a\u{123});
       int y = a\N{LATIN SMALL LETTER A WITH ACUTE});
    3) introduces -Wunicode warning (on by default) and warns for cases
       of what looks like invalid delimited escape sequence or named
       universal character escape outside of string/character literals
       and is treated as separate tokens
    
    2022-09-07  Jakub Jelinek  <jakub@redhat.com>
    
    libcpp/
            * include/cpplib.h (struct cpp_options): Add cpp_warn_unicode member.
            (enum cpp_warning_reason): Add CPP_W_UNICODE.
            * init.cc (cpp_create_reader): Initialize cpp_warn_unicode.
            * charset.cc (_cpp_valid_ucn): In possible identifier contexts, don't
            handle \u{ or \N{ specially in -std=c* modes except -std=c++2{3,b}.
            In possible identifier contexts, don't emit an error and punt
            if \N isn't followed by {, or if \N{} surrounds some lower case
            letters or _.  In possible identifier contexts when not C++23, don't
            emit an error but warning about unknown character names and treat as
            separate tokens.  When treating as separate tokens \u{ or \N{, emit
            warnings.
    gcc/
            * doc/invoke.texi (-Wno-unicode): Document.
    gcc/c-family/
            * c.opt (Winvalid-utf8): Use ObjC instead of objC.  Remove
            " in comments" from description.
            (Wunicode): New option.
    gcc/testsuite/
            * c-c++-common/cpp/delimited-escape-seq-4.c: New test.
            * c-c++-common/cpp/delimited-escape-seq-5.c: New test.
            * c-c++-common/cpp/delimited-escape-seq-6.c: New test.
            * c-c++-common/cpp/delimited-escape-seq-7.c: New test.
            * c-c++-common/cpp/named-universal-char-escape-5.c: New test.
            * c-c++-common/cpp/named-universal-char-escape-6.c: New test.
            * c-c++-common/cpp/named-universal-char-escape-7.c: New test.
            * g++.dg/cpp23/named-universal-char-escape1.C: New test.
            * g++.dg/cpp23/named-universal-char-escape2.C: New test.

Diff:
---
 gcc/c-family/c.opt                                 |  8 +-
 gcc/doc/invoke.texi                                |  8 +-
 .../c-c++-common/cpp/delimited-escape-seq-4.c      | 13 ++++
 .../c-c++-common/cpp/delimited-escape-seq-5.c      | 13 ++++
 .../c-c++-common/cpp/delimited-escape-seq-6.c      | 13 ++++
 .../c-c++-common/cpp/delimited-escape-seq-7.c      | 13 ++++
 .../cpp/named-universal-char-escape-5.c            | 17 +++++
 .../cpp/named-universal-char-escape-6.c            | 17 +++++
 .../cpp/named-universal-char-escape-7.c            | 17 +++++
 .../g++.dg/cpp23/named-universal-char-escape1.C    | 16 ++++
 .../g++.dg/cpp23/named-universal-char-escape2.C    | 18 +++++
 libcpp/charset.cc                                  | 86 ++++++++++++++++++----
 libcpp/include/cpplib.h                            |  7 +-
 libcpp/init.cc                                     |  1 +
 14 files changed, 228 insertions(+), 19 deletions(-)

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 4515664aa59..1c7f89eeb94 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -822,8 +822,8 @@ C ObjC C++ ObjC++ CPP(warn_invalid_pch) CppReason(CPP_W_INVALID_PCH) Var(cpp_war
 Warn about PCH files that are found but not used.
 
 Winvalid-utf8
-C objC C++ ObjC++ CPP(cpp_warn_invalid_utf8) CppReason(CPP_W_INVALID_UTF8) Var(warn_invalid_utf8) Init(0) Warning
-Warn about invalid UTF-8 characters in comments.
+C ObjC C++ ObjC++ CPP(cpp_warn_invalid_utf8) CppReason(CPP_W_INVALID_UTF8) Var(warn_invalid_utf8) Init(0) Warning
+Warn about invalid UTF-8 characters.
 
 Wjump-misses-init
 C ObjC Var(warn_jump_misses_init) Warning LangEnabledby(C ObjC,Wc++-compat)
@@ -1345,6 +1345,10 @@ Wundef
 C ObjC C++ ObjC++ CPP(warn_undef) CppReason(CPP_W_UNDEF) Var(cpp_warn_undef) Init(0) Warning
 Warn if an undefined macro is used in an #if directive.
 
+Wunicode
+C ObjC C++ ObjC++ CPP(cpp_warn_unicode) CppReason(CPP_W_UNICODE) Var(warn_unicode) Init(1) Warning
+Warn about invalid forms of delimited or named escape sequences.
+
 Wuninitialized
 C ObjC C++ ObjC++ LTO LangEnabledBy(C ObjC C++ ObjC++ LTO,Wall)
 ;
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 9d662e35316..cc631dfa261 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -365,7 +365,7 @@ Objective-C and Objective-C++ Dialects}.
 -Winfinite-recursion @gol
 -Winit-self  -Winline  -Wno-int-conversion  -Wint-in-bool-context @gol
 -Wno-int-to-pointer-cast  -Wno-invalid-memory-model @gol
--Winvalid-pch  -Winvalid-utf8 -Wjump-misses-init  @gol
+-Winvalid-pch  -Winvalid-utf8  -Wno-unicode  -Wjump-misses-init  @gol
 -Wlarger-than=@var{byte-size}  -Wlogical-not-parentheses  -Wlogical-op  @gol
 -Wlong-long  -Wno-lto-type-mismatch -Wmain  -Wmaybe-uninitialized @gol
 -Wmemset-elt-size  -Wmemset-transposed-args @gol
@@ -9578,6 +9578,12 @@ Warn if an invalid UTF-8 character is found.
 This warning is on by default for C++23 if @option{-finput-charset=UTF-8}
 is used and turned into error with @option{-pedantic-errors}.
 
+@item -Wno-unicode
+@opindex Wunicode
+@opindex Wno-unicode
+Don't diagnose invalid forms of delimited or named escape sequences which are
+treated as separate tokens.  @option{Wunicode} is enabled by default.
+
 @item -Wlong-long
 @opindex Wlong-long
 @opindex Wno-long-long
diff --git a/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-4.c b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-4.c
new file mode 100644
index 00000000000..107051fcce3
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-4.c
@@ -0,0 +1,13 @@
+/* P2290R3 - Delimited escape sequences */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=gnu99 -Wno-c++-compat" { target c } } */
+/* { dg-options "-std=gnu++20" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\u{});		/* { dg-warning "empty delimited escape sequence; treating it as separate tokens" } */
+int c = a\u{);		/* { dg-warning "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{; treating it as separate tokens" } */
+int d = a\u{12XYZ});	/* { dg-warning "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{12; treating it as separate tokens" } */
+int e = a\u123);
+int f = a\U1234567);
diff --git a/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-5.c b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-5.c
new file mode 100644
index 00000000000..e04f519674e
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-5.c
@@ -0,0 +1,13 @@
+/* P2290R3 - Delimited escape sequences */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=c17 -Wno-c++-compat" { target c } } */
+/* { dg-options "-std=c++23" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\u{});		/* { dg-warning "empty delimited escape sequence; treating it as separate tokens" "" { target c++23 } } */
+int c = a\u{);		/* { dg-warning "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{; treating it as separate tokens" "" { target c++23 } } */
+int d = a\u{12XYZ});	/* { dg-warning "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{12; treating it as separate tokens" "" { target c++23 } } */
+int e = a\u123);
+int f = a\U1234567);
diff --git a/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-6.c b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-6.c
new file mode 100644
index 00000000000..f2a4e939094
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-6.c
@@ -0,0 +1,13 @@
+/* P2290R3 - Delimited escape sequences */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=gnu99 -Wno-c++-compat -Wno-unicode" { target c } } */
+/* { dg-options "-std=gnu++20 -Wno-unicode" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\u{});		/* { dg-bogus "empty delimited escape sequence; treating it as separate tokens" } */
+int c = a\u{);		/* { dg-bogus "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{; treating it as separate tokens" } */
+int d = a\u{12XYZ});	/* { dg-bogus "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{12; treating it as separate tokens" } */
+int e = a\u123);
+int f = a\U1234567);
diff --git a/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-7.c b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-7.c
new file mode 100644
index 00000000000..e2f0da4e4d4
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-7.c
@@ -0,0 +1,13 @@
+/* P2290R3 - Delimited escape sequences */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=c17 -Wno-c++-compat -Wno-unicode" { target c } } */
+/* { dg-options "-std=c++23 -Wno-unicode" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\u{});		/* { dg-bogus "empty delimited escape sequence; treating it as separate tokens" } */
+int c = a\u{);		/* { dg-bogus "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{; treating it as separate tokens" } */
+int d = a\u{12XYZ});	/* { dg-bogus "'\\\\u\\\{' not terminated with '\\\}' after \\\\u\\\{12; treating it as separate tokens" } */
+int e = a\u123);
+int f = a\U1234567);
diff --git a/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-5.c b/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-5.c
new file mode 100644
index 00000000000..a1c53c7f649
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-5.c
@@ -0,0 +1,17 @@
+/* P2071R2 - Named universal character escapes */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=gnu99 -Wno-c++-compat" { target c } } */
+/* { dg-options "-std=gnu++20" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\N{});				/* { dg-warning "empty named universal character escape sequence; treating it as separate tokens" } */
+int c = a\N{);				/* { dg-warning "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{; treating it as separate tokens" } */
+int d = a\N);
+int e = a\NARG);
+int f = a\N{abc});				/* { dg-warning "\\\\N\\\{abc\\\} is not a valid universal character; treating it as separate tokens" } */
+int g = a\N{ABC.123});				/* { dg-warning "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{ABC; treating it as separate tokens" } */
+int h = a\N{NON-EXISTENT CHAR});	/* { dg-warning "\\\\N\\\{NON-EXISTENT CHAR\\\} is not a valid universal character; treating it as separate tokens" } */
+int i = a\N{Latin_Small_Letter_A_With_Acute});	/* { dg-warning "\\\\N\\\{Latin_Small_Letter_A_With_Acute\\\} is not a valid universal character; treating it as separate tokens" } */
+					/* { dg-message "did you mean \\\\N\\\{LATIN SMALL LETTER A WITH ACUTE\\\}\\?" "" { target *-*-* } .-1 } */
diff --git a/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-6.c b/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-6.c
new file mode 100644
index 00000000000..a6a5a102aac
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-6.c
@@ -0,0 +1,17 @@
+/* P2071R2 - Named universal character escapes */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=c17 -Wno-c++-compat" { target c } } */
+/* { dg-options "-std=c++20" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\N{});
+int c = a\N{);
+int d = a\N);
+int e = a\NARG);
+int f = a\N{abc});
+int g = a\N{ABC.123});
+int h = a\N{NON-EXISTENT CHAR});	/* { dg-bogus "is not a valid universal character" } */
+int i = a\N{Latin_Small_Letter_A_With_Acute});
+int j = a\N{LATIN SMALL LETTER A WITH ACUTE});
diff --git a/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-7.c b/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-7.c
new file mode 100644
index 00000000000..e6142bff63c
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/named-universal-char-escape-7.c
@@ -0,0 +1,17 @@
+/* P2071R2 - Named universal character escapes */
+/* { dg-do compile } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=gnu99 -Wno-c++-compat -Wno-unicode" { target c } } */
+/* { dg-options "-std=gnu++20 -Wno-unicode" { target c++ } } */
+
+#define z(x) 0
+#define a z(
+int b = a\N{});				/* { dg-bogus "empty named universal character escape sequence; treating it as separate tokens" } */
+int c = a\N{);				/* { dg-bogus "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{; treating it as separate tokens" } */
+int d = a\N);
+int e = a\NARG);
+int f = a\N{abc});				/* { dg-bogus "\\\\N\\\{abc\\\} is not a valid universal character; treating it as separate tokens" } */
+int g = a\N{ABC.123});				/* { dg-bogus "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{ABC; treating it as separate tokens" } */
+int h = a\N{NON-EXISTENT CHAR});	/* { dg-bogus "\\\\N\\\{NON-EXISTENT CHAR\\\} is not a valid universal character; treating it as separate tokens" } */
+int i = a\N{Latin_Small_Letter_A_With_Acute});	/* { dg-bogus "\\\\N\\\{Latin_Small_Letter_A_With_Acute\\\} is not a valid universal character; treating it as separate tokens" } */
+					/* { dg-bogus "did you mean \\\\N\\\{LATIN SMALL LETTER A WITH ACUTE\\\}\\?" "" { target *-*-* } .-1 } */
diff --git a/gcc/testsuite/g++.dg/cpp23/named-universal-char-escape1.C b/gcc/testsuite/g++.dg/cpp23/named-universal-char-escape1.C
new file mode 100644
index 00000000000..fe494824ed4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/named-universal-char-escape1.C
@@ -0,0 +1,16 @@
+// P2071R2 - Named universal character escapes
+// { dg-do compile }
+// { dg-require-effective-target wchar }
+
+#define z(x) 0
+#define a z(
+int b = a\N{});				// { dg-warning "empty named universal character escape sequence; treating it as separate tokens" "" { target c++23 } }
+int c = a\N{);				// { dg-warning "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{; treating it as separate tokens" "" { target c++23 } }
+int d = a\N);
+int e = a\NARG);
+int f = a\N{abc});			// { dg-warning "\\\\N\\\{abc\\\} is not a valid universal character; treating it as separate tokens" "" { target c++23 } }
+int g = a\N{ABC.123});			// { dg-warning "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{ABC; treating it as separate tokens" "" { target c++23 } }
+int h = a\N{NON-EXISTENT CHAR});	// { dg-error "is not a valid universal character" "" { target c++23 } }
+					// { dg-error "was not declared in this scope" "" { target c++23 } .-1 }
+int i = a\N{Latin_Small_Letter_A_With_Acute});	// { dg-warning "\\\\N\\\{Latin_Small_Letter_A_With_Acute\\\} is not a valid universal character; treating it as separate tokens" "" { target c++23 } }
+					// { dg-message "did you mean \\\\N\\\{LATIN SMALL LETTER A WITH ACUTE\\\}\\?" "" { target c++23 } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp23/named-universal-char-escape2.C b/gcc/testsuite/g++.dg/cpp23/named-universal-char-escape2.C
new file mode 100644
index 00000000000..8699e098c88
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/named-universal-char-escape2.C
@@ -0,0 +1,18 @@
+// P2071R2 - Named universal character escapes
+// { dg-do compile }
+// { dg-require-effective-target wchar }
+// { dg-options "" }
+
+#define z(x) 0
+#define a z(
+int b = a\N{});				// { dg-warning "empty named universal character escape sequence; treating it as separate tokens" }
+int c = a\N{);				// { dg-warning "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{; treating it as separate tokens" }
+int d = a\N);
+int e = a\NARG);
+int f = a\N{abc});			// { dg-warning "\\\\N\\\{abc\\\} is not a valid universal character; treating it as separate tokens" }
+int g = a\N{ABC.123});			// { dg-warning "'\\\\N\\\{' not terminated with '\\\}' after \\\\N\\\{ABC; treating it as separate tokens" }
+int h = a\N{NON-EXISTENT CHAR});	// { dg-error "is not a valid universal character" "" { target c++23 } }
+					// { dg-error "was not declared in this scope" "" { target c++23 } .-1 }
+					// { dg-warning "\\\\N\\\{NON-EXISTENT CHAR\\\} is not a valid universal character; treating it as separate tokens" "" { target c++20_down } .-2 }
+int i = a\N{Latin_Small_Letter_A_With_Acute});	// { dg-warning "\\\\N\\\{Latin_Small_Letter_A_With_Acute\\\} is not a valid universal character; treating it as separate tokens" }
+					// { dg-message "did you mean \\\\N\\\{LATIN SMALL LETTER A WITH ACUTE\\\}\\?" "" { target *-*-* } .-1 }
diff --git a/libcpp/charset.cc b/libcpp/charset.cc
index c9656dbbe15..6834969a919 100644
--- a/libcpp/charset.cc
+++ b/libcpp/charset.cc
@@ -1448,7 +1448,11 @@ _cpp_valid_ucn (cpp_reader *pfile, const uchar **pstr,
   if (str[-1] == 'u')
     {
       length = 4;
-      if (str < limit && *str == '{')
+      if (str < limit
+	  && *str == '{'
+	  && (!identifier_pos
+	      || CPP_OPTION (pfile, delimited_escape_seqs)
+	      || !CPP_OPTION (pfile, std)))
 	{
 	  str++;
 	  /* Magic value to indicate no digits seen.  */
@@ -1462,8 +1466,22 @@ _cpp_valid_ucn (cpp_reader *pfile, const uchar **pstr,
   else if (str[-1] == 'N')
     {
       length = 4;
+      if (identifier_pos
+	  && !CPP_OPTION (pfile, delimited_escape_seqs)
+	  && CPP_OPTION (pfile, std))
+	{
+	  *cp = 0;
+	  return false;
+	}
       if (str == limit || *str != '{')
-	cpp_error (pfile, CPP_DL_ERROR, "'\\N' not followed by '{'");
+	{
+	  if (identifier_pos)
+	    {
+	      *cp = 0;
+	      return false;
+	    }
+	  cpp_error (pfile, CPP_DL_ERROR, "'\\N' not followed by '{'");
+	}
       else
 	{
 	  str++;
@@ -1489,15 +1507,19 @@ _cpp_valid_ucn (cpp_reader *pfile, const uchar **pstr,
 
 	  if (str < limit && *str == '}')
 	    {
-	      if (name == str && identifier_pos)
+	      if (identifier_pos && name == str)
 		{
+		  cpp_warning (pfile, CPP_W_UNICODE,
+			       "empty named universal character escape "
+			       "sequence; treating it as separate tokens");
 		  *cp = 0;
 		  return false;
 		}
 	      if (name == str)
 		cpp_error (pfile, CPP_DL_ERROR,
 			   "empty named universal character escape sequence");
-	      else if (!CPP_OPTION (pfile, delimited_escape_seqs)
+	      else if ((!identifier_pos || strict)
+		       && !CPP_OPTION (pfile, delimited_escape_seqs)
 		       && CPP_OPTION (pfile, cpp_pedantic))
 		cpp_error (pfile, CPP_DL_PEDWARN,
 			   "named universal character escapes are only valid "
@@ -1515,27 +1537,51 @@ _cpp_valid_ucn (cpp_reader *pfile, const uchar **pstr,
 					   uname2c_tree, NULL);
 		  if (result == (cppchar_t) -1)
 		    {
-		      cpp_error (pfile, CPP_DL_ERROR,
-				 "\\N{%.*s} is not a valid universal "
-				 "character", (int) (str - name), name);
+		      bool ret = true;
+		      if (identifier_pos
+			  && (!CPP_OPTION (pfile, delimited_escape_seqs)
+			      || !strict))
+			ret = cpp_warning (pfile, CPP_W_UNICODE,
+					   "\\N{%.*s} is not a valid "
+					   "universal character; treating it "
+					   "as separate tokens",
+					   (int) (str - name), name);
+		      else
+			cpp_error (pfile, CPP_DL_ERROR,
+				   "\\N{%.*s} is not a valid universal "
+				   "character", (int) (str - name), name);
 
 		      /* Try to do a loose name lookup according to
 			 Unicode loose matching rule UAX44-LM2.  */
 		      char canon_name[uname2c_max_name_len + 1];
 		      result = _cpp_uname2c_uax44_lm2 ((const char *) name,
 						       str - name, canon_name);
-		      if (result != (cppchar_t) -1)
+		      if (result != (cppchar_t) -1 && ret)
 			cpp_error (pfile, CPP_DL_NOTE,
 				   "did you mean \\N{%s}?", canon_name);
 		      else
-			result = 0x40;
+			result = 0xC0;
+		      if (identifier_pos
+			  && (!CPP_OPTION (pfile, delimited_escape_seqs)
+			      || !strict))
+			{
+			  *cp = 0;
+			  return false;
+			}
 		    }
 		}
 	      str++;
 	      extend_char_range (char_range, loc_reader);
 	    }
 	  else if (identifier_pos)
-	    length = 1;
+	    {
+	      cpp_warning (pfile, CPP_W_UNICODE,
+			   "'\\N{' not terminated with '}' after %.*s; "
+			   "treating it as separate tokens",
+			   (int) (str - base), base);
+	      *cp = 0;
+	      return false;
+	    }
 	  else
 	    {
 	      cpp_error (pfile, CPP_DL_ERROR,
@@ -1584,12 +1630,17 @@ _cpp_valid_ucn (cpp_reader *pfile, const uchar **pstr,
       }
     while (--length);
 
-  if (delimited
-      && str < limit
-      && *str == '}'
-      && (length != 32 || !identifier_pos))
+  if (delimited && str < limit && *str == '}')
     {
-      if (length == 32)
+      if (length == 32 && identifier_pos)
+	{
+	  cpp_warning (pfile, CPP_W_UNICODE,
+		       "empty delimited escape sequence; "
+		       "treating it as separate tokens");
+	  *cp = 0;
+	  return false;
+	}
+      else if (length == 32)
 	cpp_error (pfile, CPP_DL_ERROR,
 		   "empty delimited escape sequence");
       else if (!CPP_OPTION (pfile, delimited_escape_seqs)
@@ -1607,6 +1658,11 @@ _cpp_valid_ucn (cpp_reader *pfile, const uchar **pstr,
      error message in that case.  */
   if (length && identifier_pos)
     {
+      if (delimited)
+	cpp_warning (pfile, CPP_W_UNICODE,
+		     "'\\u{' not terminated with '}' after %.*s; "
+		     "treating it as separate tokens",
+		     (int) (str - base), base);
       *cp = 0;
       return false;
     }
diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index 1a3fb19d797..c25bcf215e0 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -565,6 +565,10 @@ struct cpp_options
      2 if it should be a pedwarn.  */
   unsigned char cpp_warn_invalid_utf8;
 
+  /* True if libcpp should warn about invalid forms of delimited or named
+     escape sequences.  */
+  bool cpp_warn_unicode;
+
   /* True if -finput-charset= option has been used explicitly.  */
   bool cpp_input_charset_explicit;
 
@@ -675,7 +679,8 @@ enum cpp_warning_reason {
   CPP_W_CXX20_COMPAT,
   CPP_W_EXPANSION_TO_DEFINED,
   CPP_W_BIDIRECTIONAL,
-  CPP_W_INVALID_UTF8
+  CPP_W_INVALID_UTF8,
+  CPP_W_UNICODE
 };
 
 /* Callback for header lookup for HEADER, which is the name of a
diff --git a/libcpp/init.cc b/libcpp/init.cc
index 3e5601a5d96..629252445e1 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -228,6 +228,7 @@ cpp_create_reader (enum c_lang lang, cpp_hash_table *table,
   CPP_OPTION (pfile, warn_date_time) = 0;
   CPP_OPTION (pfile, cpp_warn_bidirectional) = bidirectional_unpaired;
   CPP_OPTION (pfile, cpp_warn_invalid_utf8) = 0;
+  CPP_OPTION (pfile, cpp_warn_unicode) = 1;
   CPP_OPTION (pfile, cpp_input_charset_explicit) = 0;
 
   /* Default CPP arithmetic to something sensible for the host for the