Support C11 Unicode string literals and character constants (#452)

* Support C11 Unicode string literals and character constants

* Add tests for C11 string literals and character constants

* Better error message for ill-formed universal character names
E.g. \u followed by fewer than 4 hex digits, or \U followed by fewer than 8 hex digits.

* Add new warning `invalid-utf8` for byte sequences that are not valid UTF8.
The warning is activated but not fatal by default.

* Warn on uses of C11 Unicode character constants and string literals
This uses the `c11-extensions` warning, which is off by default.

* Support preprocessing option -finput-charset= for GNU toolchains

1 files changed, 50 insertions, 0 deletions

diff --git a/test/regression/charlit.c b/test/regression/charlit.c
new file mode 100644
index 00000000..5a7e0916
--- /dev/null
+++ b/test/regression/charlit.c
@@ -0,0 +1,50 @@
+#include <stdio.h>
+#include <wchar.h>
+#include <uchar.h>
+
+unsigned char c1 = 'a';
+char16_t c2 = u'a';;
+char32_t c3 = U'a';;
+wchar_t c4 = L'a';;
+
+unsigned char d1 = '\xFE';
+char16_t d2 = u'\xFE';;
+char32_t d3 = U'\xFE';;
+wchar_t d4 = L'\xFE';;
+
+unsigned char e1 = '\x1234'; // warning but no error
+char16_t e2 = u'\x1234';
+char32_t e3 = U'\x1234';
+wchar_t e4 = L'\x1234';
+
+unsigned char f1 = 'é';      // CompCert tolerance
+char16_t f2 = u'é';
+char32_t f3 = U'é';
+wchar_t f4 = L'é';
+
+unsigned char g1 = '猫';     // CompCert tolerance + warning
+char16_t g2 = u'猫';
+char32_t g3 = U'猫';
+wchar_t g4 = L'猫';
+
+unsigned char h1 = '🍌';     // CompCert tolerance + warning
+char16_t h2 = u'🍌';         // CompCert tolerance + warning
+char32_t h3 = U'🍌';
+wchar_t h4 = L'🍌';
+
+int m1 = 'ab';
+int m2 = '\x01\x02\x03\x04';
+int m3 = 'éè';              // CompCert tolerance
+
+#define PRINT(x) printf("%s: %x\n", #x, x)
+
+int main()
+{
+  PRINT(c1); PRINT(c2); PRINT(c3); PRINT(c4);
+  PRINT(d1); PRINT(d2); PRINT(d3); PRINT(d4);
+  PRINT(e1); PRINT(e2); PRINT(e3); PRINT(e4);
+  PRINT(f1); PRINT(f2); PRINT(f3); PRINT(f4);
+  PRINT(g1); PRINT(g2); PRINT(g3); PRINT(g4);
+  PRINT(h1); PRINT(h2); PRINT(h3); PRINT(h4);
+  PRINT(m1); PRINT(m2); PRINT(m3);
+}