http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Bug #: 55425 Summary: constexpr does not work in many situations (both built-in and user supplied) Classification: Unclassified Product: gcc Version: 4.7.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: m8r-ug8...@mailinator.com constexpr expressions/functions do not work in many cases because of compiler wrongly interpreting the code. Seen in gcc-MinGW 4.7.0 and 4.7.1. Situation 1: __func__ -------------------------- A return statement is not a return statement if the returned value is __func__ (also true for non-standard identifiers like __PRETTY_FUNCTION__). // good //static const char func[] = "function-name"; //constexpr const char* x() { return func; } // bad constexpr const char* x() { return __func__;} int main() { __builtin_puts(x()); return 0; } Compiler output: ---------------- error: body of constexpr function 'constexpr const char* x()' not a return-statement Cross-reference to corresponding MinGW ticket: http://sourceforge.net/tracker/index.php?func=detail&aid=3471328&group_id=2435&atid=102435 Situation 2: user literals -------------------------- The (obviously constant) string that the compiler builds from the literal is not constant according to the compiler: #include <stdio.h> constexpr int valid_bin_number(const char* c) { return *c ? ((*c == '1' || *c == '0') ? valid_bin_number(c+1) : false ) : true; } unsigned int operator"" _bin(const char* str) { static_assert(valid_bin_number(str), "not a binary number"); unsigned int ret = 0; for(unsigned int i = 0; str[i] != '\0'; ++i) { char digit = str[i]; ret = ret * 2 + (digit - '0'); } return ret; } int main() { unsigned int a = 10000_bin; (void) a; return 0; } Compiler output: ---------------- In function 'unsigned int operator"" _bin(const char*)': error: non-constant condition for static assertion error: 'str' is not a constant expression Cross-reference to corresponding MinGW ticket: http://sourceforge.net/tracker/?func=detail&atid=102435&aid=3582841&group_id=2435 Situation 3: __m128i type -------------------------- Assigning a literal value to a constexpr __m128 fails because the literal is not a literal. #include <emmintrin.h> constexpr unsigned int a[] = { 5, 3}; // works (of course) constexpr float b[] = { 1.1, 3.7 }; // works, and no warning?! __m128i c[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8, 0x7795b179 } }; const __m128i d[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8, 0x7795b179 } }; static const __m128i e[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8, 0x7795b179 } }; namespace { const __m128i f[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8, 0x7795b179 } }; } constexpr __m128i g[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8, 0x7795b179 } }; // <---- fails int main() { return 0; } Compiler output: ---------------- error: the type 'const __m128i [] {aka const __vector(2) long long int []}' of constexpr variable 'g' is not literal