__builtin_constant_p: properly test, fix behaviour

We previously only compile-time tested what required a run-time check of
assertions, so move this to the "cbmc" regression test suite. Also,
extend it by behaviour that distinguishes it from actual C-standard
specified constant expressions. Finally, fix the behaviour for string
literals.

regression/cbmc/gcc_builtin_constant_p1/main.c

@@ -0,0 +1,45 @@
+#include <assert.h>
+
+enum
+{
+  E1 = 1
+} var;
+
+struct whatnot
+{
+} whatnot_var;
+
+int main()
+{
+  // this is gcc only
+
+#ifdef __GNUC__
+  assert(__builtin_constant_p("some string"));
+  assert(__builtin_constant_p(1.0f));
+  assert(__builtin_constant_p(E1));
+  assert(!__builtin_constant_p(var));
+  assert(!__builtin_constant_p(main));
+  assert(!__builtin_constant_p(whatnot_var));
+  assert(!__builtin_constant_p(&var));
+  assert(__builtin_constant_p(__builtin_constant_p(var)));
+
+  // The following are not constant expressions in the sense of the C standard
+  // and GCC wouldn't deem them constant expressions either, but they are
+  // subject to GCC's constant folding. See also regression test ansi-c/sizeof6.
+  // Clang's behaviour, however, is somewhat different. See
+  // https://github.com/llvm/llvm-project/issues/55946 for further examples of
+  // where they differ.
+  int j;
+#  ifndef __clang__
+  assert(__builtin_constant_p(j * 0));
+  assert(__builtin_constant_p(j - j));
+  assert(__builtin_constant_p(j ? 0ll : 0ll));
+#  endif
+  assert(__builtin_constant_p(0 ? j : 0ll));
+
+  // side-effects are _not_ evaluated
+  int i = 0;
+  assert(!__builtin_constant_p(i++));
+  assert(i == 0);
+#endif
+}

regression/ansi-c/gcc_builtin_constant_p1/test.desc → regression/cbmc/gcc_builtin_constant_p1/test.desc

@@ -1,6 +1,7 @@
-CORE gcc-only broken-test-c++-front-end
+CORE
 main.c
 
+^VERIFICATION SUCCESSFUL$
 ^EXIT=0$
 ^SIGNAL=0$
 --

src/ansi-c/c_typecheck_expr.cpp

@@ -3682,6 +3682,8 @@ exprt c_typecheck_baset::do_special_functions(
     {
       is_constant=true;
     }
+    else if(tmp1.id() == ID_string_constant)
+      is_constant = true;
     else
       is_constant=tmp1.is_constant();