C front-end: typecheck conditional operator over string literal and 0

0 is a valid null-pointer constant, even in the absence of an explicit
type cast.

Author: tautschnig

regression/ansi-c/null_pointer_constant1/main.c

@@ -0,0 +1,6 @@
+int main(int argc, char *argv[])
+{
+  // Literal 0 is a valid null-pointer constant, which makes this conditional
+  // operator well-formed.
+  char *maybe_str = argc > 1 ? "args" : 0;
+}

regression/ansi-c/null_pointer_constant1/test.desc

@@ -0,0 +1,8 @@
+CORE test-c++-front-end
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/c_typecheck_expr.cpp

@@ -1594,6 +1594,20 @@ void c_typecheck_baset::typecheck_expr_trinary(if_exprt &expr)
     return;
   }
 
+  if(operands[1].id() == ID_string_constant)
+  {
+    implicit_typecast(
+      operands[1],
+      pointer_type(to_array_type(operands[1].type()).element_type()));
+  }
+
+  if(operands[2].id() == ID_string_constant)
+  {
+    implicit_typecast(
+      operands[2],
+      pointer_type(to_array_type(operands[2].type()).element_type()));
+  }
+
   if(operands[1].type().id()==ID_pointer &&
      operands[2].type().id()!=ID_pointer)
     implicit_typecast(operands[2], operands[1].type());