Update [[nodiscard]] default for streaming to match established convention

We already do this for assignment: As with assignment, the convention for streaming operators is to return a reference to the stream, to permit chaining. But ignoring that return value at the end of each chain is always expected and always correct, and so those return values should always be discardable.

Author: hsutter

regression-tests/test-results/pure2-enum.cpp

@@ -157,7 +157,7 @@ inline constexpr skat_game skat_game::null = 23;
     return "invalid skat_game value"; 
     }
 
-    [[nodiscard]] auto operator<<(std::ostream& o, cpp2::in<skat_game> val) -> std::ostream&{o << CPP2_UFCS_0(to_string, val);return o; }
+    auto operator<<(std::ostream& o, cpp2::in<skat_game> val) -> std::ostream&{o << CPP2_UFCS_0(to_string, val);return o; }
 #line 15 "pure2-enum.cpp2"
     auto duality::flip(cpp2::in<duality> val) & -> void{
         if (val == first) {
@@ -199,7 +199,7 @@ inline constexpr duality duality::second = 1;
     return "invalid duality value"; 
     }
 
-    [[nodiscard]] auto operator<<(std::ostream& o, cpp2::in<duality> val) -> std::ostream&{o << CPP2_UFCS_0(to_string, val);return o; }
+    auto operator<<(std::ostream& o, cpp2::in<duality> val) -> std::ostream&{o << CPP2_UFCS_0(to_string, val);return o; }
 inline constexpr cpp2::u8 file_attributes::enumeration_count = 4;
 
 constexpr file_attributes::file_attributes(auto const& val)
@@ -250,7 +250,7 @@ inline constexpr file_attributes file_attributes::none = 0;
     return ret + ")"; 
     }
 
-    [[nodiscard]] auto operator<<(std::ostream& o, cpp2::in<file_attributes> val) -> std::ostream&{o << CPP2_UFCS_0(to_string, val);return o; }
+    auto operator<<(std::ostream& o, cpp2::in<file_attributes> val) -> std::ostream&{o << CPP2_UFCS_0(to_string, val);return o; }
 #line 33 "pure2-enum.cpp2"
 auto main() -> int{
     auto janus {duality::second}; 

source/build.info

@@ -1 +1 @@
-"8923:1630"
+"8924:1404"

source/cppfront.cpp

@@ -5666,6 +5666,10 @@ class cppfront
                         !emit_as_friend         // can't have an attribute on a friend declaration-not-definition
                         || printer.get_phase() != printer.phase1_type_defs_func_decls
                         )
+                    && !(
+                        n.name()
+                        && is_streaming_operator(n.name()->as_string_view())
+                        )
                     )
                 {
                     printer.print_cpp2( "[[nodiscard]] ", n.position() );