fix(to_cpp1): don't emit [[nodiscard]] for pre-increment/decrement (#883)

* fix(to_cpp1): don't emit `[[nodiscard]]` for pre-increment/decrement

* Rename the new function, and move part of the test to a sema enforcement

* Tweak assert/check order

---------

Co-authored-by: Herb Sutter <[email protected]>

Author: JohelEGP

source/parse.h

@@ -2229,6 +2229,9 @@ struct function_type_node
     auto is_comparison() const
         -> bool;
 
+    auto is_increment_or_decrement() const
+        -> bool;
+
     auto is_compound_assignment() const
         -> bool;
 
@@ -3197,6 +3200,16 @@ struct declaration_node
         return false;
     }
 
+    auto is_increment_or_decrement() const
+        -> bool
+    {
+        if (auto func = std::get_if<a_function>(&type)) {
+            return (*func)->is_increment_or_decrement();
+        }
+        //  else
+        return false;
+    }
+
     auto is_compound_assignment() const
         -> bool
     {
@@ -3709,6 +3722,20 @@ auto function_type_node::is_comparison() const
 }
 
 
+auto function_type_node::is_increment_or_decrement() const
+    -> bool
+{
+    if (
+        my_decl->has_name("operator++")
+        || my_decl->has_name("operator--")
+        )
+    {
+        return true;
+    }
+    return false;
+}
+
+
 auto function_type_node::is_compound_assignment() const
     -> bool
 {

source/sema.h

@@ -1523,6 +1523,34 @@ class sema
     }
 
 
+    auto check(function_type_node const& n)
+        -> bool
+    {
+        //  An increment/decrement function must have a single parameter that is 'inout this'
+        if (
+            (
+                n.my_decl->has_name("operator++")
+                || n.my_decl->has_name("operator--")
+            )
+            &&
+            (
+               (*n.parameters).ssize() != 1
+            || !(*n.parameters)[0]->has_name("this")
+            || (*n.parameters)[0]->direction() != passing_style::inout
+            )
+            )
+        {
+            errors.emplace_back(
+                n.position(),
+                "a user-defined " + n.my_decl->name()->to_string() + " must have a single 'inout this' parameter"
+            );
+            return false;
+        }
+
+        return true;
+    }
+
+
     auto check(statement_node const& n)
         -> bool
     {

source/to_cpp1.h

@@ -4596,6 +4596,10 @@ class cppfront
     {   STACKINSTR
         assert(n.parameters);
 
+        if (!sema.check(n)) {
+            return;
+        }
+
         if (
             is_main
             && n.parameters->parameters.size() > 0
@@ -6093,6 +6097,7 @@ class cppfront
                     func->has_non_void_return_type()
                     && !func->is_assignment()
                     && !func->is_compound_assignment()
+                    && !func->is_increment_or_decrement()
                     && (
                         printer.get_phase() == printer.phase1_type_defs_func_decls
                         || n.has_initializer()  // so we're printing it in phase 2