auto without return type cleanup

- Fix undefined behaviour
- Document and test workaround for class methods
- More informative WARN_CPP14_AUTO warning message

See issue #2446

Doc/Manual/CPlusPlus14.html

@@ -54,13 +54,13 @@ int b = 0b101011;
 </pre>
 </div>
 
-<H3><a href="CPlusPlus14_return_type_deduction">8.2.2 Return type deduction</a></H3>
+<H3><a name="CPlusPlus14_return_type_deduction">8.2.2 Return type deduction</a></H3>
 
 <p>
 C++14 added the ability to specify <tt>auto</tt> for the return type of a function
 and have the compiler deduce it from the body of the function (in C++11 you had
 to explicitly specify a trailing return type if you used <tt>auto</tt> for the
-return type.
+return type).
 </p>
 
 <p>
@@ -79,20 +79,36 @@ an actual example:
 </p>
 
 <div class="code"><pre>
-std::tuple<int, int> va_static_cast();
+std::tuple&lt;int, int&gt; va_static_cast();
 %ignore va_static_cast();
 #pragma SWIG nowarn=SWIGWARN_CPP14_AUTO
 
 %inline %{
-#include <tuple>
+#include &lt;tuple&gt;
 
-auto va_static_cast()
-{
+auto va_static_cast() {
     return std::make_tuple(0, 0);
 }
 %}
 </pre></div>
 
+<p>
+For member methods the trick is to use <tt>%extend</tt> to redeclare the method and call it as follows:
+</p>
+
+<div class="code"><pre>
+%extend X {
+  const char * a() const { return $self-&gt;a(); }
+}
+%inline %{
+struct X {
+  auto a() const {
+    return "a string";
+  }
+};
+%}
+</pre></div>
+
 <p>
 <b>Compatibility note:</b> SWIG-4.2.0 first introduced support for functions declared with an auto return without a trailing return type.
 </p>

Doc/Manual/Warnings.html

@@ -440,7 +440,7 @@ example.i(4) : Syntax error in input(1).
 <li>327. Extern template ignored.
 <li>340. Lambda expressions and closures are not fully supported yet.
 <li>344. Unable to deduce decltype for '<em>expr</em>'.
-<li>345. Unable to deduce return type for '<em>name</em>'.
+<li>345. Unable to deduce return type for auto in '<em>name</em>' (ignored).
 <li>346. Unable to deduce type for variable '<em>name</em>'.
 <li>350. operator new ignored.
 <li>351. operator delete ignored.

Examples/test-suite/cpp14_auto_return_type.i

@@ -7,14 +7,17 @@
 // declaration with an explicit return type in the interface file.
 
 namespace teca_variant_array_util {
+// Workaround to wrap a global function containing an auto return type with no trailing return type
 int va_static_cast();
 %ignore va_static_cast();
 
-// Not seeing a way to supply a declaration for a class method that makes
-// these work, but they can be ignored at least.
+// TODO: %ignore does not work as the declaration is not added to the parse tree
 %ignore X::a();
-%ignore X::b();
 %ignore X::s();
+// Workaround to wrap a class method containing an auto return type with no trailing return type
+%extend X {
+  const char * a() const { return $self->a(); }
+}
 }
 #pragma SWIG nowarn=SWIGWARN_CPP14_AUTO
 
@@ -27,8 +30,8 @@ auto va_static_cast()
   return 42;
 }
 struct X {
-  auto a() {
-    return "a";
+  auto a() const {
+    return "a string";
   }
   auto e() const {
     return 2.71828;

Examples/test-suite/python/cpp14_auto_return_type_runme.py

@@ -0,0 +1,10 @@
+from cpp14_auto_return_type import *
+
+sc = va_static_cast()
+if sc != 42:
+    raise RuntimeError("va_static_cast fail {}".format(sc))
+
+x = X()
+a = x.a()
+if a != "a string":
+    raise RuntimeError("x.a fail {}".format(a))

Source/CParse/parser.y

@@ -3360,7 +3360,8 @@ c_decl  : storage_class type declarator cpp_const initializer c_decl_tail {
             * to wrap.
             */
 	   | storage_class AUTO declarator cpp_const LBRACE {
-	      Swig_warning(WARN_CPP14_AUTO, cparse_file, cparse_line, "Unable to deduce return type for '%s'.\n", $3.id);
+	      Swig_warning(WARN_CPP14_AUTO, cparse_file, cparse_line, "Unable to deduce return type for auto in '%s' (ignored).\n", $3.id);
+	      $$ = 0;
 	      if (skip_balanced('{','}') < 0) Exit(EXIT_FAILURE);
 	   }
 	   /* C++11 auto variable declaration. */