Slightly better decltype() support for expressions

decltype now accepts C++ expressions instead of just an ID, such as:

  int i,j;
  ...  decltype(i+j) ...
  ...  decltype(&i) ...

These result in a warning for non-trivial expressions which SWIG cannot evaluate:

  Warning 344: Unable to deduce decltype for 'i+j'.

See 'Type Inference' in CPlusPlus.html for workarounds.

Issue #1589
Issue #1590

2 files changed, 34 insertions, 4 deletions

diff --git a/Doc/Manual/CPlusPlus11.html b/Doc/Manual/CPlusPlus11.html
index db47ee9dc..50402cefd 100644
--- a/Doc/Manual/CPlusPlus11.html
+++ b/Doc/Manual/CPlusPlus11.html
@@ -720,18 +720,45 @@ AltStruct var2{2, 4.3};   // calls the constructor
 <H3><a name="CPlusPlus11_type_inference">7.2.6 Type inference</a></H3>
 
 
-<p>SWIG supports <tt>decltype()</tt> with some limitations. Single
-variables are allowed, however, expressions are not supported yet. For
+<p>SWIG supports <tt>decltype()</tt> with limitations. Single
+variables are allowed, however, non-trivial expressions are not supported very well. For
 example, the following code will work:</p>
 <div class="code"><pre>
 int i;
 decltype(i) j;
 </pre></div>
 
-<p>However, using an expression inside the decltype results in syntax error:</p>
+<p>SWIG is able to deduce that the variable, <tt>i</tt>, is type <tt>int</tt>.
+</p>
+
+<p>
+Using a non-trivial expression for the decltype results in a warning:</p>
+<div class="code"><pre>
+int i; int j;
+decltype(i+j) k;  // Warning 344: Unable to deduce decltype for 'i+j'.
+</pre></div>
+
+<p>
+This warning should be viewed as  a prompt to add in a manual ignore of the variable/function as
+in most cases the generated code will not compile.
+For the example above, ignore the symbol that uses decltype and perhaps additionally
+suppress the warning as follows:
+</p>
+
+<div class="code"><pre>
+#pragma SWIG nowarn=SWIGWARN_CPP11_DECLTYPE
+%ignore k;
+</pre></div>
+
+<p>
+If an ignore is not acceptable, a workaround is to redefine the symbol with the actual type, for example:
+</p>
+
 <div class="code"><pre>
+int k; // define k with the actual type
+%ignore k; // ignore the real definition of k
 int i; int j;
-decltype(i+j) k;  // syntax error
+decltype(i+j) k;  // Warning 344: Unable to deduce decltype for 'i+j'.
 </pre></div>
 
 <p>SWIG does not support <tt>auto</tt> as a type specifier for variables, only
diff --git a/Doc/Manual/Warnings.html b/Doc/Manual/Warnings.html
index 9d5361dae..1cdf37ec2 100644
--- a/Doc/Manual/Warnings.html
+++ b/Doc/Manual/Warnings.html
@@ -438,6 +438,9 @@ example.i(4) : Syntax error in input(1).
 <li>325. Nested <em>kind</em> not currently supported (<em>name</em> ignored).
 <li>326. Deprecated %extend name used - the <em>kind</em> name '<em>name</em>' should be used instead of the typedef name '<em>name</em>'.
 <li>327. Extern template ignored.
+<li>340. Lambda expressions and closures are not fully supported yet.
+<li>343. Only the first variadic template argument is currently supported.
+<li>344. Unable to deduce decltype for '<em>expr</em>'.
 <li>350. operator new ignored.
 <li>351. operator delete ignored.
 <li>352. operator+ ignored.