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

CHANGES.current

@@ -7,6 +7,20 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.2.0 (in progress)
 ===========================
 
+2022-11-26: wsfulton
+            #1589 #1590 Slightly better decltype() support for expressions, 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.
+
+
 2022-11-25: wsfulton
             #961 Fix syntax error parsing unnamed template parameters with a default value.
 

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;  // syntax error
+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;  // 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

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.

Examples/test-suite/common.mk

@@ -624,6 +624,7 @@ CPP11_TEST_CASES += \
 	cpp11_thread_local \
 	cpp11_template_double_brackets \
 	cpp11_template_explicit \
+	cpp11_template_parameters_decltype \
 	cpp11_template_typedefs \
 	cpp11_type_traits \
 	cpp11_type_aliasing \

Examples/test-suite/cpp11_decltype.i

@@ -9,7 +9,7 @@
     int i;
     decltype(i) j;
 
-    auto foo( decltype(i) a ) -> decltype(i) {
+    auto get_number(decltype(i) a) -> decltype(i) {
       if (a==5)
         return 10;
       else
@@ -17,3 +17,27 @@
     }
   };
 %}
+
+
+// These are ignored as unable to deduce decltype for (i+j)
+%ignore B::k;
+%ignore B::get_number_sum;
+%ignore B::get_number_address;
+#pragma SWIG nowarn=SWIGWARN_CPP11_DECLTYPE
+
+%inline %{
+  class B {
+  public:
+    int i;
+    decltype(i) j;
+    decltype(i+j) k;
+
+    auto get_number_sum(decltype(i+j) a) -> decltype(i+j) {
+      return i+j;
+    }
+
+    auto get_number_address(decltype(&i) a) -> decltype(&i) {
+      return &i;
+    }
+  };
+%}

Examples/test-suite/python/cpp11_decltype_runme.py

@@ -9,10 +9,10 @@ a.j = 10
 if a.j != 10:
     raise RuntimeError("Assignment to a.j failed.")
 
-b = a.foo(5)
+b = a.get_int(5)
 if b != 10:
-    raise RuntimeError("foo(5) should return 10.")
+    raise RuntimeError("get_int(5) should return 10.")
 
-b = a.foo(6)
+b = a.get_int(6)
 if b != 0:
-    raise RuntimeError("foo(6) should return 0.")
+    raise RuntimeError("get_int(6) should return 0.")

Source/CParse/parser.y

@@ -6241,11 +6241,12 @@ type_right     : primitive_type { $$ = $1;
                }
                ;
 
-decltype       : DECLTYPE LPAREN idcolon RPAREN {
-                 Node *n = Swig_symbol_clookup($3,0);
+decltype       : DECLTYPE LPAREN expr RPAREN {
+                 Node *n = Swig_symbol_clookup($3.val, 0);
                  if (!n) {
-		   Swig_error(cparse_file, cparse_line, "Identifier %s not defined.\n", $3);
-                   $$ = $3;
+		   Swig_warning(WARN_CPP11_DECLTYPE, cparse_file, cparse_line, "Unable to deduce decltype for '%s'.\n", $3.val);
+
+		   $$ = NewStringf("decltype(%s)", $3.val);
                  } else {
                    $$ = Getattr(n, "type");
                  }

Source/Include/swigwarn.h

@@ -99,6 +99,7 @@
 #define WARN_CPP11_ALIAS_DECLARATION  341  /* redundant now */
 #define WARN_CPP11_ALIAS_TEMPLATE     342  /* redundant now */
 #define WARN_CPP11_VARIADIC_TEMPLATE  343
+#define WARN_CPP11_DECLTYPE           344
 
 #define WARN_IGNORE_OPERATOR_NEW        350	/* new */
 #define WARN_IGNORE_OPERATOR_DELETE     351	/* delete */