Support deducing decltype for false and true

CHANGES.current

@@ -1,12 +1,12 @@
-Below are the changes for the current release.
-See the CHANGES file for changes in older releases.
-See the RELEASENOTES file for a summary of changes in each release.
-Issue # numbers mentioned below can be found on Github. For more details, add
 the issue number to the end of the URL: https://github.com/swig/swig/issues/
 
 Version 4.2.0 (in progress)
 ===========================
 
+2023-05-20: olly
+	    Support deducing the type for `decltype(false)` and
+	    `decltype(true)`.
+
 2023-05-19: olly
 	    #2446 Add support for auto without trailing return type, which is a
 	    C++14 feature.

Doc/Manual/CPlusPlus11.html

@@ -720,9 +720,9 @@ 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 limitations. Single
-variables are allowed, however, non-trivial expressions are not supported very well. For
-example, the following code will work:</p>
+<p><tt>decltype()</tt> is supported with limitations.  SWIG can parse
+all uses, but can only deduce the type for single variables, <tt>true</tt> and
+<tt>false</tt>. For example, the following code will work:</p>
 <div class="code"><pre>
 int i;
 decltype(i) j;

Examples/test-suite/cpp11_decltype.i

@@ -26,11 +26,14 @@
 #pragma SWIG nowarn=SWIGWARN_CPP11_DECLTYPE
 
 %inline %{
+#define DECLARE(VAR, VAL) decltype(VAL) VAR = VAL
   class B {
   public:
     int i;
     decltype(i) j;
     decltype(i+j) k;
+    DECLARE(a, false);
+    DECLARE(b, true);
 
     auto get_number_sum(decltype(i+j) a) -> decltype(i+j) {
       return i+j;
@@ -39,5 +42,9 @@
     auto get_number_address(decltype(&i) a) -> decltype(&i) {
       return &i;
     }
+
+    auto negate(decltype(true) b) -> decltype(b) {
+      return !b;
+    }
   };
 %}

Examples/test-suite/php/cpp11_decltype_runme.php

@@ -16,8 +16,18 @@ check::equal($a->i, 5, 'Assignment to $a->i failed.');
 $a->j = 10;
 check::equal($a->j, 10, 'Assignment to $a->j failed.');
 
-$b = $a->get_number(5);
-check::equal($b, 10, 'get_number(5) should return 10.');
+$n = $a->get_number(5);
+check::equal($n, 10, 'get_number(5) should return 10.');
 
-$b = $a->get_number(6);
-check::equal($b, 0, 'get_number(6) should return 0.');
+$n = $a->get_number(6);
+check::equal($n, 0, 'get_number(6) should return 0.');
+
+$b = new B();
+
+check::equal($b->a, false);
+
+check::equal($b->b, true);
+
+check::equal($b->negate(true), false);
+
+check::equal($b->negate(false), true);

Examples/test-suite/python/cpp11_decltype_runme.py

@@ -9,10 +9,24 @@ a.j = 10
 if a.j != 10:
     raise RuntimeError("Assignment to a.j failed.")
 
-b = a.get_number(5)
-if b != 10:
+n = a.get_number(5)
+if n != 10:
     raise RuntimeError("get_number(5) should return 10.")
 
-b = a.get_number(6)
-if b != 0:
+n = a.get_number(6)
+if n != 0:
     raise RuntimeError("get_number(6) should return 0.")
+
+b = cpp11_decltype.B()
+
+if b.a != False:
+    raise RuntimeError("b.a should be False")
+
+if b.b != True:
+    raise RuntimeError("b.b should be True")
+
+if b.negate(True) != False:
+    raise RuntimeError("b.negate(True) should return False")
+
+if b.negate(False) != True:
+    raise RuntimeError("b.negate(False) should return True")

Source/CParse/parser.y

@@ -6253,12 +6253,14 @@ decltype       : DECLTYPE LPAREN {
 
 decltypeexpr   : expr RPAREN {
 		 Node *n = Swig_symbol_clookup($1.val, 0);
-		 if (!n) {
+		 if (n) {
+		   $$ = Getattr(n, "type");
+		 } else if (Equal($1.val, "true") || Equal($1.val, "false")) {
+		   $$ = NewString("bool");
+		 } else {
 		   Swig_warning(WARN_CPP11_DECLTYPE, cparse_file, cparse_line, "Unable to deduce decltype for '%s'.\n", $1.val);
 
 		   $$ = NewStringf("decltype(%s)", $1.val);
-		 } else {
-		   $$ = Getattr(n, "type");
 		 }
 	       }
 	       | error RPAREN {