Fix incorrectly deduced type for function call

We were incorrectly deducing the internal type code T_USER when it
should have been T_UNKNOWN (or the code for the actual return type
but currently SWIG can't deduce the return type of a function call).

Bug introduced in 4.2.0 by b5ca500b40.

Fixes #2783

CHANGES.current

@@ -7,6 +7,10 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.2.1 (in progress)
 ===========================
 
+2024-01-31: olly
+	    #2783 Fix incorrectly deduced type for function call.  Regression
+	    introduced in 4.2.0.
+
 2024-01-27: wsfulton
             [Python] Fix compilation error when wrapping two or more classes that
             have the same friend operator overload when the classes are in a namespace.

Source/CParse/parser.y

@@ -6882,7 +6882,15 @@ exprcompound   : expr PLUS expr {
 		 }
 		 $$.val = NewStringf("%s%s",qty,scanner_ccode);
 		 Clear(scanner_ccode);
+		 // Try to deduce the type - this could be a C++ "constructor
+		 // cast" such as `double(4)` or a function call such as
+		 // `some_func()`.  In the latter case we get T_USER, but that
+		 // is wrong so we map it to T_UNKNOWN until we can actually
+		 // deduce the return type of a function call (which is
+		 // complicated because the return type can vary between
+		 // overloaded forms).
 		 $$.type = SwigType_type(qty);
+		 if ($$.type == T_USER) $$.type = T_UNKNOWN;
 		 Delete(qty);
                }
                ;