Fix a preprocessor expression evaluation bug

A subexpression in parentheses lost its string/int type flag and
instead used whatever type was left in the stack entry from
previous use.  In practice we mostly got away with this because
most preprocessor expressions are integer, but it could have
resulted in a preprocessor expression incorrectly evaluating as
zero.  If -Wextra was in use you got a warning:

Warning 202: Error: 'Can't mix strings and integers in expression'

Fixes swig#1384

Author: ojwb

CHANGES.current

@@ -7,6 +7,17 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-02-23: olly
+	    #1384 Fix a preprocessor expression evaluation bug.  A
+	    subexpression in parentheses lost its string/int type flag and
+	    instead used whatever type was left in the stack entry from
+	    previous use.  In practice we mostly got away with this because
+	    most preprocessor expressions are integer, but it could have
+	    resulted in a preprocessor expression incorrectly evaluating as
+	    zero.  If -Wextra was in use you got a warning:
+
+	    Warning 202: Error: 'Can't mix strings and integers in expression'
+
 2022-02-20: wsfulton
             Fix %warnfilter warning suppress for warning 315 SWIGWARN_PARSE_USING_UNDEF.
 

Examples/test-suite/preproc.i

@@ -11,6 +11,22 @@
 #pragma SWIG nowarn=890                                      /* lots of Go name conflicts */
 #pragma SWIG nowarn=206                                      /* Unexpected tokens after #endif directive. */
 
+/* Regression test: in SWIG < 4.1.0 this triggered the two #error cases.
+ * Github issue #1384
+ */
+#if "" != ""
+#endif
+#if 1 && (!0)
+// Should go here
+#else
+# error BUG
+#endif
+#if ((("" == ""))) || (1 && (!1))
+// Should go here
+#else
+# error BUG
+#endif
+
 %{
 #if defined(__clang__)
 /*Suppress: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]*/

Source/Preprocessor/expr.c

@@ -406,6 +406,7 @@ int Preprocessor_expr(DOH *s, int *error) {
 	  goto extra_rparen;
 	stack[sp - 1].op = EXPR_VALUE;
 	stack[sp - 1].value = stack[sp].value;
+	stack[sp - 1].svalue = stack[sp].svalue;
 	sp--;
 	break;
       default: