ParseXS: disallow const on non-C++ methods

It's possible for an XSUB's name to include a class, in which case the
XS parser will treat it as a C++ method and declare a THIS variable.
It's also possible for an XSUB to have a postfix 'const' modifier to
mimic similar in C++ method declarations. For example this XS declaration:

    void
    X::Y::foo(...) const

causes this C++ variable to be declared:

    const X__Y * THIS = ...;

The 'const' is supposed to be an optional modifier to a C++ XSUB but the
parser currently allows it it on plain XSUBs, e.g.

    void
    foo(...) const

which leads to 'uninit var' warnings during parsing, and to badly-formed
C code or weird errors about missing typemap entries.

This commit makes it an error to have 'const' on a non-C++ XSUB.

Author: iabyn

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS/Node.pm

@@ -877,8 +877,17 @@ sub parse {
         unless $func_header =~
             /^(?:([\w:]*)::)?(\w+)\s*\(\s*(.*?)\s*\)\s*(const)?\s*(;\s*)?$/s;
 
-    my ($class, $name, $params_text) = ($1, $2, $3);
-    $class = "$4 $class" if $4;
+    my ($class, $name, $params_text, $const) = ($1, $2, $3, $4);
+
+    if (defined $const) {
+        if (defined $class) {
+            $class = "$const $class";
+        }
+        else {
+            $pxs->blurt("const modifier only allowed on XSUBs which are C++ methods");
+            undef $const;
+        }
+    }
 
     if ($return_type->{static} and !defined $class)
     {

dist/ExtUtils-ParseXS/t/001-basic.t

@@ -1202,6 +1202,16 @@ EOF
             [ 0, 0, qr/\QTHIS->hhh(ddd)/,                    "autocall" ],
         ],
 
+        [
+            "C++: only const",
+            [
+                'void',
+                'foo() const',
+            ],
+            [ 1, 0, qr/\Qconst modifier only allowed on XSUBs which are C++ methods/,
+                "got expected err" ],
+        ],
+
         [
             "",
             [