parser: do no allow var names with __; cgen: use __argc

vlib/builtin/array.v

@@ -506,12 +506,12 @@ pub fn (a []char) index(v char) int {
 // []int.reduce executes a given reducer function on each element of the array,
 // resulting in a single output value.
 pub fn (a []int) reduce(iter fn(accum, curr int)int, accum_start int) int {
-	mut _accum := accum_start
+	mut accum_ := accum_start
 	for i in a {
-		_accum = iter(_accum, i)
+		accum_ = iter(accum_, i)
 	}
 
-	return _accum
+	return accum_
 }
 
 // array_eq<T> checks if two arrays contain all the same elements in the same order.

vlib/v/gen/cgen.v

@@ -757,9 +757,9 @@ fn (g mut Gen) gen_fn_decl(it ast.FnDecl) {
 	is_main := it.name == 'main'
 	if is_main {
 		if g.pref.os == .windows {
-			g.write('int wmain(int argc, wchar_t *argv[], wchar_t *envp[]')
+			g.write('int wmain(int __argc, wchar_t *__argv[], wchar_t *__envp[]')
 		} else {
-			g.write('int ${it.name}(int argc, char** argv')
+			g.write('int ${it.name}(int __argc, char** __argv')
 		}
 	}
 	else {
@@ -817,9 +817,9 @@ fn (g mut Gen) gen_fn_decl(it ast.FnDecl) {
 				g.writeln('free(_const_os__args.data); // empty, inited in _vinit()')
 			}
 			if g.pref.os == .windows {
-				g.writeln('_const_os__args = os__init_os_args_wide(argc, argv);')
+				g.writeln('_const_os__args = os__init_os_args_wide(__argc, __argv);')
 			}	else {
-				g.writeln('_const_os__args = os__init_os_args(argc, (byteptr*)argv);')
+				g.writeln('_const_os__args = os__init_os_args(__argc, (byteptr*)__argv);')
 			}
 		}
 	}

vlib/v/parser/parser.v

@@ -1686,6 +1686,9 @@ fn (p mut Parser) assign_stmt() ast.Stmt {
 			p.error('unknown variable `$ident.name`')
 		}
 		if is_decl && ident.kind != .blank_ident {
+			if ident.name.starts_with('__') {
+				p.error('variable names cannot start with `__`')
+			}
 			if p.scope.known_var(ident.name) {
 				p.error('redefinition of `$ident.name`')
 			}