Core: jQuery("#") is bad syntax

src/core.js

@@ -19,6 +19,7 @@ jQuery.fn.init = function( selector, context, rootjQuery ) {
 		if ( match[ 3 ] ) {
 			migrateWarn("$(html) HTML text after last tag is ignored");
 		}
+
 		// Consistently reject any HTML-like string starting with a hash (#9521)
 		// Note that this may break jQuery 1.6.x code that otherwise would work.
 		if ( match[ 0 ].charAt( 0 ) === "#" ) {
@@ -37,6 +38,12 @@ jQuery.fn.init = function( selector, context, rootjQuery ) {
 		}
 	}
 
+	// jQuery( "#" ) is a bogus ID selector, but it returned an empty set before jQuery 3.0
+	if ( selector === "#" ) {
+		migrateWarn( "jQuery( '#' ) is not a valid selector" );
+		selector = [];
+	}
+
 	ret = oldInit.apply( this, arguments );
 
 	// Fill in selector and context properties so .live() works

test/core.js

@@ -47,6 +47,15 @@ test( "jQuery(html) loose rules", function() {
 	}
 });
 
+test( "jQuery( '#' )", function() {
+	expect( 2 );
+
+	expectWarning( "Selector, through the jQuery constructor, nothing but hash", function() {
+		var set = jQuery( "#" );
+		equal( set.length, 0, "empty set" );
+	});
+});
+
 test( "selector state", function() {
 	expect( 18 );