Don't error if someone returns a built-in function from their module

This is getting close to the point of "don't do that, just wrap the
built-in". But since it's a regression in a recent patch, let's restore
the old behavior here.

Bug: T236092
Change-Id: Ieddc23d942bc91fd0246ae14d8a4af7719e3834f

includes/engines/LuaCommon/lualib/mw.lua

@@ -496,7 +496,16 @@ function mw.executeModule( chunk, name, frame )
 end
 
 function mw.executeFunction( chunk )
-	local frame = getfenv( chunk ).mw.getCurrentFrame()
+	local getCurrentFrame = getfenv( chunk ).mw.getCurrentFrame
+	local frame
+	if getCurrentFrame then
+		-- Normal case
+		frame = getCurrentFrame()
+	else
+		-- If someone assigns a built-in method to the module's return table,
+		-- its env won't have mw.getCurrentFrame()
+		frame = newFrame( 'current', 'parent' )
+	end
 
 	if executeFunctionDepth == 0 then
 		-- math.random is defined as using C's rand(), and C's rand() uses 1 as

tests/phpunit/engines/LuaCommon/CommonTest.php

@@ -811,6 +811,23 @@ class Scribunto_LuaCommonTest extends Scribunto_LuaEngineTestBase {
 			$this->assertTrue( mb_check_encoding( $err, 'UTF-8' ), 'JS config vars are UTF-8' );
 		}
 	}
+
+	public function testT236092() {
+		$engine = $this->getEngine();
+		$parser = $engine->getParser();
+		$pp = $parser->getPreprocessor();
+
+		$this->extraModules['Module:T236092'] = '
+			local p = {}
+			p.foo = mw.isSubsting
+			return p
+		';
+
+		$frame = $pp->newFrame();
+		$text = $frame->expand( $pp->preprocessToObj( ">{{#invoke:T236092|foo}}<" ) );
+		$text = $parser->mStripState->unstripBoth( $text );
+		$this->assertSame( '>false<', $text );
+	}
 }
 
 class Scribunto_LuaCommonTestsLibrary extends Scribunto_LuaLibraryBase {