fix: Invert reference type null check in makeIsTrueish (#1719)

Author: surma

Diff for: src/compiler.ts

@@ -9941,7 +9941,10 @@ export class Compiler extends DiagnosticEmitter {
       case TypeKind.EQREF:
       case TypeKind.DATAREF:
       case TypeKind.I31REF: {
-        return module.ref_is(RefIsOp.RefIsNull, expr);
+        // Needs to be true (i.e. not zero) when the ref is _not_ null,
+        // which means `ref.is_null` returns false (i.e. zero).
+        return module.unary(UnaryOp.EqzI32, module.ref_is(RefIsOp.RefIsNull, expr));
+
       }
       default: {
         assert(false);

Diff for: tests/compiler/features/reference-types.js

@@ -9,6 +9,12 @@ exports.preInstantiate = function(imports, exports) {
     external: function(a) {
       return a;
     },
+    somethingReal() {
+      return {};
+    },
+    somethingNull() {
+      return null;
+    },
     someObject: {
       theKey: "Hello world!"
     },

Diff for: tests/compiler/features/reference-types.ts

@@ -1,6 +1,8 @@
 // can use externref as a parameter or return type
 
 export declare function external(a: externref): externref;
+export declare function somethingReal(): externref;
+export declare function somethingNull(): externref;
 
 export function internal(a: externref): externref {
   const b = external(a);
@@ -26,6 +28,44 @@ console.log(someObject);
 console.log(someKey);
 console.log(Reflect.get(someObject, someKey));
 
+// Truthiness conversion
+if(!somethingReal()) {
+  assert(false);
+}
+if(!somethingNull()) {
+  // nop
+} else {
+  assert(false);
+}
+if(somethingReal()) {
+  // nop
+} else {
+  assert(false);
+}
+if(somethingNull()) {
+  assert(false);
+}
+
+// Explicit null checks (don’t work yet)
+/*
+if(somethingReal() !== null) {
+  // nop
+} else {
+  assert(false);
+}
+if(somethingReal() === null) {
+  assert(false);
+}
+if(somethingNull() === null) {
+  // nop
+} else {
+  assert(false);
+}
+if(somethingNull() !== null) {
+  assert(false);
+}
+*/
+
 // can represent and recognize 'null'
 
 var funcGlobal: funcref;