this can be nullish

src/compiler/checker.ts

@@ -39818,6 +39818,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             case SyntaxKind.NewExpression:
             case SyntaxKind.PropertyAccessExpression:
             case SyntaxKind.YieldExpression:
+            case SyntaxKind.ThisKeyword:
                 return PredicateSemantics.Sometimes;
             case SyntaxKind.BinaryExpression:
                 // List of operators that can produce null/undefined:

tests/baselines/reference/predicateSemantics.errors.txt

@@ -73,4 +73,8 @@ predicateSemantics.ts(36,8): error TS2872: This kind of expression is always tru
 
     // Should be OK
     console.log((cond || undefined) && 1 / cond);
-
+
+    function foo(this: Object | undefined) {
+        // Should be OK
+        return this ?? 0;
+    }

tests/baselines/reference/predicateSemantics.js

@@ -40,7 +40,11 @@ while ((({}))) { }
 
 // Should be OK
 console.log((cond || undefined) && 1 / cond);
-
+
+function foo(this: Object | undefined) {
+    // Should be OK
+    return this ?? 0;
+}
 
 //// [predicateSemantics.js]
 var _a, _b, _c, _d, _e, _f;
@@ -80,3 +84,7 @@ while (({})) { }
 while ((({}))) { }
 // Should be OK
 console.log((cond || undefined) && 1 / cond);
+function foo() {
+    // Should be OK
+    return this !== null && this !== void 0 ? this : 0;
+}

tests/baselines/reference/predicateSemantics.symbols

@@ -70,3 +70,12 @@ console.log((cond || undefined) && 1 / cond);
 >undefined : Symbol(undefined)
 >cond : Symbol(cond, Decl(predicateSemantics.ts, 0, 11))
 
+function foo(this: Object | undefined) {
+>foo : Symbol(foo, Decl(predicateSemantics.ts, 38, 45))
+>this : Symbol(this, Decl(predicateSemantics.ts, 40, 13))
+>Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    // Should be OK
+    return this ?? 0;
+>this : Symbol(this, Decl(predicateSemantics.ts, 40, 13))
+}

tests/baselines/reference/predicateSemantics.types

@@ -219,3 +219,18 @@ console.log((cond || undefined) && 1 / cond);
 >cond : any
 >     : ^^^
 
+function foo(this: Object | undefined) {
+>foo : (this: Object | undefined) => Object | 0
+>    : ^    ^^                  ^^^^^^^^^^^^^^^
+>this : Object
+>     : ^^^^^^
+
+    // Should be OK
+    return this ?? 0;
+>this ?? 0 : 0 | Object
+>          : ^^^^^^^^^^
+>this : Object
+>     : ^^^^^^
+>0 : 0
+>  : ^
+}

tests/cases/compiler/predicateSemantics.ts

@@ -37,3 +37,8 @@ while ((({}))) { }
 
 // Should be OK
 console.log((cond || undefined) && 1 / cond);
+
+function foo(this: Object | undefined) {
+    // Should be OK
+    return this ?? 0;
+}