Make for..in expressions allowed to be null/undefined (#28348)

src/compiler/checker.ts

@@ -4766,7 +4766,7 @@ namespace ts {
             // A variable declared in a for..in statement is of type string, or of type keyof T when the
             // right hand expression is of a type parameter type.
             if (isVariableDeclaration(declaration) && declaration.parent.parent.kind === SyntaxKind.ForInStatement) {
-                const indexType = getIndexType(checkNonNullExpression(declaration.parent.parent.expression));
+                const indexType = getIndexType(getNonNullableTypeIfNeeded(checkExpression(declaration.parent.parent.expression)));
                 return indexType.flags & (TypeFlags.TypeParameter | TypeFlags.Index) ? getExtractStringType(indexType) : stringType;
             }
 
@@ -15175,6 +15175,10 @@ namespace ts {
                     }
                     return declaredType;
                 }
+                // for (const _ in ref) acts as a nonnull on ref
+                if (isVariableDeclaration(node) && node.parent.parent.kind === SyntaxKind.ForInStatement && isMatchingReference(reference, node.parent.parent.expression)) {
+                    return getNonNullableTypeIfNeeded(getTypeFromFlowType(getTypeAtFlowNode(flow.antecedent)));
+                }
                 // Assignment doesn't affect reference
                 return undefined;
             }
@@ -18489,6 +18493,14 @@ namespace ts {
             );
         }
 
+        function getNonNullableTypeIfNeeded(type: Type) {
+            const kind = (strictNullChecks ? getFalsyFlags(type) : type.flags) & TypeFlags.Nullable;
+            if (kind) {
+                return getNonNullableType(type);
+            }
+            return type;
+        }
+
         function checkNonNullType(
             type: Type,
             node: Node,
@@ -25286,7 +25298,7 @@ namespace ts {
             // Grammar checking
             checkGrammarForInOrForOfStatement(node);
 
-            const rightType = checkNonNullExpression(node.expression);
+            const rightType = getNonNullableTypeIfNeeded(checkExpression(node.expression));
             // TypeScript 1.0 spec (April 2014): 5.4
             // In a 'for-in' statement of the form
             // for (let VarDecl in Expr) Statement

tests/baselines/reference/ambientWithStatements.errors.txt

@@ -1,11 +1,10 @@
 tests/cases/compiler/ambientWithStatements.ts(2,5): error TS1036: Statements are not allowed in ambient contexts.
 tests/cases/compiler/ambientWithStatements.ts(3,5): error TS1104: A 'continue' statement can only be used within an enclosing iteration statement.
-tests/cases/compiler/ambientWithStatements.ts(7,15): error TS2531: Object is possibly 'null'.
 tests/cases/compiler/ambientWithStatements.ts(11,5): error TS1108: A 'return' statement can only be used within a function body.
 tests/cases/compiler/ambientWithStatements.ts(25,5): error TS2410: The 'with' statement is not supported. All symbols in a 'with' block will have type 'any'.
 
 
-==== tests/cases/compiler/ambientWithStatements.ts (5 errors) ====
+==== tests/cases/compiler/ambientWithStatements.ts (4 errors) ====
     declare module M {
         break;
         ~~~~~
@@ -17,8 +16,6 @@ tests/cases/compiler/ambientWithStatements.ts(25,5): error TS2410: The 'with' st
         do { } while (true);
         var x;
         for (x in null) { }
-                  ~~~~
-!!! error TS2531: Object is possibly 'null'.
         if (true) { } else { }
         1;
         L: var y;

tests/baselines/reference/forInStrictNullChecksNoError.errors.txt

@@ -0,0 +1,12 @@
+tests/cases/compiler/forInStrictNullChecksNoError.ts(5,5): error TS2533: Object is possibly 'null' or 'undefined'.
+
+
+==== tests/cases/compiler/forInStrictNullChecksNoError.ts (1 errors) ====
+    function f(x: { [key: string]: number; } | null | undefined) {
+        for (const key in x) {  // 1
+            console.log(x[key]);  // 2
+        }
+        x["no"]; // should still error
+        ~
+!!! error TS2533: Object is possibly 'null' or 'undefined'.
+    }

tests/baselines/reference/forInStrictNullChecksNoError.js

@@ -0,0 +1,15 @@
+//// [forInStrictNullChecksNoError.ts]
+function f(x: { [key: string]: number; } | null | undefined) {
+    for (const key in x) {  // 1
+        console.log(x[key]);  // 2
+    }
+    x["no"]; // should still error
+}
+
+//// [forInStrictNullChecksNoError.js]
+function f(x) {
+    for (var key in x) { // 1
+        console.log(x[key]); // 2
+    }
+    x["no"]; // should still error
+}

tests/baselines/reference/forInStrictNullChecksNoError.symbols

@@ -0,0 +1,20 @@
+=== tests/cases/compiler/forInStrictNullChecksNoError.ts ===
+function f(x: { [key: string]: number; } | null | undefined) {
+>f : Symbol(f, Decl(forInStrictNullChecksNoError.ts, 0, 0))
+>x : Symbol(x, Decl(forInStrictNullChecksNoError.ts, 0, 11))
+>key : Symbol(key, Decl(forInStrictNullChecksNoError.ts, 0, 17))
+
+    for (const key in x) {  // 1
+>key : Symbol(key, Decl(forInStrictNullChecksNoError.ts, 1, 14))
+>x : Symbol(x, Decl(forInStrictNullChecksNoError.ts, 0, 11))
+
+        console.log(x[key]);  // 2
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>x : Symbol(x, Decl(forInStrictNullChecksNoError.ts, 0, 11))
+>key : Symbol(key, Decl(forInStrictNullChecksNoError.ts, 1, 14))
+    }
+    x["no"]; // should still error
+>x : Symbol(x, Decl(forInStrictNullChecksNoError.ts, 0, 11))
+}

tests/baselines/reference/forInStrictNullChecksNoError.types

@@ -0,0 +1,25 @@
+=== tests/cases/compiler/forInStrictNullChecksNoError.ts ===
+function f(x: { [key: string]: number; } | null | undefined) {
+>f : (x: { [key: string]: number; } | null | undefined) => void
+>x : { [key: string]: number; } | null | undefined
+>key : string
+>null : null
+
+    for (const key in x) {  // 1
+>key : string
+>x : { [key: string]: number; } | null | undefined
+
+        console.log(x[key]);  // 2
+>console.log(x[key]) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>x[key] : number
+>x : { [key: string]: number; }
+>key : string
+    }
+    x["no"]; // should still error
+>x["no"] : number
+>x : { [key: string]: number; } | null | undefined
+>"no" : "no"
+}

tests/baselines/reference/widenedTypes.errors.txt

@@ -1,7 +1,6 @@
 tests/cases/compiler/widenedTypes.ts(1,1): error TS2358: The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.
 tests/cases/compiler/widenedTypes.ts(4,1): error TS2531: Object is possibly 'null'.
 tests/cases/compiler/widenedTypes.ts(5,7): error TS2531: Object is possibly 'null'.
-tests/cases/compiler/widenedTypes.ts(7,15): error TS2531: Object is possibly 'null'.
 tests/cases/compiler/widenedTypes.ts(9,14): error TS2695: Left side of comma operator is unused and has no side effects.
 tests/cases/compiler/widenedTypes.ts(10,1): error TS2322: Type '""' is not assignable to type 'number'.
 tests/cases/compiler/widenedTypes.ts(17,1): error TS2322: Type '""' is not assignable to type 'number'.
@@ -9,7 +8,7 @@ tests/cases/compiler/widenedTypes.ts(22,22): error TS2322: Type 'number' is not
 tests/cases/compiler/widenedTypes.ts(23,39): error TS2322: Type 'number' is not assignable to type 'string'.
 
 
-==== tests/cases/compiler/widenedTypes.ts (9 errors) ====
+==== tests/cases/compiler/widenedTypes.ts (8 errors) ====
     null instanceof (() => { });
     ~~~~
 !!! error TS2358: The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.
@@ -23,8 +22,6 @@ tests/cases/compiler/widenedTypes.ts(23,39): error TS2322: Type 'number' is not
 !!! error TS2531: Object is possibly 'null'.
     
     for (var a in null) { }
-                  ~~~~
-!!! error TS2531: Object is possibly 'null'.
     
     var t = [3, (3, null)];
                  ~

tests/cases/compiler/forInStrictNullChecksNoError.ts

@@ -0,0 +1,7 @@
+// @strictNullChecks: true
+function f(x: { [key: string]: number; } | null | undefined) {
+    for (const key in x) {  // 1
+        console.log(x[key]);  // 2
+    }
+    x["no"]; // should still error
+}