Fixes for control-flow analysis

src/compiler/binder.ts

@@ -1015,7 +1015,8 @@ namespace ts {
                 else {
                     return node.kind === SyntaxKind.BinaryExpression && (
                         (<BinaryExpression>node).operatorToken.kind === SyntaxKind.AmpersandAmpersandToken ||
-                        (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken);
+                        (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken ||
+                        (<BinaryExpression>node).operatorToken.kind === SyntaxKind.QuestionQuestionToken);
                 }
             }
         }
@@ -1440,14 +1441,6 @@ namespace ts {
             bindCondition(node.right, trueTarget, falseTarget);
         }
 
-        function bindNullishCoalescingExpression(node: BinaryExpression, trueTarget: FlowLabel, falseTarget: FlowLabel) {
-            const notNullLabel = createBranchLabel();
-            bindCondition(node.left, trueTarget, notNullLabel);
-            currentFlow = finishFlowLabel(notNullLabel);
-            bind(node.operatorToken);
-            bindCondition(node.right, trueTarget, falseTarget);
-        }
-
         function bindPrefixUnaryExpressionFlow(node: PrefixUnaryExpression) {
             if (node.operator === SyntaxKind.ExclamationToken) {
                 const saveTrueTarget = currentTrueTarget;
@@ -1480,9 +1473,6 @@ namespace ts {
                     bindLogicalExpression(node, postExpressionLabel, postExpressionLabel);
                     currentFlow = finishFlowLabel(postExpressionLabel);
                 }
-                else if (operator === SyntaxKind.QuestionQuestionToken) {
-                    bindNullishCoalescingExpression(node, currentTrueTarget!, currentFalseTarget!);
-                }
                 else {
                     bindLogicalExpression(node, currentTrueTarget!, currentFalseTarget!);
                 }

src/compiler/checker.ts

@@ -19697,7 +19697,8 @@ namespace ts {
             // will be a subtype or the same type as the argument.
             function narrowType(type: Type, expr: Expression, assumeTrue: boolean): Type {
                 // for `a?.b`, we emulate a synthetic `a !== null && a !== undefined` condition for `a`
-                if (isOptionalChainRoot(expr.parent)) {
+                if (isOptionalChainRoot(expr.parent) ||
+                    isBinaryExpression(expr.parent) && expr.parent.operatorToken.kind === SyntaxKind.QuestionQuestionToken && expr.parent.left === expr) {
                     return narrowTypeByOptionality(type, expr, assumeTrue);
                 }
                 switch (expr.kind) {

tests/baselines/reference/controlFlowNullishCoalesce.errors.txt

@@ -0,0 +1,19 @@
+tests/cases/conformance/controlFlow/controlFlowNullishCoalesce.ts(4,1): error TS2454: Variable 'a' is used before being assigned.
+
+
+==== tests/cases/conformance/controlFlow/controlFlowNullishCoalesce.ts (1 errors) ====
+    // assignments in shortcutting rhs
+    let a: number;
+    o ?? (a = 1);
+    a.toString();
+    ~
+!!! error TS2454: Variable 'a' is used before being assigned.
+
+    // assignment flow
+    declare const o: { x: number } | undefined;
+    let x: { x: number } | boolean;
+    if (x = o ?? true) {
+        x;
+    }
+
+

tests/baselines/reference/controlFlowNullishCoalesce.js

@@ -0,0 +1,25 @@
+//// [controlFlowNullishCoalesce.ts]
+// assignments in shortcutting rhs
+let a: number;
+o ?? (a = 1);
+a.toString();
+
+// assignment flow
+declare const o: { x: number } | undefined;
+let x: { x: number } | boolean;
+if (x = o ?? true) {
+    x;
+}
+
+
+
+//// [controlFlowNullishCoalesce.js]
+"use strict";
+// assignments in shortcutting rhs
+var a;
+(o !== null && o !== void 0 ? o : (a = 1));
+a.toString();
+var x;
+if (x = (o !== null && o !== void 0 ? o : true)) {
+    x;
+}

tests/baselines/reference/controlFlowNullishCoalesce.symbols

@@ -0,0 +1,32 @@
+=== tests/cases/conformance/controlFlow/controlFlowNullishCoalesce.ts ===
+// assignments in shortcutting rhs
+let a: number;
+>a : Symbol(a, Decl(controlFlowNullishCoalesce.ts, 1, 3))
+
+o ?? (a = 1);
+>o : Symbol(o, Decl(controlFlowNullishCoalesce.ts, 6, 13))
+>a : Symbol(a, Decl(controlFlowNullishCoalesce.ts, 1, 3))
+
+a.toString();
+>a.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
+>a : Symbol(a, Decl(controlFlowNullishCoalesce.ts, 1, 3))
+>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
+
+// assignment flow
+declare const o: { x: number } | undefined;
+>o : Symbol(o, Decl(controlFlowNullishCoalesce.ts, 6, 13))
+>x : Symbol(x, Decl(controlFlowNullishCoalesce.ts, 6, 18))
+
+let x: { x: number } | boolean;
+>x : Symbol(x, Decl(controlFlowNullishCoalesce.ts, 7, 3))
+>x : Symbol(x, Decl(controlFlowNullishCoalesce.ts, 7, 8))
+
+if (x = o ?? true) {
+>x : Symbol(x, Decl(controlFlowNullishCoalesce.ts, 7, 3))
+>o : Symbol(o, Decl(controlFlowNullishCoalesce.ts, 6, 13))
+
+    x;
+>x : Symbol(x, Decl(controlFlowNullishCoalesce.ts, 7, 3))
+}
+
+

tests/baselines/reference/controlFlowNullishCoalesce.types

@@ -0,0 +1,40 @@
+=== tests/cases/conformance/controlFlow/controlFlowNullishCoalesce.ts ===
+// assignments in shortcutting rhs
+let a: number;
+>a : number
+
+o ?? (a = 1);
+>o ?? (a = 1) : { x: number; } | 1
+>o : { x: number; } | undefined
+>(a = 1) : 1
+>a = 1 : 1
+>a : number
+>1 : 1
+
+a.toString();
+>a.toString() : string
+>a.toString : (radix?: number | undefined) => string
+>a : number
+>toString : (radix?: number | undefined) => string
+
+// assignment flow
+declare const o: { x: number } | undefined;
+>o : { x: number; } | undefined
+>x : number
+
+let x: { x: number } | boolean;
+>x : boolean | { x: number; }
+>x : number
+
+if (x = o ?? true) {
+>x = o ?? true : true | { x: number; }
+>x : boolean | { x: number; }
+>o ?? true : true | { x: number; }
+>o : { x: number; } | undefined
+>true : true
+
+    x;
+>x : true | { x: number; }
+}
+
+

tests/baselines/reference/nullishCoalescingOperator11.errors.txt

@@ -1,17 +1,12 @@
 tests/cases/conformance/expressions/nullishCoalescingOperator/nullishCoalescingOperator11.ts(3,18): error TS2533: Object is possibly 'null' or 'undefined'.
-tests/cases/conformance/expressions/nullishCoalescingOperator/nullishCoalescingOperator11.ts(3,22): error TS2339: Property 'toFixed' does not exist on type '"" | 0'.
-  Property 'toFixed' does not exist on type '""'.
 
 
-==== tests/cases/conformance/expressions/nullishCoalescingOperator/nullishCoalescingOperator11.ts (2 errors) ====
+==== tests/cases/conformance/expressions/nullishCoalescingOperator/nullishCoalescingOperator11.ts (1 errors) ====
     declare const f11: 1 | 0 | '' | null | undefined;
 
     let g11 = f11 ?? f11.toFixed()
                      ~~~
 !!! error TS2533: Object is possibly 'null' or 'undefined'.
-                         ~~~~~~~
-!!! error TS2339: Property 'toFixed' does not exist on type '"" | 0'.
-!!! error TS2339:   Property 'toFixed' does not exist on type '""'.
 
 
 

tests/baselines/reference/nullishCoalescingOperator11.types

@@ -9,7 +9,7 @@ let g11 = f11 ?? f11.toFixed()
 >f11 : "" | 0 | 1 | null | undefined
 >f11.toFixed() : any
 >f11.toFixed : any
->f11 : "" | 0 | null | undefined
+>f11 : null | undefined
 >toFixed : any
 
 

tests/baselines/reference/nullishCoalescingOperator5.types

@@ -20,7 +20,7 @@ a ?? b || c;
 a || b ?? c;
 >a || b ?? c : string | undefined
 >a || b : string | undefined
->a : string
+>a : string | undefined
 >b : string | undefined
 >c : string | undefined
 

tests/cases/conformance/controlFlow/controlFlowNullishCoalesce.ts

@@ -0,0 +1,14 @@
+// @strict: true
+
+// assignments in shortcutting rhs
+let a: number;
+o ?? (a = 1);
+a.toString();
+
+// assignment flow
+declare const o: { x: number } | undefined;
+let x: { x: number } | boolean;
+if (x = o ?? true) {
+    x;
+}
+