Fix isExhaustiveSwitchStatement to better handle circularities (#51095)

* Fix isExhaustiveSwitchStatement to better handle circularities

* Add regression test

Author: ahejlsberg

src/compiler/checker.ts

@@ -33328,7 +33328,17 @@ namespace ts {
 
         function isExhaustiveSwitchStatement(node: SwitchStatement): boolean {
             const links = getNodeLinks(node);
-            return links.isExhaustive !== undefined ? links.isExhaustive : (links.isExhaustive = computeExhaustiveSwitchStatement(node));
+            if (links.isExhaustive === undefined) {
+                links.isExhaustive = 0;  // Indicate resolution is in process
+                const exhaustive = computeExhaustiveSwitchStatement(node);
+                if (links.isExhaustive === 0) {
+                    links.isExhaustive = exhaustive;
+                }
+            }
+            else if (links.isExhaustive === 0) {
+                links.isExhaustive = false;  // Resolve circularity to false
+            }
+            return links.isExhaustive;
         }
 
         function computeExhaustiveSwitchStatement(node: SwitchStatement): boolean {
@@ -33337,7 +33347,7 @@ namespace ts {
                 if (!witnesses) {
                     return false;
                 }
-                const operandConstraint = getBaseConstraintOrType(getTypeOfExpression((node.expression as TypeOfExpression).expression));
+                const operandConstraint = getBaseConstraintOrType(checkExpressionCached((node.expression as TypeOfExpression).expression));
                 // Get the not-equal flags for all handled cases.
                 const notEqualFacts = getNotEqualFactsFromTypeofSwitch(0, 0, witnesses);
                 if (operandConstraint.flags & TypeFlags.AnyOrUnknown) {
@@ -33347,7 +33357,7 @@ namespace ts {
                 // A missing not-equal flag indicates that the type wasn't handled by some case.
                 return !someType(operandConstraint, t => (getTypeFacts(t) & notEqualFacts) === notEqualFacts);
             }
-            const type = getTypeOfExpression(node.expression);
+            const type = checkExpressionCached(node.expression);
             if (!isLiteralType(type)) {
                 return false;
             }

src/compiler/types.ts

@@ -5571,7 +5571,7 @@ namespace ts {
         deferredNodes?: Set<Node>;          // Set of nodes whose checking has been deferred
         capturedBlockScopeBindings?: Symbol[]; // Block-scoped bindings captured beneath this part of an IterationStatement
         outerTypeParameters?: TypeParameter[]; // Outer type parameters of anonymous object type
-        isExhaustive?: boolean;             // Is node an exhaustive switch statement
+        isExhaustive?: boolean | 0;         // Is node an exhaustive switch statement (0 indicates in-process resolution)
         skipDirectInference?: true;         // Flag set by the API `getContextualType` call on a node when `Completions` is passed to force the checker to skip making inferences to a node's type
         declarationRequiresScopeChange?: boolean; // Set by `useOuterVariableScopeInParameter` in checker when downlevel emit would change the name resolution scope inside of a parameter.
         serializedTypes?: ESMap<string, TypeNode & {truncating?: boolean, addedLength: number}>; // Collection of types serialized at this location

tests/baselines/reference/exhaustiveSwitchCheckCircularity.errors.txt

@@ -0,0 +1,25 @@
+tests/cases/compiler/exhaustiveSwitchCheckCircularity.ts(14,26): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+
+
+==== tests/cases/compiler/exhaustiveSwitchCheckCircularity.ts (1 errors) ====
+    // Repro from #47539
+    
+    declare function isNever(n: never): boolean;
+    
+    function f() {
+        let foo: "aaa" | "bbb" = "aaa";
+        while (true) {
+            switch (foo) {
+                case "aaa":
+            }
+            if (foo === "aaa") {
+                foo = "bbb";
+            }
+            else if (isNever(foo)) {  // Error expected
+                             ~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+                break;
+            }
+        }
+    }
+    

tests/baselines/reference/exhaustiveSwitchCheckCircularity.js

@@ -0,0 +1,38 @@
+//// [exhaustiveSwitchCheckCircularity.ts]
+// Repro from #47539
+
+declare function isNever(n: never): boolean;
+
+function f() {
+    let foo: "aaa" | "bbb" = "aaa";
+    while (true) {
+        switch (foo) {
+            case "aaa":
+        }
+        if (foo === "aaa") {
+            foo = "bbb";
+        }
+        else if (isNever(foo)) {  // Error expected
+            break;
+        }
+    }
+}
+
+
+//// [exhaustiveSwitchCheckCircularity.js]
+"use strict";
+// Repro from #47539
+function f() {
+    var foo = "aaa";
+    while (true) {
+        switch (foo) {
+            case "aaa":
+        }
+        if (foo === "aaa") {
+            foo = "bbb";
+        }
+        else if (isNever(foo)) { // Error expected
+            break;
+        }
+    }
+}

tests/baselines/reference/exhaustiveSwitchCheckCircularity.symbols

@@ -0,0 +1,34 @@
+=== tests/cases/compiler/exhaustiveSwitchCheckCircularity.ts ===
+// Repro from #47539
+
+declare function isNever(n: never): boolean;
+>isNever : Symbol(isNever, Decl(exhaustiveSwitchCheckCircularity.ts, 0, 0))
+>n : Symbol(n, Decl(exhaustiveSwitchCheckCircularity.ts, 2, 25))
+
+function f() {
+>f : Symbol(f, Decl(exhaustiveSwitchCheckCircularity.ts, 2, 44))
+
+    let foo: "aaa" | "bbb" = "aaa";
+>foo : Symbol(foo, Decl(exhaustiveSwitchCheckCircularity.ts, 5, 7))
+
+    while (true) {
+        switch (foo) {
+>foo : Symbol(foo, Decl(exhaustiveSwitchCheckCircularity.ts, 5, 7))
+
+            case "aaa":
+        }
+        if (foo === "aaa") {
+>foo : Symbol(foo, Decl(exhaustiveSwitchCheckCircularity.ts, 5, 7))
+
+            foo = "bbb";
+>foo : Symbol(foo, Decl(exhaustiveSwitchCheckCircularity.ts, 5, 7))
+        }
+        else if (isNever(foo)) {  // Error expected
+>isNever : Symbol(isNever, Decl(exhaustiveSwitchCheckCircularity.ts, 0, 0))
+>foo : Symbol(foo, Decl(exhaustiveSwitchCheckCircularity.ts, 5, 7))
+
+            break;
+        }
+    }
+}
+

tests/baselines/reference/exhaustiveSwitchCheckCircularity.types

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/exhaustiveSwitchCheckCircularity.ts ===
+// Repro from #47539
+
+declare function isNever(n: never): boolean;
+>isNever : (n: never) => boolean
+>n : never
+
+function f() {
+>f : () => void
+
+    let foo: "aaa" | "bbb" = "aaa";
+>foo : "aaa" | "bbb"
+>"aaa" : "aaa"
+
+    while (true) {
+>true : true
+
+        switch (foo) {
+>foo : "aaa" | "bbb"
+
+            case "aaa":
+>"aaa" : "aaa"
+        }
+        if (foo === "aaa") {
+>foo === "aaa" : boolean
+>foo : "aaa" | "bbb"
+>"aaa" : "aaa"
+
+            foo = "bbb";
+>foo = "bbb" : "bbb"
+>foo : "aaa" | "bbb"
+>"bbb" : "bbb"
+        }
+        else if (isNever(foo)) {  // Error expected
+>isNever(foo) : boolean
+>isNever : (n: never) => boolean
+>foo : "bbb"
+
+            break;
+        }
+    }
+}
+

tests/cases/compiler/exhaustiveSwitchCheckCircularity.ts

@@ -0,0 +1,20 @@
+// @strict: true
+
+// Repro from #47539
+
+declare function isNever(n: never): boolean;
+
+function f() {
+    let foo: "aaa" | "bbb" = "aaa";
+    while (true) {
+        switch (foo) {
+            case "aaa":
+        }
+        if (foo === "aaa") {
+            foo = "bbb";
+        }
+        else if (isNever(foo)) {  // Error expected
+            break;
+        }
+    }
+}