Defer switch exhaustiveness checks (microsoft#35000)

* Defer switch exhaustiveness checks until they're actually needed

* Add regression test

* Accept new baselines

Author: ahejlsberg

src/compiler/checker.ts

@@ -19294,9 +19294,6 @@ namespace ts {
                     else if (containsMatchingReferenceDiscriminant(reference, expr)) {
                         type = declaredType;
                     }
-                    else if (flow.clauseStart === flow.clauseEnd && isExhaustiveSwitchStatement(flow.switchStatement)) {
-                        return unreachableNeverType;
-                    }
                 }
                 return createFlowType(type, isIncomplete(flowType));
             }
@@ -19305,13 +19302,19 @@ namespace ts {
                 const antecedentTypes: Type[] = [];
                 let subtypeReduction = false;
                 let seenIncomplete = false;
+                let bypassFlow: FlowSwitchClause | undefined;
                 for (const antecedent of flow.antecedents!) {
                     if (antecedent.flags & FlowFlags.PreFinally && (<PreFinallyFlow>antecedent).lock.locked) {
                         // if flow correspond to branch from pre-try to finally and this branch is locked - this means that
                         // we initially have started following the flow outside the finally block.
                         // in this case we should ignore this branch.
                         continue;
                     }
+                    if (!bypassFlow && antecedent.flags & FlowFlags.SwitchClause && (<FlowSwitchClause>antecedent).clauseStart === (<FlowSwitchClause>antecedent).clauseEnd) {
+                        // The antecedent is the bypass branch of a potentially exhaustive switch statement.
+                        bypassFlow = <FlowSwitchClause>antecedent;
+                        continue;
+                    }
                     const flowType = getTypeAtFlowNode(antecedent);
                     const type = getTypeFromFlowType(flowType);
                     // If the type at a particular antecedent path is the declared type and the
@@ -19332,6 +19335,25 @@ namespace ts {
                         seenIncomplete = true;
                     }
                 }
+                if (bypassFlow) {
+                    const flowType = getTypeAtFlowNode(bypassFlow.antecedent);
+                    const type = getTypeFromFlowType(flowType);
+                    // If the bypass flow contributes a type we haven't seen yet and the switch statement
+                    // isn't exhaustive, process the bypass flow type. Since exhaustiveness checks increase
+                    // the risk of circularities, we only want to perform them when they make a difference.
+                    if (!contains(antecedentTypes, type) && !isExhaustiveSwitchStatement(bypassFlow.switchStatement)) {
+                        if (type === declaredType && declaredType === initialType) {
+                            return type;
+                        }
+                        antecedentTypes.push(type);
+                        if (!isTypeSubsetOf(type, declaredType)) {
+                            subtypeReduction = true;
+                        }
+                        if (isIncomplete(flowType)) {
+                            seenIncomplete = true;
+                        }
+                    }
+                }
                 return createFlowType(getUnionOrEvolvingArrayType(antecedentTypes, subtypeReduction ? UnionReduction.Subtype : UnionReduction.Literal), seenIncomplete);
             }
 

tests/baselines/reference/exhaustiveSwitchStatements1.errors.txt

@@ -212,4 +212,17 @@ tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts(7,9): error T
             case Animal.CAT: return Animal.CAT
         }
     }
+    
+    // Repro from #34840
+    
+    function foo() {
+        const foo: number | undefined = 0;
+        while (true) {
+            const stats = foo;
+            switch (stats) {
+                case 1: break;
+                case 2: break;
+            }
+        }
+    }
 

tests/baselines/reference/exhaustiveSwitchStatements1.js

@@ -207,6 +207,19 @@ function expression(): Animal {
         case Animal.CAT: return Animal.CAT
     }
 }
+
+// Repro from #34840
+
+function foo() {
+    const foo: number | undefined = 0;
+    while (true) {
+        const stats = foo;
+        switch (stats) {
+            case 1: break;
+            case 2: break;
+        }
+    }
+}
 
 
 //// [exhaustiveSwitchStatements1.js]
@@ -405,6 +418,17 @@ function expression() {
         case Animal.CAT: return Animal.CAT;
     }
 }
+// Repro from #34840
+function foo() {
+    var foo = 0;
+    while (true) {
+        var stats = foo;
+        switch (stats) {
+            case 1: break;
+            case 2: break;
+        }
+    }
+}
 
 
 //// [exhaustiveSwitchStatements1.d.ts]
@@ -469,3 +493,4 @@ declare const zoo: {
     animal: Animal;
 } | undefined;
 declare function expression(): Animal;
+declare function foo(): void;

tests/baselines/reference/exhaustiveSwitchStatements1.symbols

@@ -543,3 +543,25 @@ function expression(): Animal {
     }
 }
 
+// Repro from #34840
+
+function foo() {
+>foo : Symbol(foo, Decl(exhaustiveSwitchStatements1.ts, 207, 1))
+
+    const foo: number | undefined = 0;
+>foo : Symbol(foo, Decl(exhaustiveSwitchStatements1.ts, 212, 9))
+
+    while (true) {
+        const stats = foo;
+>stats : Symbol(stats, Decl(exhaustiveSwitchStatements1.ts, 214, 13))
+>foo : Symbol(foo, Decl(exhaustiveSwitchStatements1.ts, 212, 9))
+
+        switch (stats) {
+>stats : Symbol(stats, Decl(exhaustiveSwitchStatements1.ts, 214, 13))
+
+            case 1: break;
+            case 2: break;
+        }
+    }
+}
+

tests/baselines/reference/exhaustiveSwitchStatements1.types

@@ -634,3 +634,31 @@ function expression(): Animal {
     }
 }
 
+// Repro from #34840
+
+function foo() {
+>foo : () => void
+
+    const foo: number | undefined = 0;
+>foo : number | undefined
+>0 : 0
+
+    while (true) {
+>true : true
+
+        const stats = foo;
+>stats : number
+>foo : number
+
+        switch (stats) {
+>stats : number
+
+            case 1: break;
+>1 : 1
+
+            case 2: break;
+>2 : 2
+        }
+    }
+}
+

tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts

@@ -210,3 +210,16 @@ function expression(): Animal {
         case Animal.CAT: return Animal.CAT
     }
 }
+
+// Repro from #34840
+
+function foo() {
+    const foo: number | undefined = 0;
+    while (true) {
+        const stats = foo;
+        switch (stats) {
+            case 1: break;
+            case 2: break;
+        }
+    }
+}