Cherry-pick PR microsoft#38398 into release-3.9 (microsoft#38400)

TypeScript Bot · ahejlsberg · web-flow · commit d906a471dd37 · 2020-05-07T23:50:20.000-07:00
Component commits: 99c5c09 Properly finalize evolving array type in getTypeAtFlowCall b355cd4 Add regression test Co-authored-by: Anders Hejlsberg <andersh@microsoft.com>
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -20291,7 +20291,7 @@ namespace ts {
                     const predicate = getTypePredicateOfSignature(signature);
                     if (predicate && (predicate.kind === TypePredicateKind.AssertsThis || predicate.kind === TypePredicateKind.AssertsIdentifier)) {
                         const flowType = getTypeAtFlowNode(flow.antecedent);
-                        const type = getTypeFromFlowType(flowType);
+                        const type = finalizeEvolvingArrayType(getTypeFromFlowType(flowType));
                         const narrowedType = predicate.type ? narrowTypeByTypePredicate(type, predicate, flow.node, /*assumeTrue*/ true) :
                             predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex >= 0 && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :
                             type;
diff --git a/tests/baselines/reference/evolvingArrayTypeInAssert.js b/tests/baselines/reference/evolvingArrayTypeInAssert.js
@@ -0,0 +1,23 @@
+//// [evolvingArrayTypeInAssert.ts]
+export function unsafeCast<T>(_value: unknown): asserts _value is T { }
+
+function yadda() {
+    let out = [];
+    out.push(100)
+    unsafeCast<any>(out);
+    return out;
+}
+
+
+//// [evolvingArrayTypeInAssert.js]
+"use strict";
+exports.__esModule = true;
+exports.unsafeCast = void 0;
+function unsafeCast(_value) { }
+exports.unsafeCast = unsafeCast;
+function yadda() {
+    var out = [];
+    out.push(100);
+    unsafeCast(out);
+    return out;
+}
diff --git a/tests/baselines/reference/evolvingArrayTypeInAssert.symbols b/tests/baselines/reference/evolvingArrayTypeInAssert.symbols
@@ -0,0 +1,27 @@
+=== tests/cases/compiler/evolvingArrayTypeInAssert.ts ===
+export function unsafeCast<T>(_value: unknown): asserts _value is T { }
+>unsafeCast : Symbol(unsafeCast, Decl(evolvingArrayTypeInAssert.ts, 0, 0))
+>T : Symbol(T, Decl(evolvingArrayTypeInAssert.ts, 0, 27))
+>_value : Symbol(_value, Decl(evolvingArrayTypeInAssert.ts, 0, 30))
+>_value : Symbol(_value, Decl(evolvingArrayTypeInAssert.ts, 0, 30))
+>T : Symbol(T, Decl(evolvingArrayTypeInAssert.ts, 0, 27))
+
+function yadda() {
+>yadda : Symbol(yadda, Decl(evolvingArrayTypeInAssert.ts, 0, 71))
+
+    let out = [];
+>out : Symbol(out, Decl(evolvingArrayTypeInAssert.ts, 3, 7))
+
+    out.push(100)
+>out.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>out : Symbol(out, Decl(evolvingArrayTypeInAssert.ts, 3, 7))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+
+    unsafeCast<any>(out);
+>unsafeCast : Symbol(unsafeCast, Decl(evolvingArrayTypeInAssert.ts, 0, 0))
+>out : Symbol(out, Decl(evolvingArrayTypeInAssert.ts, 3, 7))
+
+    return out;
+>out : Symbol(out, Decl(evolvingArrayTypeInAssert.ts, 3, 7))
+}
+
diff --git a/tests/baselines/reference/evolvingArrayTypeInAssert.types b/tests/baselines/reference/evolvingArrayTypeInAssert.types
@@ -0,0 +1,28 @@
+=== tests/cases/compiler/evolvingArrayTypeInAssert.ts ===
+export function unsafeCast<T>(_value: unknown): asserts _value is T { }
+>unsafeCast : <T>(_value: unknown) => asserts _value is T
+>_value : unknown
+
+function yadda() {
+>yadda : () => number[]
+
+    let out = [];
+>out : any[]
+>[] : never[]
+
+    out.push(100)
+>out.push(100) : number
+>out.push : (...items: any[]) => number
+>out : any[]
+>push : (...items: any[]) => number
+>100 : 100
+
+    unsafeCast<any>(out);
+>unsafeCast<any>(out) : void
+>unsafeCast : <T>(_value: unknown) => asserts _value is T
+>out : number[]
+
+    return out;
+>out : number[]
+}
+
diff --git a/tests/cases/compiler/evolvingArrayTypeInAssert.ts b/tests/cases/compiler/evolvingArrayTypeInAssert.ts
@@ -0,0 +1,10 @@
+// @strict: true
+
+export function unsafeCast<T>(_value: unknown): asserts _value is T { }
+
+function yadda() {
+    let out = [];
+    out.push(100)
+    unsafeCast<any>(out);
+    return out;
+}
