Allow Not-null assertion fallback to assume initialize

src/compiler/checker.ts

@@ -23414,9 +23414,11 @@ namespace ts {
                 node.parent.kind === SyntaxKind.NonNullExpression ||
                 declaration.kind === SyntaxKind.VariableDeclaration && (<VariableDeclaration>declaration).exclamationToken ||
                 declaration.flags & NodeFlags.Ambient;
-            const initialType = assumeInitialized ? (isParameter ? removeOptionalityFromDeclaredType(type, declaration as VariableLikeDeclaration) : type) :
-                type === autoType || type === autoArrayType ? undefinedType :
-                getOptionalType(type);
+            const initialType = assumeInitialized
+                ? isParameter
+                    ? removeOptionalityFromDeclaredType(type, declaration as VariableLikeDeclaration)
+                    : (type === autoType || type === autoArrayType ? undefinedType : type)
+                : (type === autoType || type === autoArrayType ? undefinedType : getOptionalType(type));
             const flowType = getFlowTypeOfReference(node, type, initialType, flowContainer, !assumeInitialized);
             // A variable is considered uninitialized when it is possible to analyze the entire control flow graph
             // from declaration to use, and when the variable's declared type doesn't include undefined but the

tests/baselines/reference/initializerWithNonNullAssertion.js

@@ -0,0 +1,31 @@
+//// [initializerWithNonNullAssertion.ts]
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function test(numbers: number[]) {
+    let last;
+
+    for (const n of numbers) {
+        if (n % 2) {
+            return n;
+        }
+
+        last = n;
+    }
+
+    return last!;
+}
+
+
+//// [initializerWithNonNullAssertion.js]
+// https://github.com/microsoft/TypeScript/issues/19577
+function test(numbers) {
+    var last;
+    for (var _i = 0, numbers_1 = numbers; _i < numbers_1.length; _i++) {
+        var n = numbers_1[_i];
+        if (n % 2) {
+            return n;
+        }
+        last = n;
+    }
+    return last;
+}

tests/baselines/reference/initializerWithNonNullAssertion.symbols

@@ -0,0 +1,30 @@
+=== tests/cases/compiler/initializerWithNonNullAssertion.ts ===
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function test(numbers: number[]) {
+>test : Symbol(test, Decl(initializerWithNonNullAssertion.ts, 0, 0))
+>numbers : Symbol(numbers, Decl(initializerWithNonNullAssertion.ts, 2, 14))
+
+    let last;
+>last : Symbol(last, Decl(initializerWithNonNullAssertion.ts, 3, 7))
+
+    for (const n of numbers) {
+>n : Symbol(n, Decl(initializerWithNonNullAssertion.ts, 5, 14))
+>numbers : Symbol(numbers, Decl(initializerWithNonNullAssertion.ts, 2, 14))
+
+        if (n % 2) {
+>n : Symbol(n, Decl(initializerWithNonNullAssertion.ts, 5, 14))
+
+            return n;
+>n : Symbol(n, Decl(initializerWithNonNullAssertion.ts, 5, 14))
+        }
+
+        last = n;
+>last : Symbol(last, Decl(initializerWithNonNullAssertion.ts, 3, 7))
+>n : Symbol(n, Decl(initializerWithNonNullAssertion.ts, 5, 14))
+    }
+
+    return last!;
+>last : Symbol(last, Decl(initializerWithNonNullAssertion.ts, 3, 7))
+}
+

tests/baselines/reference/initializerWithNonNullAssertion.types

@@ -0,0 +1,34 @@
+=== tests/cases/compiler/initializerWithNonNullAssertion.ts ===
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function test(numbers: number[]) {
+>test : (numbers: number[]) => number
+>numbers : number[]
+
+    let last;
+>last : any
+
+    for (const n of numbers) {
+>n : number
+>numbers : number[]
+
+        if (n % 2) {
+>n % 2 : number
+>n : number
+>2 : 2
+
+            return n;
+>n : number
+        }
+
+        last = n;
+>last = n : number
+>last : any
+>n : number
+    }
+
+    return last!;
+>last! : number
+>last : number
+}
+

tests/cases/compiler/initializerWithNonNullAssertion.ts

@@ -0,0 +1,16 @@
+// @noImplicitAny: true
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function test(numbers: number[]) {
+    let last;
+
+    for (const n of numbers) {
+        if (n % 2) {
+            return n;
+        }
+
+        last = n;
+    }
+
+    return last!;
+}