fix(19577): fix regression with fully inferred types and non-null assertions

src/compiler/checker.ts

@@ -27275,6 +27275,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const isOuterVariable = flowContainer !== declarationContainer;
         const isSpreadDestructuringAssignmentTarget = node.parent && node.parent.parent && isSpreadAssignment(node.parent) && isDestructuringAssignmentTarget(node.parent.parent);
         const isModuleExports = symbol.flags & SymbolFlags.ModuleExports;
+        const typeIsAutomatic = type === autoType || type === autoArrayType;
+        const isAutomaticTypeInNonNull = typeIsAutomatic && node.parent.kind === SyntaxKind.NonNullExpression;
         // When the control flow originates in a function expression or arrow function and we are referencing
         // a const variable or parameter from an outer function, we extend the origin of the control flow
         // analysis to include the immediately enclosing function.
@@ -27292,10 +27294,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             node.parent.kind === SyntaxKind.NonNullExpression ||
             declaration.kind === SyntaxKind.VariableDeclaration && (declaration as VariableDeclaration).exclamationToken ||
             declaration.flags & NodeFlags.Ambient;
-        const initialType = assumeInitialized ? (isParameter ? removeOptionalityFromDeclaredType(type, declaration as VariableLikeDeclaration) : type) :
-            type === autoType || type === autoArrayType ? undefinedType :
-            getOptionalType(type);
-        const flowType = getFlowTypeOfReference(node, type, initialType, flowContainer);
+        const initialType = isAutomaticTypeInNonNull ? undefinedType :
+            assumeInitialized ? (isParameter ? removeOptionalityFromDeclaredType(type, declaration as VariableLikeDeclaration) : type) :
+            typeIsAutomatic ? undefinedType : getOptionalType(type);
+        const flowType = isAutomaticTypeInNonNull ? getNonNullableType(getFlowTypeOfReference(node, type, initialType, flowContainer)) :
+            getFlowTypeOfReference(node, type, initialType, flowContainer);
         // 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
         // control flow based type does include undefined.

tests/baselines/reference/nonNullFullInference.js

@@ -0,0 +1,63 @@
+//// [nonNullFullInference.ts]
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function testNonNullInference(numbers: number[]) {
+    let last;
+
+    for (const n of numbers) {
+        if (n % 2) {
+            return n;
+        }
+
+        last = n;
+    }
+
+    last;
+    last!;
+}
+
+function testNonNullInferenceWithArrays(numbers: number[]) {
+    let result;
+    const arr = [];
+
+    for (const n of numbers) {
+        if (n % 2) {
+            return [n];
+        }
+
+        arr.push(n);
+        result = arr;
+    }
+
+    result;
+    result!;
+}
+
+//// [nonNullFullInference.js]
+// https://github.com/microsoft/TypeScript/issues/19577
+function testNonNullInference(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;
+    }
+    last;
+    last;
+}
+function testNonNullInferenceWithArrays(numbers) {
+    var result;
+    var arr = [];
+    for (var _i = 0, numbers_2 = numbers; _i < numbers_2.length; _i++) {
+        var n = numbers_2[_i];
+        if (n % 2) {
+            return [n];
+        }
+        arr.push(n);
+        result = arr;
+    }
+    result;
+    result;
+}

tests/baselines/reference/nonNullFullInference.symbols

@@ -0,0 +1,71 @@
+=== tests/cases/compiler/nonNullFullInference.ts ===
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function testNonNullInference(numbers: number[]) {
+>testNonNullInference : Symbol(testNonNullInference, Decl(nonNullFullInference.ts, 0, 0))
+>numbers : Symbol(numbers, Decl(nonNullFullInference.ts, 2, 30))
+
+    let last;
+>last : Symbol(last, Decl(nonNullFullInference.ts, 3, 7))
+
+    for (const n of numbers) {
+>n : Symbol(n, Decl(nonNullFullInference.ts, 5, 14))
+>numbers : Symbol(numbers, Decl(nonNullFullInference.ts, 2, 30))
+
+        if (n % 2) {
+>n : Symbol(n, Decl(nonNullFullInference.ts, 5, 14))
+
+            return n;
+>n : Symbol(n, Decl(nonNullFullInference.ts, 5, 14))
+        }
+
+        last = n;
+>last : Symbol(last, Decl(nonNullFullInference.ts, 3, 7))
+>n : Symbol(n, Decl(nonNullFullInference.ts, 5, 14))
+    }
+
+    last;
+>last : Symbol(last, Decl(nonNullFullInference.ts, 3, 7))
+
+    last!;
+>last : Symbol(last, Decl(nonNullFullInference.ts, 3, 7))
+}
+
+function testNonNullInferenceWithArrays(numbers: number[]) {
+>testNonNullInferenceWithArrays : Symbol(testNonNullInferenceWithArrays, Decl(nonNullFullInference.ts, 15, 1))
+>numbers : Symbol(numbers, Decl(nonNullFullInference.ts, 17, 40))
+
+    let result;
+>result : Symbol(result, Decl(nonNullFullInference.ts, 18, 7))
+
+    const arr = [];
+>arr : Symbol(arr, Decl(nonNullFullInference.ts, 19, 9))
+
+    for (const n of numbers) {
+>n : Symbol(n, Decl(nonNullFullInference.ts, 21, 14))
+>numbers : Symbol(numbers, Decl(nonNullFullInference.ts, 17, 40))
+
+        if (n % 2) {
+>n : Symbol(n, Decl(nonNullFullInference.ts, 21, 14))
+
+            return [n];
+>n : Symbol(n, Decl(nonNullFullInference.ts, 21, 14))
+        }
+
+        arr.push(n);
+>arr.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arr : Symbol(arr, Decl(nonNullFullInference.ts, 19, 9))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>n : Symbol(n, Decl(nonNullFullInference.ts, 21, 14))
+
+        result = arr;
+>result : Symbol(result, Decl(nonNullFullInference.ts, 18, 7))
+>arr : Symbol(arr, Decl(nonNullFullInference.ts, 19, 9))
+    }
+
+    result;
+>result : Symbol(result, Decl(nonNullFullInference.ts, 18, 7))
+
+    result!;
+>result : Symbol(result, Decl(nonNullFullInference.ts, 18, 7))
+}

tests/baselines/reference/nonNullFullInference.types

@@ -0,0 +1,82 @@
+=== tests/cases/compiler/nonNullFullInference.ts ===
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function testNonNullInference(numbers: number[]) {
+>testNonNullInference : (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
+    }
+
+    last;
+>last : number
+
+    last!;
+>last! : number
+>last : number
+}
+
+function testNonNullInferenceWithArrays(numbers: number[]) {
+>testNonNullInferenceWithArrays : (numbers: number[]) => number[]
+>numbers : number[]
+
+    let result;
+>result : any
+
+    const arr = [];
+>arr : any[]
+>[] : undefined[]
+
+    for (const n of numbers) {
+>n : number
+>numbers : number[]
+
+        if (n % 2) {
+>n % 2 : number
+>n : number
+>2 : 2
+
+            return [n];
+>[n] : number[]
+>n : number
+        }
+
+        arr.push(n);
+>arr.push(n) : number
+>arr.push : (...items: any[]) => number
+>arr : any[]
+>push : (...items: any[]) => number
+>n : number
+
+        result = arr;
+>result = arr : number[]
+>result : any
+>arr : number[]
+    }
+
+    result;
+>result : number[]
+
+    result!;
+>result! : number[]
+>result : number[]
+}

tests/cases/compiler/nonNullFullInference.ts

@@ -0,0 +1,34 @@
+// @noImplicitAny: true
+// https://github.com/microsoft/TypeScript/issues/19577
+
+function testNonNullInference(numbers: number[]) {
+    let last;
+
+    for (const n of numbers) {
+        if (n % 2) {
+            return n;
+        }
+
+        last = n;
+    }
+
+    last;
+    last!;
+}
+
+function testNonNullInferenceWithArrays(numbers: number[]) {
+    let result;
+    const arr = [];
+
+    for (const n of numbers) {
+        if (n % 2) {
+            return [n];
+        }
+
+        arr.push(n);
+        result = arr;
+    }
+
+    result;
+    result!;
+}