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Port TypeScript#63071: Mark downlevelIteration as removed #2820

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2afc8b4
Initial plan
Copilot Feb 18, 2026
5f02cfd
Add removed diagnostic for downlevelIteration and skip tests with it set
Copilot Feb 18, 2026
12cb34b
Merge branch 'main' into copilot/add-removed-diag-for-downleveliteration
jakebailey Mar 4, 2026
c8f59d0
Clean up getIteratedTypeOrElementType
jakebailey Mar 4, 2026
8d00152
Unskip tests and just let the errors flow
jakebailey Mar 4, 2026
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23 changes: 5 additions & 18 deletions internal/checker/checker.go
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Expand Up @@ -5910,17 +5910,10 @@ func (c *Checker) getIteratedTypeOrElementType(use IterationUse, inputType *Type
}
return nil
}
// TODO: remove ScriptTargetES2015
iterableExists := c.getGlobalIterableType() != c.emptyGenericType
uplevelIteration := c.languageVersion >= core.ScriptTargetES2015 && iterableExists
downlevelIteration := !uplevelIteration && c.compilerOptions.DownlevelIteration == core.TSTrue
possibleOutOfBounds := c.compilerOptions.NoUncheckedIndexedAccess == core.TSTrue && use&IterationUsePossiblyOutOfBounds != 0
// Get the iterated type of an `Iterable<T>` or `IterableIterator<T>` only in ES2015
// or higher, when inside of an async generator or for-await-if, or when
// downlevelIteration is requested.
if uplevelIteration || downlevelIteration || allowAsyncIterables {
// We only report errors for an invalid iterable type in ES2015 or higher.
iterationTypes := c.getIterationTypesOfIterable(inputType, use, core.IfElse(uplevelIteration, errorNode, nil))
if iterableExists || allowAsyncIterables {
iterationTypes := c.getIterationTypesOfIterable(inputType, use, core.IfElse(iterableExists, errorNode, nil))
if checkAssignability {
if iterationTypes.nextType != nil {
var diagnostic *diagnostics.Message
Expand All @@ -5939,7 +5932,7 @@ func (c *Checker) getIteratedTypeOrElementType(use IterationUse, inputType *Type
}
}
}
if iterationTypes.yieldType != nil || uplevelIteration {
if iterationTypes.yieldType != nil || iterableExists {
if iterationTypes.yieldType == nil {
return nil
}
Expand Down Expand Up @@ -5988,7 +5981,7 @@ func (c *Checker) getIteratedTypeOrElementType(use IterationUse, inputType *Type
// number and string input is allowed, we want to say that number is not an
// array type or a string type.
allowsStrings := use&IterationUseAllowsStringInputFlag != 0 && !hasStringConstituent
defaultDiagnostic, maybeMissingAwait := c.getIterationDiagnosticDetails(use, inputType, allowsStrings, downlevelIteration)
defaultDiagnostic, maybeMissingAwait := c.getIterationDiagnosticDetails(use, inputType, allowsStrings)
c.errorAndMaybeSuggestAwait(errorNode, maybeMissingAwait && c.getAwaitedTypeOfPromise(arrayType) != nil, defaultDiagnostic, c.TypeToString(arrayType))
}
if hasStringConstituent {
Expand Down Expand Up @@ -6490,13 +6483,7 @@ func (c *Checker) reportTypeNotIterableError(errorNode *ast.Node, t *Type, allow
return c.errorAndMaybeSuggestAwait(errorNode, suggestAwait, message, c.TypeToString(t))
}

func (c *Checker) getIterationDiagnosticDetails(use IterationUse, inputType *Type, allowsStrings bool, downlevelIteration bool) (*diagnostics.Message, bool) {
if downlevelIteration {
if allowsStrings {
return diagnostics.Type_0_is_not_an_array_type_or_a_string_type_or_does_not_have_a_Symbol_iterator_method_that_returns_an_iterator, true
}
return diagnostics.Type_0_is_not_an_array_type_or_does_not_have_a_Symbol_iterator_method_that_returns_an_iterator, true
}
func (c *Checker) getIterationDiagnosticDetails(use IterationUse, inputType *Type, allowsStrings bool) (*diagnostics.Message, bool) {
yieldType := c.getIterationTypeOfIterable(use, IterationTypeKindYield, inputType, nil /*errorNode*/)
if yieldType != nil {
return diagnostics.Type_0_can_only_be_iterated_through_when_using_the_downlevelIteration_flag_or_with_a_target_of_es2015_or_higher, false
Expand Down
4 changes: 4 additions & 0 deletions internal/compiler/program.go
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Expand Up @@ -696,6 +696,10 @@ func (p *Program) verifyCompilerOptions() {
createRemovedOptionDiagnostic("allowSyntheticDefaultImports", "false", "")
}

if !options.DownlevelIteration.IsUnknown() {
createRemovedOptionDiagnostic("downlevelIteration", "", "")
}

if options.StrictPropertyInitialization.IsTrue() && !options.GetStrictOptionValue(options.StrictNullChecks) {
createDiagnosticForOptionName(diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "strictPropertyInitialization", "strictNullChecks")
}
Expand Down
3 changes: 2 additions & 1 deletion internal/core/compileroptions.go
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Expand Up @@ -29,7 +29,6 @@ type CompilerOptions struct {
EmitDeclarationOnly Tristate `json:"emitDeclarationOnly,omitzero"`
EmitBOM Tristate `json:"emitBOM,omitzero"`
EmitDecoratorMetadata Tristate `json:"emitDecoratorMetadata,omitzero"`
DownlevelIteration Tristate `json:"downlevelIteration,omitzero"`
Declaration Tristate `json:"declaration,omitzero"`
DeclarationDir string `json:"declarationDir,omitzero"`
DeclarationMap Tristate `json:"declarationMap,omitzero"`
Expand Down Expand Up @@ -124,6 +123,8 @@ type CompilerOptions struct {
// Deprecated: Do not use outside of options parsing and validation.
BaseUrl string `json:"baseUrl,omitzero"`
// Deprecated: Do not use outside of options parsing and validation.
DownlevelIteration Tristate `json:"downlevelIteration,omitzero"`
// Deprecated: Do not use outside of options parsing and validation.
ESModuleInterop Tristate `json:"esModuleInterop,omitzero"`
// Deprecated: Do not use outside of options parsing and validation.
OutFile string `json:"outFile,omitzero"`
Expand Down
11 changes: 11 additions & 0 deletions ...ence/submodule/compiler/blockScopedBindingsInDownlevelGenerator(target=es2015).errors.txt
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@@ -0,0 +1,11 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== blockScopedBindingsInDownlevelGenerator.ts (0 errors) ====
function* a() {
for (const i of [1,2,3]) {
(() => i)()
yield i
}
}
15 changes: 7 additions & 8 deletions ...submodule/compiler/blockScopedBindingsInDownlevelGenerator(target=es2015).errors.txt.diff
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Expand Up @@ -5,11 +5,10 @@
-
-
-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
-==== blockScopedBindingsInDownlevelGenerator.ts (0 errors) ====
- function* a() {
- for (const i of [1,2,3]) {
- (() => i)()
- yield i
- }
- }
+<no content>
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
+
+
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== blockScopedBindingsInDownlevelGenerator.ts (0 errors) ====
function* a() {
for (const i of [1,2,3]) {
8 changes: 8 additions & 0 deletions ...e/submodule/compiler/sourceMapValidationVarInDownLevelGenerator(target=es2015).errors.txt
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@@ -0,0 +1,8 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== sourceMapValidationVarInDownLevelGenerator.ts (0 errors) ====
function * f() {
var x = 1, y;
}
12 changes: 7 additions & 5 deletions ...module/compiler/sourceMapValidationVarInDownLevelGenerator(target=es2015).errors.txt.diff
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Expand Up @@ -5,8 +5,10 @@
-
-
-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
-==== sourceMapValidationVarInDownLevelGenerator.ts (0 errors) ====
- function * f() {
- var x = 1, y;
- }
+<no content>
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
+
+
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== sourceMapValidationVarInDownLevelGenerator.ts (0 errors) ====
function * f() {
var x = 1, y;
8 changes: 8 additions & 0 deletions testdata/baselines/reference/submodule/conformance/ES5For-of33(target=es2015).errors.txt
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@@ -0,0 +1,8 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of33.ts (0 errors) ====
for (var v of ['a', 'b', 'c']) {
console.log(v);
}
12 changes: 7 additions & 5 deletions ...data/baselines/reference/submodule/conformance/ES5For-of33(target=es2015).errors.txt.diff
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Expand Up @@ -5,8 +5,10 @@
-
-
-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
-==== ES5For-of33.ts (0 errors) ====
- for (var v of ['a', 'b', 'c']) {
- console.log(v);
- }
+<no content>
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
+
+
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of33.ts (0 errors) ====
for (var v of ['a', 'b', 'c']) {
console.log(v);
2 changes: 2 additions & 0 deletions testdata/baselines/reference/submodule/conformance/ES5For-of34(target=es2015).errors.txt
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@@ -1,6 +1,8 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
ES5For-of34.ts(4,6): error TS2322: Type 'string' is not assignable to type 'number'.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of34.ts (1 errors) ====
function foo() {
return { x: 0 };
Expand Down
2 changes: 2 additions & 0 deletions ...data/baselines/reference/submodule/conformance/ES5For-of34(target=es2015).errors.txt.diff
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Expand Up @@ -2,10 +2,12 @@
+++ new.ES5For-of34(target=es2015).errors.txt
@@= skipped -0, +0 lines =@@
-error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
ES5For-of34.ts(4,6): error TS2322: Type 'string' is not assignable to type 'number'.


-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of34.ts (1 errors) ====
function foo() {
return { x: 0 };
2 changes: 2 additions & 0 deletions testdata/baselines/reference/submodule/conformance/ES5For-of35(target=es2015).errors.txt
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@@ -1,7 +1,9 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
ES5For-of35.ts(1,13): error TS2339: Property 'x' does not exist on type 'Number'.
ES5For-of35.ts(1,23): error TS2339: Property 'y' does not exist on type 'Number'.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of35.ts (2 errors) ====
for (const {x: a = 0, y: b = 1} of [2, 3]) {
~
Expand Down
2 changes: 2 additions & 0 deletions ...data/baselines/reference/submodule/conformance/ES5For-of35(target=es2015).errors.txt.diff
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Expand Up @@ -2,11 +2,13 @@
+++ new.ES5For-of35(target=es2015).errors.txt
@@= skipped -0, +0 lines =@@
-error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
ES5For-of35.ts(1,13): error TS2339: Property 'x' does not exist on type 'Number'.
ES5For-of35.ts(1,23): error TS2339: Property 'y' does not exist on type 'Number'.


-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of35.ts (2 errors) ====
for (const {x: a = 0, y: b = 1} of [2, 3]) {
~
2 changes: 2 additions & 0 deletions testdata/baselines/reference/submodule/conformance/ES5For-of36(target=es2015).errors.txt
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@@ -1,6 +1,8 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
ES5For-of36.ts(1,10): error TS2488: Type 'number' must have a '[Symbol.iterator]()' method that returns an iterator.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of36.ts (1 errors) ====
for (let [a = 0, b = 1] of [2, 3]) {
~~~~~~~~~~~~~~
Expand Down
2 changes: 2 additions & 0 deletions ...data/baselines/reference/submodule/conformance/ES5For-of36(target=es2015).errors.txt.diff
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Expand Up @@ -2,10 +2,12 @@
+++ new.ES5For-of36(target=es2015).errors.txt
@@= skipped -0, +0 lines =@@
-error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
ES5For-of36.ts(1,10): error TS2488: Type 'number' must have a '[Symbol.iterator]()' method that returns an iterator.


-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of36.ts (1 errors) ====
for (let [a = 0, b = 1] of [2, 3]) {
~~~~~~~~~~~~~~
20 changes: 20 additions & 0 deletions testdata/baselines/reference/submodule/conformance/ES5For-of37(target=es2015).errors.txt
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@@ -0,0 +1,20 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of37.ts (0 errors) ====
// https://github.com/microsoft/TypeScript/issues/30083

for (const i of [0, 1, 2, 3, 4]) {
try {
// Ensure catch binding for the following loop is reset per iteration:
for (const j of [1, 2, 3]) {
if (i === 2) {
throw new Error('ERR');
}
}
console.log(i);
} catch (err) {
console.log('E %s %s', i, err);
}
}
24 changes: 7 additions & 17 deletions ...data/baselines/reference/submodule/conformance/ES5For-of37(target=es2015).errors.txt.diff
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Expand Up @@ -5,20 +5,10 @@
-
-
-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
-==== ES5For-of37.ts (0 errors) ====
- // https://github.com/microsoft/TypeScript/issues/30083
-
- for (const i of [0, 1, 2, 3, 4]) {
- try {
- // Ensure catch binding for the following loop is reset per iteration:
- for (const j of [1, 2, 3]) {
- if (i === 2) {
- throw new Error('ERR');
- }
- }
- console.log(i);
- } catch (err) {
- console.log('E %s %s', i, err);
- }
- }
+<no content>
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
+
+
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== ES5For-of37.ts (0 errors) ====
// https://github.com/microsoft/TypeScript/issues/30083

28 changes: 28 additions & 0 deletions ...s/reference/submodule/conformance/arrayLiteralSpreadES5iterable(target=es2015).errors.txt
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@@ -0,0 +1,28 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== arrayLiteralSpreadES5iterable.ts (0 errors) ====
function f0() {
var a = [1, 2, 3];
var a1 = [...a];
var a2 = [1, ...a];
var a3 = [1, 2, ...a];
var a4 = [...a, 1];
var a5 = [...a, 1, 2];
var a6 = [1, 2, ...a, 1, 2];
var a7 = [1, ...a, 2, ...a];
var a8 = [...a, ...a, ...a];
}

function f1() {
var a = [1, 2, 3];
var b = ["hello", ...a, true];
var b: (string | number | boolean)[];
}

function f2() {
var a = [...[...[...[...[...[]]]]]];
var b = [...[...[...[...[...[5]]]]]];
}

32 changes: 7 additions & 25 deletions ...erence/submodule/conformance/arrayLiteralSpreadES5iterable(target=es2015).errors.txt.diff
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Expand Up @@ -5,28 +5,10 @@
-
-
-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
-==== arrayLiteralSpreadES5iterable.ts (0 errors) ====
- function f0() {
- var a = [1, 2, 3];
- var a1 = [...a];
- var a2 = [1, ...a];
- var a3 = [1, 2, ...a];
- var a4 = [...a, 1];
- var a5 = [...a, 1, 2];
- var a6 = [1, 2, ...a, 1, 2];
- var a7 = [1, ...a, 2, ...a];
- var a8 = [...a, ...a, ...a];
- }
-
- function f1() {
- var a = [1, 2, 3];
- var b = ["hello", ...a, true];
- var b: (string | number | boolean)[];
- }
-
- function f2() {
- var a = [...[...[...[...[...[]]]]]];
- var b = [...[...[...[...[...[5]]]]]];
- }
-
+<no content>
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
+
+
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== arrayLiteralSpreadES5iterable.ts (0 errors) ====
function f0() {
var a = [1, 2, 3];
12 changes: 12 additions & 0 deletions ...elines/reference/submodule/conformance/asyncArrowFunction11_es5(target=es2015).errors.txt
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@@ -0,0 +1,12 @@
error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.


!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== asyncArrowFunction11_es5.ts (0 errors) ====
// https://github.com/Microsoft/TypeScript/issues/24722
class A {
b = async (...args: any[]) => {
await Promise.resolve();
const obj = { ["a"]: () => this }; // computed property name after `await` triggers case
};
}
16 changes: 7 additions & 9 deletions ...s/reference/submodule/conformance/asyncArrowFunction11_es5(target=es2015).errors.txt.diff
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Expand Up @@ -5,12 +5,10 @@
-
-
-!!! error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
-==== asyncArrowFunction11_es5.ts (0 errors) ====
- // https://github.com/Microsoft/TypeScript/issues/24722
- class A {
- b = async (...args: any[]) => {
- await Promise.resolve();
- const obj = { ["a"]: () => this }; // computed property name after `await` triggers case
- };
- }
+<no content>
+error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
+
+
+!!! error TS5102: Option 'downlevelIteration' has been removed. Please remove it from your configuration.
==== asyncArrowFunction11_es5.ts (0 errors) ====
// https://github.com/Microsoft/TypeScript/issues/24722
class A {
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