--erasableSyntaxOnly

src/compiler/checker.ts

@@ -41290,6 +41290,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         checkVariableLikeDeclaration(node);
         const func = getContainingFunction(node)!;
         if (hasSyntacticModifier(node, ModifierFlags.ParameterPropertyModifier)) {
+            if (compilerOptions.erasableSyntaxOnly) {
+                error(node, Diagnostics.This_syntax_is_not_allowed_when_erasableSyntaxOnly_is_enabled);
+            }
             if (!(func.kind === SyntaxKind.Constructor && nodeIsPresent(func.body))) {
                 error(node, Diagnostics.A_parameter_property_is_only_allowed_in_a_constructor_implementation);
             }
@@ -47504,6 +47507,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         checkExportsOnMergedDeclarations(node);
         node.members.forEach(checkEnumMember);
 
+        if (compilerOptions.erasableSyntaxOnly && (node.flags & NodeFlags.Ambient) === 0) {
+            error(node, Diagnostics.This_syntax_is_not_allowed_when_erasableSyntaxOnly_is_enabled);
+        }
+
         computeEnumMemberValues(node);
 
         // Spec 2014 - Section 9.3:
@@ -47643,6 +47650,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 && !inAmbientContext
                 && isInstantiatedModule(node, shouldPreserveConstEnums(compilerOptions))
             ) {
+                if (compilerOptions.erasableSyntaxOnly) {
+                    error(node.name, Diagnostics.This_syntax_is_not_allowed_when_erasableSyntaxOnly_is_enabled);
+                }
+
                 if (getIsolatedModules(compilerOptions) && !getSourceFileOfNode(node).externalModuleIndicator) {
                     // This could be loosened a little if needed. The only problem we are trying to avoid is unqualified
                     // references to namespace members declared in other files. But use of namespaces is discouraged anyway,

src/compiler/commandLineParser.ts

@@ -855,6 +855,15 @@ const commandOptionsWithoutBuild: CommandLineOption[] = [
         affectsBuildInfo: true,
         affectsSemanticDiagnostics: true,
     },
+    {
+        name: "erasableSyntaxOnly",
+        type: "boolean",
+        category: Diagnostics.Interop_Constraints,
+        description: Diagnostics.Do_not_allow_runtime_constructs_that_are_not_part_of_ECMAScript,
+        defaultValueDescription: false,
+        affectsBuildInfo: true,
+        affectsSemanticDiagnostics: true,
+    },
 
     // Strict Type Checks
     {

src/compiler/diagnosticMessages.json

@@ -971,7 +971,10 @@
         "category": "Error",
         "code": 1293
     },
-
+    "This syntax is not allowed when 'erasableSyntaxOnly' is enabled.": {
+        "category": "Error",
+        "code": 1294
+    },
     "'with' statements are not allowed in an async function block.": {
         "category": "Error",
         "code": 1300
@@ -6467,11 +6470,14 @@
         "category": "Message",
         "code": 6719
     },
-
     "Built-in iterators are instantiated with a 'TReturn' type of 'undefined' instead of 'any'.": {
         "category": "Message",
         "code": 6720
     },
+    "Do not allow runtime constructs that are not part of ECMAScript.": {
+        "category": "Message",
+        "code": 6721
+    },
     "Default catch clause variables as 'unknown' instead of 'any'.": {
         "category": "Message",
         "code": 6803

src/compiler/types.ts

@@ -7502,6 +7502,7 @@ export interface CompilerOptions {
     /** Paths used to compute primary types search locations */
     typeRoots?: string[];
     verbatimModuleSyntax?: boolean;
+    erasableSyntaxOnly?: boolean;
     /** @internal */ version?: boolean;
     /** @internal */ watch?: boolean;
     esModuleInterop?: boolean;

tests/baselines/reference/api/typescript.d.ts

@@ -7102,6 +7102,7 @@ declare namespace ts {
         /** Paths used to compute primary types search locations */
         typeRoots?: string[];
         verbatimModuleSyntax?: boolean;
+        erasableSyntaxOnly?: boolean;
         esModuleInterop?: boolean;
         useDefineForClassFields?: boolean;
         [option: string]: CompilerOptionsValue | TsConfigSourceFile | undefined;

tests/baselines/reference/config/initTSConfig/Default initialized TSConfig/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with --help/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with --watch/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with advanced options/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with enum value compiler options/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with files options/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/initTSConfig/Initialized TSConfig with list compiler options/tsconfig.json

@@ -77,6 +77,7 @@
     // "isolatedModules": true,                          /* Ensure that each file can be safely transpiled without relying on other imports. */
     // "verbatimModuleSyntax": true,                     /* Do not transform or elide any imports or exports not marked as type-only, ensuring they are written in the output file's format based on the 'module' setting. */
     // "isolatedDeclarations": true,                     /* Require sufficient annotation on exports so other tools can trivially generate declaration files. */
+    // "erasableSyntaxOnly": true,                       /* Do not allow runtime constructs that are not part of ECMAScript. */
     // "allowSyntheticDefaultImports": true,             /* Allow 'import x from y' when a module doesn't have a default export. */
     "esModuleInterop": true,                             /* Emit additional JavaScript to ease support for importing CommonJS modules. This enables 'allowSyntheticDefaultImports' for type compatibility. */
     // "preserveSymlinks": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */

tests/baselines/reference/config/showConfig/Shows tsconfig for single option/erasableSyntaxOnly/tsconfig.json

@@ -0,0 +1,5 @@
+{
+    "compilerOptions": {
+        "erasableSyntaxOnly": true
+    }
+}

tests/baselines/reference/erasableSyntaxOnly.errors.txt

@@ -0,0 +1,67 @@
+erasableSyntaxOnly.ts(3,17): error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+erasableSyntaxOnly.ts(6,11): error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+erasableSyntaxOnly.ts(10,11): error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+erasableSyntaxOnly.ts(16,6): error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+erasableSyntaxOnly.ts(20,12): error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+
+
+==== erasableSyntaxOnly.ts (5 errors) ====
+    class MyClassErr {
+        // No parameter properties
+        constructor(public foo: string) { }
+                    ~~~~~~~~~~~~~~~~~~
+!!! error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+    }
+
+    namespace IllegalBecauseInstantiated {
+              ~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+        export const m = 1;
+    }
+
+    namespace AlsoIllegalBecauseInstantiated {
+              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+        class PrivateClass {
+
+        }
+    }
+
+    enum NotLegalEnum {
+         ~~~~~~~~~~~~
+!!! error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+        B = 1
+    }
+
+    const enum NotLegalConstEnum {
+               ~~~~~~~~~~~~~~~~~
+!!! error TS1294: This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
+        C = 2
+    }
+
+    // No errors after this point
+    class MyClassOk {
+        // Not a parameter property, ok
+        constructor(foo: string) { }
+    }
+    namespace NotInstantiated {
+        export interface JustAType { }
+        export type ATypeInANamespace = {};
+    }
+    declare namespace AmbientIsNotInstantiated {
+        export const stillOk = 12;
+    }
+
+    declare enum LegalEnum {
+        A = 1
+    }
+
+    declare namespace AmbientStuff {
+        namespace Nested {
+            export const stillOk = 12;
+        }
+        enum EnumInAmbientContext {
+            B = 1
+        }
+    }
+