Flag resolution mode assertions in non-nightly builds

src/compiler/checker.ts

@@ -40718,6 +40718,10 @@ namespace ts {
                 const validForTypeAssertions = isExclusivelyTypeOnlyImportOrExport(declaration);
                 const override = getResolutionModeOverrideForClause(declaration.assertClause, validForTypeAssertions ? grammarErrorOnNode : undefined);
                 if (validForTypeAssertions && override) {
+                    if (!isNightly()) {
+                        grammarErrorOnNode(declaration.assertClause, Diagnostics.Resolution_mode_assertions_are_unstable_Use_nightly_TypeScript_to_silence_this_error_Try_updating_with_npm_install_D_typescript_next);
+                    }
+
                     if (getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.Node16 && getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.NodeNext) {
                         return grammarErrorOnNode(declaration.assertClause, Diagnostics.Resolution_modes_are_only_supported_when_moduleResolution_is_node16_or_nodenext);
                     }

src/compiler/diagnosticMessages.json

@@ -3880,6 +3880,10 @@
         "category": "Error",
         "code": 4124
     },
+    "Resolution mode assertions are unstable. Use nightly TypeScript to silence this error. Try updating with 'npm install -D typescript@next'.": {
+        "category": "Error",
+        "code": 4125
+    },
 
     "The current host does not support the '{0}' option.": {
         "category": "Error",

src/compiler/program.ts

@@ -593,7 +593,7 @@ namespace ts {
     }
 
     /* @internal */
-    export function getResolutionModeOverrideForClause(clause: AssertClause | undefined, grammarErrorOnNode?: (node: Node, diagnostic: DiagnosticMessage) => boolean) {
+    export function getResolutionModeOverrideForClause(clause: AssertClause | undefined, grammarErrorOnNode?: (node: Node, diagnostic: DiagnosticMessage) => void) {
         if (!clause) return undefined;
         if (length(clause.elements) !== 1) {
             grammarErrorOnNode?.(clause, Diagnostics.Type_import_assertions_should_have_exactly_one_key_resolution_mode_with_value_import_or_require);

src/compiler/transformers/declarations.ts

@@ -733,7 +733,7 @@ namespace ts {
                     decl.modifiers,
                     decl.importClause,
                     rewriteModuleSpecifier(decl, decl.moduleSpecifier),
-                    getResolutionModeOverrideForClause(decl.assertClause) ? decl.assertClause : undefined
+                    getResolutionModeOverrideForClauseInNightly(decl.assertClause)
                 );
             }
             // The `importClause` visibility corresponds to the default's visibility.
@@ -745,7 +745,7 @@ namespace ts {
                     decl.importClause.isTypeOnly,
                     visibleDefaultBinding,
                     /*namedBindings*/ undefined,
-                ), rewriteModuleSpecifier(decl, decl.moduleSpecifier), getResolutionModeOverrideForClause(decl.assertClause) ? decl.assertClause : undefined);
+                ), rewriteModuleSpecifier(decl, decl.moduleSpecifier), getResolutionModeOverrideForClauseInNightly(decl.assertClause));
             }
             if (decl.importClause.namedBindings.kind === SyntaxKind.NamespaceImport) {
                 // Namespace import (optionally with visible default)
@@ -755,7 +755,7 @@ namespace ts {
                     decl.importClause.isTypeOnly,
                     visibleDefaultBinding,
                     namedBindings,
-                ), rewriteModuleSpecifier(decl, decl.moduleSpecifier), getResolutionModeOverrideForClause(decl.assertClause) ? decl.assertClause : undefined) : undefined;
+                ), rewriteModuleSpecifier(decl, decl.moduleSpecifier), getResolutionModeOverrideForClauseInNightly(decl.assertClause)) : undefined;
             }
             // Named imports (optionally with visible default)
             const bindingList = mapDefined(decl.importClause.namedBindings.elements, b => resolver.isDeclarationVisible(b) ? b : undefined);
@@ -771,7 +771,7 @@ namespace ts {
                         bindingList && bindingList.length ? factory.updateNamedImports(decl.importClause.namedBindings, bindingList) : undefined,
                     ),
                     rewriteModuleSpecifier(decl, decl.moduleSpecifier),
-                    getResolutionModeOverrideForClause(decl.assertClause) ? decl.assertClause : undefined
+                    getResolutionModeOverrideForClauseInNightly(decl.assertClause)
                 );
             }
             // Augmentation of export depends on import
@@ -782,12 +782,23 @@ namespace ts {
                     decl.modifiers,
                     /*importClause*/ undefined,
                     rewriteModuleSpecifier(decl, decl.moduleSpecifier),
-                    getResolutionModeOverrideForClause(decl.assertClause) ? decl.assertClause : undefined
+                    getResolutionModeOverrideForClauseInNightly(decl.assertClause)
                 );
             }
             // Nothing visible
         }
 
+        function getResolutionModeOverrideForClauseInNightly(assertClause: AssertClause | undefined) {
+            const mode = getResolutionModeOverrideForClause(assertClause);
+            if (mode !== undefined) {
+                if (!isNightly()) {
+                    context.addDiagnostic(createDiagnosticForNode(assertClause!, Diagnostics.Resolution_mode_assertions_are_unstable_Use_nightly_TypeScript_to_silence_this_error_Try_updating_with_npm_install_D_typescript_next));
+                }
+                return assertClause;
+            }
+            return undefined;
+        }
+
         function transformAndReplaceLatePaintedStatements(statements: NodeArray<Statement>): NodeArray<Statement> {
             // This is a `while` loop because `handleSymbolAccessibilityError` can see additional import aliases marked as visible during
             // error handling which must now be included in the output and themselves checked for errors.

src/compiler/utilities.ts

@@ -4121,6 +4121,10 @@ namespace ts {
         return indentStrings[1].length;
     }
 
+    export function isNightly() {
+        return stringContains(version, "-dev") || stringContains(version, "-insiders");
+    }
+
     export function createTextWriter(newLine: string): EmitTextWriter {
         let output: string;
         let indent: number;