Handle .mts and .cts files in @babel/preset-typescript (#13838)

packages/babel-helper-fixtures/src/index.ts

@@ -64,7 +64,7 @@ function shouldIgnore(name, ignore?: Array<string>) {
   );
 }
 
-const EXTENSIONS = [".js", ".mjs", ".ts", ".tsx"];
+const EXTENSIONS = [".js", ".mjs", ".ts", ".tsx", ".cts", ".mts"];
 
 function findFile(filepath: string, allowJSON?: boolean) {
   const matches = [];
@@ -134,6 +134,7 @@ function pushTask(taskName, taskDir, suite, suiteName) {
         ? taskOpts.BABEL_8_BREAKING === false
         : taskOpts.BABEL_8_BREAKING === true),
     options: taskOpts,
+    doNotSetSourceType: taskOpts.DO_NOT_SET_SOURCE_TYPE,
     externalHelpers:
       taskOpts.externalHelpers ??
       !!tryResolve("@babel/plugin-external-helpers"),
@@ -162,6 +163,7 @@ function pushTask(taskName, taskDir, suite, suiteName) {
   };
 
   delete taskOpts.BABEL_8_BREAKING;
+  delete taskOpts.DO_NOT_SET_SOURCE_TYPE;
 
   // If there's node requirement, check it before pushing task
   if (taskOpts.minNodeVersion) {

packages/babel-helper-transform-fixture-test-runner/src/index.ts

@@ -230,6 +230,7 @@ function run(task) {
     expect: expected,
     exec,
     options: opts,
+    doNotSetSourceType,
     optionsDir,
     validateLogs,
     ignoreOutput,
@@ -245,7 +246,7 @@ function run(task) {
       filename: self.loc,
       filenameRelative: self.filename,
       sourceFileName: self.filename,
-      sourceType: "script",
+      ...(doNotSetSourceType ? {} : { sourceType: "script" }),
       babelrc: false,
       configFile: false,
       inputSourceMap: task.inputSourceMap || undefined,

packages/babel-parser/src/plugins/typescript/index.js

@@ -135,6 +135,10 @@ const TSErrors = makeErrorTemplates(
       "Private elements cannot have an accessibility modifier ('%0').",
     ReadonlyForMethodSignature:
       "'readonly' modifier can only appear on a property declaration or index signature.",
+    ReservedArrowTypeParam:
+      "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`.",
+    ReservedTypeAssertion:
+      "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead.",
     SetAccesorCannotHaveOptionalParameter:
       "A 'set' accessor cannot have an optional parameter.",
     SetAccesorCannotHaveRestParameter:
@@ -359,12 +363,14 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     tsParseDelimitedList<T: N.Node>(
       kind: ParsingContext,
       parseElement: () => T,
+      refTrailingCommaPos?: { value: number },
     ): T[] {
       return nonNull(
         this.tsParseDelimitedListWorker(
           kind,
           parseElement,
           /* expectSuccess */ true,
+          refTrailingCommaPos,
         ),
       );
     }
@@ -377,13 +383,16 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       kind: ParsingContext,
       parseElement: () => ?T,
       expectSuccess: boolean,
+      refTrailingCommaPos?: { value: number },
     ): ?(T[]) {
       const result = [];
+      let trailingCommaPos = -1;
 
       for (;;) {
         if (this.tsIsListTerminator(kind)) {
           break;
         }
+        trailingCommaPos = -1;
 
         const element = parseElement();
         if (element == null) {
@@ -392,6 +401,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         result.push(element);
 
         if (this.eat(tt.comma)) {
+          trailingCommaPos = this.state.lastTokStart;
           continue;
         }
 
@@ -406,6 +416,10 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         return undefined;
       }
 
+      if (refTrailingCommaPos) {
+        refTrailingCommaPos.value = trailingCommaPos;
+      }
+
       return result;
     }
 
@@ -414,6 +428,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       parseElement: () => T,
       bracket: boolean,
       skipFirstToken: boolean,
+      refTrailingCommaPos?: { value: number },
     ): T[] {
       if (!skipFirstToken) {
         if (bracket) {
@@ -423,7 +438,11 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         }
       }
 
-      const result = this.tsParseDelimitedList(kind, parseElement);
+      const result = this.tsParseDelimitedList(
+        kind,
+        parseElement,
+        refTrailingCommaPos,
+      );
 
       if (bracket) {
         this.expect(tt.bracketR);
@@ -524,15 +543,21 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         this.unexpected();
       }
 
+      const refTrailingCommaPos = { value: -1 };
+
       node.params = this.tsParseBracketedList(
         "TypeParametersOrArguments",
         this.tsParseTypeParameter.bind(this),
         /* bracket */ false,
         /* skipFirstToken */ true,
+        refTrailingCommaPos,
       );
       if (node.params.length === 0) {
         this.raise(node.start, TSErrors.EmptyTypeParameters);
       }
+      if (refTrailingCommaPos.value !== -1) {
+        this.addExtra(node, "trailingComma", refTrailingCommaPos.value);
+      }
       return this.finishNode(node, "TSTypeParameterDeclaration");
     }
 
@@ -1403,6 +1428,10 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     tsParseTypeAssertion(): N.TsTypeAssertion {
+      if (this.getPluginOption("typescript", "disallowAmbiguousJSXLike")) {
+        this.raise(this.state.start, TSErrors.ReservedTypeAssertion);
+      }
+
       const node: N.TsTypeAssertion = this.startNode();
       const _const = this.tsTryNextParseConstantContext();
       node.typeAnnotation = _const || this.tsNextThenParseType();
@@ -2854,7 +2883,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
 
       // Either way, we're looking at a '<': tt.jsxTagStart or relational.
 
-      let typeParameters: N.TsTypeParameterDeclaration;
+      let typeParameters: ?N.TsTypeParameterDeclaration;
       state = state || this.state.clone();
 
       const arrow = this.tryParse(abort => {
@@ -2878,7 +2907,13 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }, state);
 
       /*:: invariant(arrow.node != null) */
-      if (!arrow.error && !arrow.aborted) return arrow.node;
+      if (!arrow.error && !arrow.aborted) {
+        // This error is reported outside of the this.tryParse call so that
+        // in case of <T>(x) => 2, we don't consider <T>(x) as a type assertion
+        // because of this error.
+        if (typeParameters) this.reportReservedArrowTypeParam(typeParameters);
+        return arrow.node;
+      }
 
       if (!jsx) {
         // Try parsing a type cast instead of an arrow function.
@@ -2903,6 +2938,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       if (arrow.node) {
         /*:: invariant(arrow.failState) */
         this.state = arrow.failState;
+        if (typeParameters) this.reportReservedArrowTypeParam(typeParameters);
         return arrow.node;
       }
 
@@ -2919,6 +2955,16 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       throw jsx?.error || arrow.error || typeCast?.error;
     }
 
+    reportReservedArrowTypeParam(node: any) {
+      if (
+        node.params.length === 1 &&
+        !node.extra?.trailingComma &&
+        this.getPluginOption("typescript", "disallowAmbiguousJSXLike")
+      ) {
+        this.raise(node.start, TSErrors.ReservedArrowTypeParam);
+      }
+    }
+
     // Handle type assertions
     parseMaybeUnary(refExpressionErrors?: ?ExpressionErrors): N.Expression {
       if (!this.hasPlugin("jsx") && this.isRelational("<")) {

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": [["typescript", { "disallowAmbiguousJSXLike": true }]]
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-assertion/input.ts

@@ -0,0 +1 @@
+<T>x;

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-assertion/output.json

@@ -0,0 +1,38 @@
+{
+  "type": "File",
+  "start":0,"end":5,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":5}},
+  "errors": [
+    "SyntaxError: This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead. (1:0)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":5,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":5}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":5,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":5}},
+        "expression": {
+          "type": "TSTypeAssertion",
+          "start":0,"end":4,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":4}},
+          "typeAnnotation": {
+            "type": "TSTypeReference",
+            "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2}},
+            "typeName": {
+              "type": "Identifier",
+              "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2},"identifierName":"T"},
+              "name": "T"
+            }
+          },
+          "expression": {
+            "type": "Identifier",
+            "start":3,"end":4,"loc":{"start":{"line":1,"column":3},"end":{"line":1,"column":4},"identifierName":"x"},
+            "name": "x"
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-babel-7/input.ts

@@ -0,0 +1,2 @@
+<T>() => 1;
+<T>(x) => 1;

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-babel-7/options.json

@@ -0,0 +1,3 @@
+{
+  "BABEL_8_BREAKING": false
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-babel-7/output.json

@@ -0,0 +1,87 @@
+{
+  "type": "File",
+  "start":0,"end":24,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":12}},
+  "errors": [
+    "SyntaxError: This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (1:0)",
+    "SyntaxError: This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (2:0)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":24,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":12}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":11,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":11}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":0,"end":10,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":10}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [],
+          "body": {
+            "type": "NumericLiteral",
+            "start":9,"end":10,"loc":{"start":{"line":1,"column":9},"end":{"line":1,"column":10}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":0,"end":3,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":3}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2}},
+                "name": "T"
+              }
+            ]
+          }
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start":12,"end":24,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":12}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":12,"end":23,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":11}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [
+            {
+              "type": "Identifier",
+              "start":16,"end":17,"loc":{"start":{"line":2,"column":4},"end":{"line":2,"column":5},"identifierName":"x"},
+              "name": "x"
+            }
+          ],
+          "body": {
+            "type": "NumericLiteral",
+            "start":22,"end":23,"loc":{"start":{"line":2,"column":10},"end":{"line":2,"column":11}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":12,"end":15,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":3}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":13,"end":14,"loc":{"start":{"line":2,"column":1},"end":{"line":2,"column":2}},
+                "name": "T"
+              }
+            ]
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-unambiguous-babel-7/input.ts

@@ -0,0 +1,2 @@
+<T,>() => 1;
+<T,>(x) => 1;

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-unambiguous-babel-7/options.json

@@ -0,0 +1,3 @@
+{
+  "BABEL_8_BREAKING": false
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-unambiguous-babel-7/output.json

@@ -0,0 +1,89 @@
+{
+  "type": "File",
+  "start":0,"end":26,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":13}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":26,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":13}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":12,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":12}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":0,"end":11,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":11}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [],
+          "body": {
+            "type": "NumericLiteral",
+            "start":10,"end":11,"loc":{"start":{"line":1,"column":10},"end":{"line":1,"column":11}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":0,"end":4,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":4}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2}},
+                "name": "T"
+              }
+            ],
+            "extra": {
+              "trailingComma": 2
+            }
+          }
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start":13,"end":26,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":13}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":13,"end":25,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":12}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [
+            {
+              "type": "Identifier",
+              "start":18,"end":19,"loc":{"start":{"line":2,"column":5},"end":{"line":2,"column":6},"identifierName":"x"},
+              "name": "x"
+            }
+          ],
+          "body": {
+            "type": "NumericLiteral",
+            "start":24,"end":25,"loc":{"start":{"line":2,"column":11},"end":{"line":2,"column":12}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":13,"end":17,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":4}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":14,"end":15,"loc":{"start":{"line":2,"column":1},"end":{"line":2,"column":2}},
+                "name": "T"
+              }
+            ],
+            "extra": {
+              "trailingComma": 15
+            }
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-unambiguous/input.ts

@@ -0,0 +1,2 @@
+<T,>() => 1;
+<T,>(x) => 1;

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-unambiguous/options.json

@@ -0,0 +1,3 @@
+{
+  "BABEL_8_BREAKING": true
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter-unambiguous/output.json

@@ -0,0 +1,97 @@
+{
+  "type": "File",
+  "start":0,"end":26,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":13}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":26,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":13}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":12,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":12}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":0,"end":11,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":11}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [],
+          "body": {
+            "type": "NumericLiteral",
+            "start":10,"end":11,"loc":{"start":{"line":1,"column":10},"end":{"line":1,"column":11}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":0,"end":4,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":4}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2}},
+                "name": {
+                  "type": "Identifier",
+                  "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2},"identifierName":"T"},
+                  "name": "T"
+                }
+              }
+            ],
+            "extra": {
+              "trailingComma": 2
+            }
+          }
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start":13,"end":26,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":13}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":13,"end":25,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":12}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [
+            {
+              "type": "Identifier",
+              "start":18,"end":19,"loc":{"start":{"line":2,"column":5},"end":{"line":2,"column":6},"identifierName":"x"},
+              "name": "x"
+            }
+          ],
+          "body": {
+            "type": "NumericLiteral",
+            "start":24,"end":25,"loc":{"start":{"line":2,"column":11},"end":{"line":2,"column":12}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":13,"end":17,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":4}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":14,"end":15,"loc":{"start":{"line":2,"column":1},"end":{"line":2,"column":2}},
+                "name": {
+                  "type": "Identifier",
+                  "start":14,"end":15,"loc":{"start":{"line":2,"column":1},"end":{"line":2,"column":2},"identifierName":"T"},
+                  "name": "T"
+                }
+              }
+            ],
+            "extra": {
+              "trailingComma": 15
+            }
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter/input.ts

@@ -0,0 +1,2 @@
+<T>() => 1;
+<T>(x) => 1;

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter/options.json

@@ -0,0 +1,3 @@
+{
+  "BABEL_8_BREAKING": true
+}

packages/babel-parser/test/fixtures/typescript/disallow-jsx-ambiguity/type-parameter/output.json

@@ -0,0 +1,95 @@
+{
+  "type": "File",
+  "start":0,"end":24,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":12}},
+  "errors": [
+    "SyntaxError: This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (1:0)",
+    "SyntaxError: This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (2:0)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":24,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":12}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":11,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":11}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":0,"end":10,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":10}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [],
+          "body": {
+            "type": "NumericLiteral",
+            "start":9,"end":10,"loc":{"start":{"line":1,"column":9},"end":{"line":1,"column":10}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":0,"end":3,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":3}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2}},
+                "name": {
+                  "type": "Identifier",
+                  "start":1,"end":2,"loc":{"start":{"line":1,"column":1},"end":{"line":1,"column":2},"identifierName":"T"},
+                  "name": "T"
+                }
+              }
+            ]
+          }
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start":12,"end":24,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":12}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":12,"end":23,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":11}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [
+            {
+              "type": "Identifier",
+              "start":16,"end":17,"loc":{"start":{"line":2,"column":4},"end":{"line":2,"column":5},"identifierName":"x"},
+              "name": "x"
+            }
+          ],
+          "body": {
+            "type": "NumericLiteral",
+            "start":22,"end":23,"loc":{"start":{"line":2,"column":10},"end":{"line":2,"column":11}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "typeParameters": {
+            "type": "TSTypeParameterDeclaration",
+            "start":12,"end":15,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":3}},
+            "params": [
+              {
+                "type": "TSTypeParameter",
+                "start":13,"end":14,"loc":{"start":{"line":2,"column":1},"end":{"line":2,"column":2}},
+                "name": {
+                  "type": "Identifier",
+                  "start":13,"end":14,"loc":{"start":{"line":2,"column":1},"end":{"line":2,"column":2},"identifierName":"T"},
+                  "name": "T"
+                }
+              }
+            ]
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/typings/babel-parser.d.ts

@@ -185,6 +185,7 @@ export interface FlowPluginOptions {
 
 export interface TypeScriptPluginOptions {
   dts?: boolean;
+  disallowAmbiguousJSXLike?: boolean;
 }
 
 export const tokTypes: {

packages/babel-plugin-syntax-typescript/package.json

@@ -24,7 +24,8 @@
     "@babel/core": "^7.0.0-0"
   },
   "devDependencies": {
-    "@babel/core": "workspace:^"
+    "@babel/core": "workspace:^",
+    "@babel/helper-plugin-test-runner": "workspace:^"
   },
   "engines": {
     "node": ">=6.9.0"

packages/babel-plugin-syntax-typescript/src/index.js

@@ -15,7 +15,7 @@ function removePlugin(plugins, name) {
   }
 }
 
-export default declare((api, { isTSX }) => {
+export default declare((api, { isTSX, disallowAmbiguousJSXLike }) => {
   api.assertVersion(7);
 
   return {
@@ -31,7 +31,10 @@ export default declare((api, { isTSX }) => {
       // in TS depends on the extensions, and is purely dependent on 'isTSX'.
       removePlugin(plugins, "jsx");
 
-      parserOpts.plugins.push("typescript", "classProperties");
+      parserOpts.plugins.push(
+        ["typescript", { disallowAmbiguousJSXLike }],
+        "classProperties",
+      );
 
       if (!process.env.BABEL_8_BREAKING) {
         // This is enabled by default since @babel/parser 7.1.5

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": [["syntax-typescript", { "disallowAmbiguousJSXLike": true }]]
+}

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-assertion/input.ts

@@ -0,0 +1 @@
+<T>x;

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-assertion/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead. (1:0)"
+}

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-parameter-unambiguous/input.ts

@@ -0,0 +1,2 @@
+<T,>() => 1;
+<T,>(x) => 1;

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-parameter-unambiguous/output.js

@@ -0,0 +1,3 @@
+<T>() => 1;
+
+<T>(x) => 1;

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-parameter/input.ts

@@ -0,0 +1,2 @@
+<T>() => 1;
+<T>(x) => 1;

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-parameter/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (1:0)"
+}

packages/babel-plugin-syntax-typescript/test/index.js

@@ -0,0 +1,3 @@
+import runner from "@babel/helper-plugin-test-runner";
+
+runner(import.meta.url);

packages/babel-preset-typescript/src/index.js

@@ -8,6 +8,7 @@ export default declare((api, opts) => {
   const {
     allExtensions,
     allowNamespaces,
+    disallowAmbiguousJSXLike,
     isTSX,
     jsxPragma,
     jsxPragmaFrag,
@@ -16,17 +17,19 @@ export default declare((api, opts) => {
   } = normalizeOptions(opts);
 
   const pluginOptions = process.env.BABEL_8_BREAKING
-    ? isTSX => ({
+    ? (isTSX, disallowAmbiguousJSXLike) => ({
         allowNamespaces,
+        disallowAmbiguousJSXLike,
         isTSX,
         jsxPragma,
         jsxPragmaFrag,
         onlyRemoveTypeImports,
         optimizeConstEnums,
       })
-    : isTSX => ({
+    : (isTSX, disallowAmbiguousJSXLike) => ({
         allowDeclareFields: opts.allowDeclareFields,
         allowNamespaces,
+        disallowAmbiguousJSXLike,
         isTSX,
         jsxPragma,
         jsxPragmaFrag,
@@ -38,21 +41,36 @@ export default declare((api, opts) => {
     overrides: allExtensions
       ? [
           {
-            plugins: [[transformTypeScript, pluginOptions(isTSX)]],
+            plugins: [
+              [
+                transformTypeScript,
+                pluginOptions(isTSX, disallowAmbiguousJSXLike),
+              ],
+            ],
           },
         ]
-      : [
+      : // Only set 'test' if explicitly requested, since it requires that
+        // Babel is being called`
+        [
           {
-            // Only set 'test' if explicitly requested, since it requires that
-            // Babel is being called`
             test: /\.ts$/,
-            plugins: [[transformTypeScript, pluginOptions(false)]],
+            plugins: [[transformTypeScript, pluginOptions(false, false)]],
+          },
+          {
+            test: /\.mts$/,
+            sourceType: "module",
+            plugins: [[transformTypeScript, pluginOptions(false, true)]],
+          },
+          {
+            test: /\.cts$/,
+            sourceType: "script",
+            plugins: [[transformTypeScript, pluginOptions(false, true)]],
           },
           {
-            // Only set 'test' if explicitly requested, since it requires that
-            // Babel is being called`
             test: /\.tsx$/,
-            plugins: [[transformTypeScript, pluginOptions(true)]],
+            // disallowAmbiguousJSXLike is a no-op when parsing TSX, since it's
+            // always disallowed.
+            plugins: [[transformTypeScript, pluginOptions(true, false)]],
           },
         ],
   };

packages/babel-preset-typescript/src/normalize-options.js

@@ -7,6 +7,7 @@ export default function normalizeOptions(options = {}) {
   const TopLevelOptions = {
     allExtensions: "allExtensions",
     allowNamespaces: "allowNamespaces",
+    disallowAmbiguousJSXLike: "disallowAmbiguousJSXLike",
     isTSX: "isTSX",
     jsxPragma: "jsxPragma",
     jsxPragmaFrag: "jsxPragmaFrag",
@@ -54,6 +55,18 @@ export default function normalizeOptions(options = {}) {
     v.invariant(allExtensions, "isTSX:true requires allExtensions:true");
   }
 
+  const disallowAmbiguousJSXLike = v.validateBooleanOption(
+    TopLevelOptions.disallowAmbiguousJSXLike,
+    options.disallowAmbiguousJSXLike,
+    false,
+  );
+  if (disallowAmbiguousJSXLike) {
+    v.invariant(
+      allExtensions,
+      "disallowAmbiguousJSXLike:true requires allExtensions:true",
+    );
+  }
+
   const optimizeConstEnums = v.validateBooleanOption(
     TopLevelOptions.optimizeConstEnums,
     options.optimizeConstEnums,
@@ -63,6 +76,7 @@ export default function normalizeOptions(options = {}) {
   return {
     allExtensions,
     allowNamespaces,
+    disallowAmbiguousJSXLike,
     isTSX,
     jsxPragma,
     jsxPragmaFrag,

packages/babel-preset-typescript/test/fixtures/node-extensions/import-in-cts/input.cts

@@ -0,0 +1 @@
+import "x";

packages/babel-preset-typescript/test/fixtures/node-extensions/import-in-cts/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "'import' and 'export' may appear only with 'sourceType: \"module\"' (1:0)"
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/import-in-mts/input.mts

@@ -0,0 +1 @@
+import "x";

packages/babel-preset-typescript/test/fixtures/node-extensions/import-in-mts/output.mjs

@@ -0,0 +1 @@
+import "x";

packages/babel-preset-typescript/test/fixtures/node-extensions/options.json

@@ -0,0 +1,4 @@
+{
+  "DO_NOT_SET_SOURCE_TYPE": true,
+  "presets": ["typescript"]
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-cts/input.cts

@@ -0,0 +1 @@
+<T> x;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-cts/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead. (1:0)"
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-mts/input.mts

@@ -0,0 +1 @@
+<T> x;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-mts/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead. (1:0)"
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-ts/input.ts

@@ -0,0 +1 @@
+<T> x;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-ts/options.json

@@ -0,0 +1,3 @@
+{
+  "DO_NOT_SET_SOURCE_TYPE": false
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-assertion-in-ts/output.js

@@ -0,0 +1 @@
+x;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-cts/input.mts

@@ -0,0 +1 @@
+<T>() => 0;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-cts/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (1:0)"
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-mts/input.mts

@@ -0,0 +1 @@
+<T>() => 0;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-mts/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (1:0)"
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-ts/input.ts

@@ -0,0 +1 @@
+<T>() => 0;

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-ts/options.json

@@ -0,0 +1,3 @@
+{
+  "DO_NOT_SET_SOURCE_TYPE": false
+}

packages/babel-preset-typescript/test/fixtures/node-extensions/type-param-arrow-in-ts/output.js

@@ -0,0 +1 @@
+() => 0;

packages/babel-preset-typescript/test/fixtures/node-extensions/with-in-cts/input.cts

@@ -0,0 +1 @@
+with (x) {}

packages/babel-preset-typescript/test/fixtures/node-extensions/with-in-cts/output.js

@@ -0,0 +1 @@
+with (x) {}

packages/babel-preset-typescript/test/fixtures/node-extensions/with-in-mts/input.mts

@@ -0,0 +1 @@
+with (x) {}

packages/babel-preset-typescript/test/fixtures/node-extensions/with-in-mts/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "'with' in strict mode. (1:0)"
+}

packages/babel-preset-typescript/test/normalize-options.spec.js

@@ -30,16 +30,17 @@ describe("normalize options", () => {
     });
     it("default values", () => {
       expect(normalizeOptions({})).toMatchInlineSnapshot(`
-        Object {
+Object {
-          "allExtensions": false,
+  "allExtensions": false,
-          "allowNamespaces": true,
+  "allowNamespaces": true,
-          "isTSX": false,
+  "disallowAmbiguousJSXLike": false,
-          "jsxPragma": "React",
+  "isTSX": false,
-          "jsxPragmaFrag": "React.Fragment",
+  "jsxPragma": "React",
-          "onlyRemoveTypeImports": true,
+  "jsxPragmaFrag": "React.Fragment",
-          "optimizeConstEnums": false,
+  "onlyRemoveTypeImports": true,
-        }
+  "optimizeConstEnums": false,
-      `);
+}
+`);
     });
   });
   (process.env.BABEL_8_BREAKING ? describe.skip : describe)("Babel 7", () => {
@@ -78,16 +79,17 @@ describe("normalize options", () => {
     );
     it("default values", () => {
       expect(normalizeOptions({})).toMatchInlineSnapshot(`
-        Object {
+Object {
-          "allExtensions": false,
+  "allExtensions": false,
-          "allowNamespaces": true,
+  "allowNamespaces": true,
-          "isTSX": false,
+  "disallowAmbiguousJSXLike": false,
-          "jsxPragma": undefined,
+  "isTSX": false,
-          "jsxPragmaFrag": "React.Fragment",
+  "jsxPragma": undefined,
-          "onlyRemoveTypeImports": undefined,
+  "jsxPragmaFrag": "React.Fragment",
-          "optimizeConstEnums": false,
+  "onlyRemoveTypeImports": undefined,
-        }
+  "optimizeConstEnums": false,
-      `);
+}
+`);
     });
   });
 });

yarn.lock

@@ -2021,6 +2021,7 @@ __metadata:
   resolution: "@babel/plugin-syntax-typescript@workspace:packages/babel-plugin-syntax-typescript"
   dependencies:
     "@babel/core": "workspace:^"
+    "@babel/helper-plugin-test-runner": "workspace:^"
     "@babel/helper-plugin-utils": "workspace:^"
   peerDependencies:
     "@babel/core": ^7.0.0-0