Fix "sourceType: unambiguous" with TLA

packages/babel-parser/src/index.js

@@ -25,15 +25,31 @@ export function parse(input: string, options?: Options): File {
       const parser = getParser(options, input);
       const ast = parser.parse();
 
-      // Rather than try to parse as a script first, we opt to parse as a module and convert back
-      // to a script where possible to avoid having to do a full re-parse of the input content.
-      if (!parser.sawUnambiguousESM) ast.program.sourceType = "script";
-      return ast;
+      if (parser.sawUnambiguousESM) {
+        return ast;
+      }
+
+      if (!parser.ambiguousScriptDifferentAst) {
+        ast.program.sourceType = "script";
+        return ast;
+      }
+
+      // Top level await introduces code which can be both a valid script and
+      // a valid module, but which produces different ASTs:
+      //    await
+      //    0
+      // can be parser either as an AwaitExpression, or as two ExpressionStatements.
+      try {
+        options.sourceType = "script";
+        return getParser(options, input).parse();
+      } catch {
+        return ast;
+      }
     } catch (moduleError) {
       try {
         options.sourceType = "script";
         return getParser(options, input).parse();
-      } catch (scriptError) {}
+      } catch {}
 
       throw moduleError;
     }

packages/babel-parser/src/parser/base.js

@@ -13,6 +13,7 @@ export default class BaseParser {
   plugins: PluginsMap;
   filename: ?string;
   sawUnambiguousESM: boolean = false;
+  ambiguousScriptDifferentAst: boolean = false;
 
   // Initialized by Tokenizer
   state: State;

packages/babel-parser/src/parser/expression.js

@@ -141,6 +141,7 @@ export default class ExpressionParser extends LValParser {
       this.toReferencedList(node.expressions);
       return this.finishNode(node, "SequenceExpression");
     }
+
     return expr;
   }
 
@@ -2235,9 +2236,18 @@ export default class ExpressionParser extends LValParser {
       );
     }
 
+    if (!this.scope.inFunction && !this.options.allowAwaitOutsideFunction) {
+      if (this.hasPrecedingLineBreak()) {
+        this.ambiguousScriptDifferentAst = true;
+      } else {
+        this.sawUnambiguousESM = true;
+      }
+    }
+
     if (!this.state.soloAwait) {
       node.argument = this.parseMaybeUnary();
     }
+
     return this.finishNode(node, "AwaitExpression");
   }
 

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-ambiguous-allowAwaitOutsideFunction/input.js

@@ -0,0 +1 @@
+await 0

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-ambiguous-allowAwaitOutsideFunction/options.json

@@ -0,0 +1,5 @@
+{
+  "plugins": ["topLevelAwait"],
+  "sourceType": "unambiguous",
+  "allowAwaitOutsideFunction": true
+}

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-ambiguous-allowAwaitOutsideFunction/output.json

@@ -0,0 +1,85 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 7,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 1,
+      "column": 7
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 7,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 1,
+        "column": 7
+      }
+    },
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start": 0,
+        "end": 7,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 7
+          }
+        },
+        "expression": {
+          "type": "AwaitExpression",
+          "start": 0,
+          "end": 7,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 0
+            },
+            "end": {
+              "line": 1,
+              "column": 7
+            }
+          },
+          "argument": {
+            "type": "NumericLiteral",
+            "start": 6,
+            "end": 7,
+            "loc": {
+              "start": {
+                "line": 1,
+                "column": 6
+              },
+              "end": {
+                "line": 1,
+                "column": 7
+              }
+            },
+            "extra": {
+              "rawValue": 0,
+              "raw": "0"
+            },
+            "value": 0
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-ambiguous-script/input.js

@@ -0,0 +1,2 @@
+await
+0

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-ambiguous-script/options.json

@@ -0,0 +1,4 @@
+{
+  "plugins": ["topLevelAwait"],
+  "sourceType": "unambiguous"
+}

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-ambiguous-script/output.json

@@ -0,0 +1,102 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 7,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 2,
+      "column": 1
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 7,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 2,
+        "column": 1
+      }
+    },
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start": 0,
+        "end": 5,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 5
+          }
+        },
+        "expression": {
+          "type": "Identifier",
+          "start": 0,
+          "end": 5,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 0
+            },
+            "end": {
+              "line": 1,
+              "column": 5
+            },
+            "identifierName": "await"
+          },
+          "name": "await"
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start": 6,
+        "end": 7,
+        "loc": {
+          "start": {
+            "line": 2,
+            "column": 0
+          },
+          "end": {
+            "line": 2,
+            "column": 1
+          }
+        },
+        "expression": {
+          "type": "NumericLiteral",
+          "start": 6,
+          "end": 7,
+          "loc": {
+            "start": {
+              "line": 2,
+              "column": 0
+            },
+            "end": {
+              "line": 2,
+              "column": 1
+            }
+          },
+          "extra": {
+            "rawValue": 0,
+            "raw": "0"
+          },
+          "value": 0
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-module/input.js

@@ -0,0 +1 @@
+await 0

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-module/options.json

@@ -0,0 +1,4 @@
+{
+  "plugins": ["topLevelAwait"],
+  "sourceType": "unambiguous"
+}

packages/babel-parser/test/fixtures/experimental/top-level-await/unambiguous-module/output.json

@@ -0,0 +1,85 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 7,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 1,
+      "column": 7
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 7,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 1,
+        "column": 7
+      }
+    },
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start": 0,
+        "end": 7,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 7
+          }
+        },
+        "expression": {
+          "type": "AwaitExpression",
+          "start": 0,
+          "end": 7,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 0
+            },
+            "end": {
+              "line": 1,
+              "column": 7
+            }
+          },
+          "argument": {
+            "type": "NumericLiteral",
+            "start": 6,
+            "end": 7,
+            "loc": {
+              "start": {
+                "line": 1,
+                "column": 6
+              },
+              "end": {
+                "line": 1,
+                "column": 7
+              }
+            },
+            "extra": {
+              "rawValue": 0,
+              "raw": "0"
+            },
+            "value": 0
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}