fix: Check ASI upon unusual trailing expressions (#2252)

Author: technohippy

Diff for: src/parser.ts

@@ -939,7 +939,7 @@ export class Parser extends DiagnosticEmitter {
     } while (tn.skip(Token.Comma));
 
     let ret = Node.createVariableStatement(decorators, declarations, tn.range(startPos, tn.pos));
-    tn.skip(Token.Semicolon);
+    if (!tn.skip(Token.Semicolon) && !isFor) this.checkASI(tn);
     return ret;
   }
 
@@ -1122,7 +1122,7 @@ export class Parser extends DiagnosticEmitter {
     }
 
     let ret = Node.createReturnStatement(expr, tn.range(startPos, tn.pos));
-    tn.skip(Token.Semicolon);
+    if (!tn.skip(Token.Semicolon)) this.checkASI(tn);
     return ret;
   }
 
@@ -3009,7 +3009,7 @@ export class Parser extends DiagnosticEmitter {
       }
     }
     let ret = Node.createBlockStatement(statements, tn.range(startPos, tn.pos));
-    tn.skip(Token.Semicolon);
+    if (topLevel) tn.skip(Token.Semicolon);
     return ret;
   }
 
@@ -3417,7 +3417,7 @@ export class Parser extends DiagnosticEmitter {
     let expression = this.parseExpression(tn);
     if (!expression) return null;
     let ret = Node.createThrowStatement(expression, tn.range(startPos, tn.pos));
-    tn.skip(Token.Semicolon);
+    if (!tn.skip(Token.Semicolon)) this.checkASI(tn);
     return ret;
   }
 
@@ -4401,6 +4401,18 @@ export class Parser extends DiagnosticEmitter {
     return expr;
   }
 
+  private checkASI(
+    tn: Tokenizer
+  ): void {
+    // see: https://tc39.es/ecma262/#sec-automatic-semicolon-insertion
+    let token = tn.peek(true);
+    if (tn.nextTokenOnNewLine || token == Token.EndOfFile || token == Token.CloseBrace) return;
+    this.error(
+      DiagnosticCode.Unexpected_token,
+      tn.range(tn.nextTokenPos)
+    );
+  }
+
   /** Skips over a statement on errors in an attempt to reduce unnecessary diagnostic noise. */
   skipStatement(tn: Tokenizer): void {
     tn.peek(true);

Diff for: tests/parser/asi.ts

@@ -0,0 +1,19 @@
+function failLet(y: i32): i32 {
+  let x = y 234;
+  return x + y;
+}
+
+function failReturn(): i32 {
+  return 123 456;
+}
+
+function failThrow(): i32 {
+  throw 123 456;
+}
+
+function successCloseBrace(): i32 {
+  return 123 }
+
+function successCloseParen(): i32 {
+  return ( 123 )
+}

Diff for: tests/parser/asi.ts.fixture.ts

@@ -0,0 +1,22 @@
+function failLet(y: i32): i32 {
+  let x = y;
+  234;
+  return x + y;
+}
+function failReturn(): i32 {
+  return 123;
+  456;
+}
+function failThrow(): i32 {
+  throw 123;
+  456;
+}
+function successCloseBrace(): i32 {
+  return 123;
+}
+function successCloseParen(): i32 {
+  return (123);
+}
+// ERROR 1012: "Unexpected token." in asi.ts(2,13+0)
+// ERROR 1012: "Unexpected token." in asi.ts(7,14+0)
+// ERROR 1012: "Unexpected token." in asi.ts(11,13+0)