[Parse] Diagnose missing 'in' after closure signature

lib/Parse/ParseExpr.cpp

@@ -2546,7 +2546,9 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
         skipSingle();
     }
   };
-
+
+  bool sawTopLevelArrowInLookahead = false;
+
   // If we have a leading token that may be part of the closure signature, do a
   // speculative parse to validate it and look for 'in'.
   if (Tok.isAny(
@@ -2588,6 +2590,7 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
 
         // Parse the func-signature-result, if present.
         if (consumeIf(tok::arrow)) {
+          sawTopLevelArrowInLookahead = true;
           if (!canParseType())
             return makeParserSuccess();
 
@@ -2612,17 +2615,23 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
 
       // Parse the func-signature-result, if present.
       if (consumeIf(tok::arrow)) {
+        sawTopLevelArrowInLookahead = true;
         if (!canParseType())
           return makeParserSuccess();
-
+        
         consumeEffectsSpecifiers();
       }
     }
 
     // Parse the 'in' at the end.
-    if (Tok.isNot(tok::kw_in))
-      return makeParserSuccess();
+    if (Tok.isNot(tok::kw_in)) {
+      // Even if 'in' is missing, the presence of '->' makes this look
+      // like a closure signature. There's no other valid syntax that
+      // could legally contain '->' at this position.
+      if (!sawTopLevelArrowInLookahead)
+        return makeParserSuccess();
 
+    }
     // Okay, we have a closure signature.
   } else {
     // No closure signature.

test/Parse/closure-missing-in.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-frontend -parse -verify -swift-version 5 %s
+
+func test<T>(make: () -> [T], consume: (T) -> Void) { consume(make()[0]) }
+
+func main1() {
+  test(make: { () -> [Int] // expected-error@+1 {{expected 'in' after the closure signature}}
+    return [3]
+  }, consume: { _ in })
+}
+
+
+// Resync path: there are tokens before `in` — should diagnose once, then recover.
+func main2() {
+  _ = { () -> Int
+    0 // expected-error {{unexpected tokens prior to 'in'}}
+    in
+    1
+  }
+}
+
+func main3() {
+  _ = { x, y -> Int
+    x + y // expected-error {{expected 'in' after the closure signature}}
+  }
+}
+
+func ok() {
+  _ = { (x: Int) -> Int in x + 1 }
+}

test/Parse/type_expr.swift

@@ -344,10 +344,9 @@ func testFunctionCollectionTypes() {
 
 func testInvalidArrowWithClosure() {
   _ = { undefined -> undefined2 }
-  // expected-error@-1 {{cannot find 'undefined' in scope}}
-  // expected-error@-2 {{cannot find 'undefined2' in scope}}
-  // expected-error@-3 {{expected type before '->'}}
-  // expected-error@-4 {{expected type after '->'}}
+  // expected-error@-1 {{expected 'in' after the closure signature}}
+  // expected-error@-2 {{cannot find type 'undefined2' in scope}}
+  // expected-error@-3 {{cannot infer type of closure parameter 'undefined' without a type annotation}}
 
   () -> { let x: Int = "" }
   // expected-error@-1 {{expected type after '->'}}