[Sema] Hide underscored names from typo-correction

Identifiers with a leading underscore are now hidden from typo-correction unless the typed name also begins with an underscore.

Author: adamshin

lib/Sema/TypeCheckNameLookup.cpp

@@ -602,6 +602,12 @@ static unsigned getCallEditDistance(DeclName writtenName,
   StringRef writtenBase = writtenName.getBaseName().userFacingName();
   StringRef correctedBase = correctedName.getBaseName().userFacingName();
 
+  // Don't typo-correct to a name with a leading underscore unless the typed
+  // name also begins with an underscore.
+  if (correctedBase.startswith("_") && !writtenBase.startswith("_")) {
+    return UnreasonableCallEditDistance;
+  }
+
   unsigned distance = writtenBase.edit_distance(correctedBase, maxEditDistance);
 
   // Bound the distance to UnreasonableCallEditDistance.

test/Sema/typo_correction.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -typo-correction-limit 20
+// RUN: %target-typecheck-verify-swift -typo-correction-limit 22
 // RUN: not %target-swift-frontend -typecheck -disable-typo-correction %s 2>&1 | %FileCheck %s -check-prefix=DISABLED
 // RUN: not %target-swift-frontend -typecheck -typo-correction-limit 0 %s 2>&1 | %FileCheck %s -check-prefix=DISABLED
 // RUN: not %target-swift-frontend -typecheck -DIMPORT_FAIL %s 2>&1 | %FileCheck %s -check-prefix=DISABLED
@@ -177,3 +177,17 @@ func boo() { // expected-note {{did you mean 'boo'?}}
     }
   }
 }
+
+// Don't show underscored names as typo corrections unless the typed name also
+// begins with an underscore.
+func test_underscored_no_match() {
+  let _ham = 0
+  _ = ham
+  // expected-error@-1 {{use of unresolved identifier 'ham'}}
+}
+
+func test_underscored_match() {
+  let _eggs = 4 // expected-note {{'_eggs' declared here}}
+  _ = _fggs + 1
+  // expected-error@-1 {{use of unresolved identifier '_fggs'; did you mean '_eggs'?}}
+}