[Diagnostics] Diagnose ambiguity resulting from subscript operator fix as a missing member

Since the rule is to prioritize names over types, let's diagnose
ambiguous solutions containing subscript operator fix as missing
member and list possible candidates to use.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -64,9 +64,6 @@ ERROR(ambiguous_reference_to_decl,none,
 ERROR(ambiguous_subscript,none,
       "ambiguous subscript with base type %0 and index type %1",
       (Type, Type))
-NOTE(ambiguous_subscript_candidate_requires_operator,none,
-     "candidate requires use of the subscript operator", ())
-
 ERROR(could_not_find_value_subscript,none,
       "value of type %0 has no subscripts",
       (Type))

lib/Sema/CSDiagnostics.cpp

@@ -1327,8 +1327,7 @@ bool SubscriptMisuseFailure::diagnoseAsError() {
 
 bool SubscriptMisuseFailure::diagnoseAsNote() {
   if (auto overload = getOverloadChoiceIfAvailable(getLocator())) {
-    emitDiagnostic(overload->choice.getDecl(),
-                   diag::ambiguous_subscript_candidate_requires_operator);
+    emitDiagnostic(overload->choice.getDecl(), diag::found_candidate);
     return true;
   }
   return false;

lib/Sema/ConstraintSystem.cpp

@@ -2205,8 +2205,16 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
   {
     DiagnosticTransaction transaction(TC.Diags);
 
-    TC.diagnose(commonAnchor->getLoc(), diag::ambiguous_reference_to_decl,
-                decl->getDescriptiveKind(), decl->getFullName());
+    const auto *fix = viableSolutions.front().second;
+    if (fix->getKind() == FixKind::UseSubscriptOperator) {
+      auto *UDE = cast<UnresolvedDotExpr>(commonAnchor);
+      TC.diagnose(commonAnchor->getLoc(),
+                  diag::could_not_find_subscript_member_did_you_mean,
+                  getType(UDE->getBase()));
+    } else {
+      TC.diagnose(commonAnchor->getLoc(), diag::ambiguous_reference_to_decl,
+                  decl->getDescriptiveKind(), decl->getFullName());
+    }
 
     for (const auto &viable : viableSolutions) {
       // Create scope so each applied solution is rolled back.

test/decl/subscript/subscripting.swift

@@ -266,11 +266,9 @@ func testGenSubscriptFixit(_ s0: GenSubscriptFixitTest) {
 
 struct SubscriptTest1 {
   subscript(keyword:String) -> Bool { return true }
-  // expected-note@-1 2 {{found this candidate}}
-  // expected-note@-2 2 {{candidate requires use of the subscript operator}}
+  // expected-note@-1 4 {{found this candidate}}
   subscript(keyword:String) -> String? {return nil }
-  // expected-note@-1 2 {{found this candidate}}
-  // expected-note@-2 2 {{candidate requires use of the subscript operator}}
+  // expected-note@-1 4 {{found this candidate}}
 
   subscript(arg: SubClass) -> Bool { return true } // expected-note {{declared here}}
   subscript(arg: Protocol) -> Bool { return true } // expected-note 2 {{declared here}}
@@ -303,9 +301,9 @@ func testSubscript1(_ s1 : SubscriptTest1) {
   // expected-error@-1 {{cannot invoke 'subscript' with an argument list of type '(SuperClass)'}}
   // expected-note@-2 {{overloads for 'subscript' exist with these partially matching parameter lists: (Protocol), (String), (SubClass)}}
   _ = s1.subscript("hello")
-  // expected-error@-1 {{ambiguous reference to subscript 'subscript(_:)'}}
+  // expected-error@-1 {{value of type 'SubscriptTest1' has no property or method named 'subscript'; did you mean to use the subscript operator?}}
   _ = s1.subscript("hello"
-  // expected-error@-1 {{ambiguous reference to subscript 'subscript(_:)'}}
+  // expected-error@-1 {{value of type 'SubscriptTest1' has no property or method named 'subscript'; did you mean to use the subscript operator?}}
   // expected-note@-2 {{to match this opening '('}}
 
   let _ = s1["hello"]