[ConstraintSystem] Adjust diagnoseAmbiguity to use a local slice of overloads

Compute a slice of ambiguous overload choices related to an aggregated fix
and if such a slice points to a single overload diagnose it as non-ambiguous.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -3399,6 +3399,30 @@ static bool diagnoseAmbiguity(
 
   auto &DE = cs.getASTContext().Diags;
 
+  llvm::SmallPtrSet<ValueDecl *, 4> localAmbiguity;
+  {
+    for (auto &entry : aggregateFix) {
+      const auto &solution = entry.first;
+      const auto &overload = solution->getOverloadChoice(ambiguity.locator);
+      auto *choice = overload.choice.getDeclOrNull();
+
+      // It's not possible to diagnose different kinds of overload choices.
+      if (!choice)
+        return false;
+
+      localAmbiguity.insert(choice);
+    }
+  }
+
+  if (localAmbiguity.empty())
+    return false;
+
+  // If all of the fixes are rooted in the same choice.
+  if (localAmbiguity.size() == 1) {
+    auto &primaryFix = aggregateFix.front();
+    return primaryFix.second->diagnose(*primaryFix.first);
+  }
+
   {
     auto fixKind = aggregateFix.front().second->getKind();
     if (llvm::all_of(
@@ -3413,10 +3437,7 @@ static bool diagnoseAmbiguity(
     }
   }
 
-  auto *decl = ambiguity.choices.front().getDeclOrNull();
-  if (!decl)
-    return false;
-
+  auto *decl = *localAmbiguity.begin();
   auto *commonCalleeLocator = ambiguity.locator;
 
   bool diagnosed = true;

test/Sema/enum_raw_representable.swift

@@ -204,8 +204,7 @@ func sr8150_mutable(obj: SR8150Box) {
     sr8150_helper1(opt)
     // expected-error@-1 {{cannot convert value of type 'Bar?' to expected argument type 'Double'}} {{23-23=?.rawValue ?? <#default value#>}}
     sr8150_helper1(opt ?? Bar.a)
-    // expected-error@-1 {{no exact matches in call to global function 'sr8150_helper1'}}
-    // expected-note@-2  {{candidate expects value of type 'Bar' for parameter #0}} {{20-20=(}} {{32-32=).rawValue}}
+    // expected-error@-1 {{cannot convert value of type 'Bar' to expected argument type 'Double'}} {{20-20=(}} {{32-32=).rawValue}}
     let _: Double? = opt
     // expected-error@-1 {{cannot convert value of type 'Bar?' to specified type 'Double?'}} {{25-25=?.rawValue}}
   }

test/stdlib/KeyPathAppending.swift

@@ -61,7 +61,7 @@ func mismatchedAppends<T, U, V>(readOnlyLeft: KeyPath<T, U>,
   // expected-error@-1 {{no exact matches in call to instance method 'appending'}}
 
   _ = writableRight.appending(path: readOnlyLeft)
-  // expected-error@-1 {{no exact matches in call to instance method 'appending'}}
+  // expected-error@-1 {{instance method 'appending(path:)' requires that 'KeyPath<U, V>' inherit from 'KeyPath<U, T>'}}
 
   _ = writableRight.appending(path: writableLeft)
   // expected-error@-1 {{cannot convert value of type 'WritableKeyPath<T, U>' to expected argument type 'WritableKeyPath<V, U>'}}
@@ -71,7 +71,7 @@ func mismatchedAppends<T, U, V>(readOnlyLeft: KeyPath<T, U>,
   // expected-error@-1 {{no exact matches in call to instance method 'appending'}}
 
   _ = referenceRight.appending(path: readOnlyLeft)
-  // expected-error@-1 {{no exact matches in call to instance method 'appending'}}
+  // expected-error@-1 {{instance method 'appending(path:)' requires that 'KeyPath<U, V>' inherit from 'KeyPath<U, T>'}}
 
   _ = referenceRight.appending(path: writableLeft)
   // expected-error@-1 {{cannot convert value of type 'WritableKeyPath<T, U>' to expected argument type 'WritableKeyPath<V, U>'}}