If the missing generic requirement includes type variables, don't emit an unhelpful note for it.

Fixes SR-12759

Author: gregomni

lib/Sema/TypeCheckProtocol.cpp

@@ -784,6 +784,9 @@ static Optional<RequirementMatch> findMissingGenericRequirementForSolutionFix(
   default:
     return Optional<RequirementMatch>();
   }
+  
+  if (missingType->hasTypeVariable())
+    return Optional<RequirementMatch>();
 
   auto missingRequirementMatch = [&](Type type) -> RequirementMatch {
     Requirement requirement(requirementKind, type, missingType);

test/decl/protocol/req/missing_conformance.swift

@@ -110,3 +110,31 @@ struct S3 : P12 { // expected-error {{type 'S3' does not conform to protocol 'P1
     // expected-note@-1 {{candidate can not infer 'A' = 'P11' because 'P11' is not a nominal type and so can't conform to 'P11'}}
 }
 
+// SR-12759
+struct CountSteps1<T> : Collection {
+  init(count: Int) { self.count = count }
+  var count: Int
+
+  var startIndex: Int { 0 }
+  var endIndex: Int { count }
+  func index(after i: Int) -> Int {
+    totalSteps += 1 // expected-error {{cannot find 'totalSteps' in scope}}
+    return i + 1
+  }
+  subscript(i: Int) -> Int { return i }
+}
+
+extension CountSteps1 // expected-error {{type 'CountSteps1<T>' does not conform to protocol 'RandomAccessCollection'}}
+  // expected-error@-1 {{conditional conformance of type 'CountSteps1<T>' to protocol 'RandomAccessCollection' does not imply conformance to inherited protocol 'BidirectionalCollection'}}
+  // expected-note@-2 {{did you mean to explicitly state the conformance like 'extension CountSteps1: BidirectionalCollection where ...'?}}
+  // expected-error@-3 {{type 'CountSteps1<T>' does not conform to protocol 'BidirectionalCollection'}}
+  : RandomAccessCollection
+     where T : Equatable
+{
+  typealias Index = Int
+  func index(_ i: Index, offsetBy d: Int) -> Index {
+    return i + d
+  }
+}
+
+