[Constraint system] Prefer enum cases to static members when pattern matching.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -5693,6 +5693,22 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
     }
   }
 
+  // If we are pattern-matching an enum element and we found any enum elements,
+  // ignore anything that isn't an enum element.
+  bool onlyAcceptEnumElements = false;
+  if (memberLocator &&
+      memberLocator->isLastElement<LocatorPathElt::PatternMatch>() &&
+      isa<EnumElementPattern>(
+          memberLocator->getLastElementAs<LocatorPathElt::PatternMatch>()
+            ->getPattern())) {
+    for (const auto &result: lookup) {
+      if (isa<EnumElementDecl>(result.getValueDecl())) {
+        onlyAcceptEnumElements = true;
+        break;
+      }
+    }
+  }
+
   // If the instance type is String bridged to NSString, compute
   // the type we'll look in for bridging.
   Type bridgedType;
@@ -5717,6 +5733,10 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       return;
     }
 
+    // If we only accept enum elements but this isn't one, ignore it.
+    if (onlyAcceptEnumElements && !isa<EnumElementDecl>(decl))
+      return;
+
     // Dig out the instance type and figure out what members of the instance type
     // we are going to see.
     auto baseTy = candidate.getBaseType();

test/stmt/if_while_var.swift

@@ -206,3 +206,16 @@ func matchImplicitTupling(pr: SomeParseResult<Int>) {
     let y: Int = x // expected-error{{cannot convert value of type '(value: String, repetitions: Int)' to specified type 'Int'}}
   }
 }
+
+// Cope with an ambiguity between a case name and a static member. Prefer the
+// case.
+enum CaseStaticAmbiguity {
+  case C(Bool)
+
+  var isC: Bool {
+    if case .C = self { return true }
+    return false
+  }
+
+  static func C(_: Int) -> CaseStaticAmbiguity { return .C(true) }
+}