[CSOptimizer] Increase score when type matches opaque type

Since erasure of a concrete type into an existential value yields
score of 1, we need to bump the score in cases where a type
passed to a generic parameter satisfies all of its protocol
conformance requirements and that parameter represents an
opaque type.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -314,8 +314,14 @@ static void determineBestChoicesInContext(
 
         if (llvm::all_of(protocolRequirements, [&](ProtocolDecl *protocol) {
               return bool(cs.lookupConformance(candidateType, protocol));
-            }))
+            })) {
+          if (auto *GP = paramType->getAs<GenericTypeParamType>()) {
+            auto *paramDecl = GP->getDecl();
+            if (paramDecl && paramDecl->isOpaqueType())
+              return 1.0;
+          }
           return 0.7;
+        }
       }
 
       // Parameter is generic, let's check whether top-level

test/Constraints/operator.swift

@@ -329,3 +329,49 @@ enum I60954 {
   }
   init?<S>(_ string: S) where S: StringProtocol {} // expected-note{{where 'S' = 'I60954'}}
 }
+
+infix operator <<<>>> : DefaultPrecedence
+
+protocol P5 {
+}
+
+struct Expr : P6 {}
+
+protocol P6: P5 {
+}
+
+extension P6 {
+  public static func <<<>>> (lhs: Self, rhs: (any P5)?) -> Expr { Expr() }
+  public static func <<<>>> (lhs: (any P5)?, rhs: Self) -> Expr { Expr() }
+  public static func <<<>>> (lhs: Self, rhs: some P6) -> Expr { Expr() }
+
+  public static prefix func ! (value: Self) -> Expr {
+    Expr()
+  }
+}
+
+extension P6 {
+  public static func != (lhs: Self, rhs: some P6) -> Expr {
+    !(lhs <<<>>> rhs) // Ok
+  }
+}
+
+do {
+  struct Value : P6 {
+  }
+
+  struct Column: P6 {
+  }
+
+  func test(col: Column, val: Value) -> Expr {
+    col <<<>>> val // Ok
+  }
+
+  func test(col: Column, val: some P6) -> Expr {
+    col <<<>>> val // Ok
+  }
+
+  func test(col: some P6, val: Value) -> Expr {
+    col <<<>>> val // Ok
+  }
+}