Sema: Fix type mixup with unbound method references

When applying a solution containing an unbound reference to
an instance method of a class, the type of the new expression
did not match the type in the constraint system.

Usually this was papered over because CSApply is sprinkled
with coerceToType() calls, but we would crash when passing
an unbound reference to a generic function, or type(of:).

Author: slavapestov

lib/Sema/ConstraintSystem.cpp

@@ -1282,7 +1282,8 @@ ConstraintSystem::getTypeOfMemberReference(
   } else {
     // For an unbound instance method reference, replace the 'Self'
     // parameter with the base type.
-    type = openedFnType->replaceSelfParameterType(baseObjTy);
+    openedType = openedFnType->replaceSelfParameterType(baseObjTy);
+    type = openedType;
   }
 
   // When accessing protocol members with an existential base, replace

test/SILGen/decls.swift

@@ -167,3 +167,25 @@ struct StructWithStaticVar {
   init() {
   }
 }
+
+// Make sure unbound method references on class hierarchies are
+// properly represented in the AST
+
+class Base {
+  func method1() -> Self { return self }
+  func method2() -> Self { return self }
+}
+
+class Derived : Base {
+  override func method2() -> Self { return self }
+}
+
+func generic<T>(arg: T) { }
+
+func unboundMethodReferences() {
+  generic(arg: Derived.method1)
+  generic(arg: Derived.method2)
+
+  _ = type(of: Derived.method1)
+  _ = type(of: Derived.method2)
+}