Sema: Correctly handle static 'let's in VarDecl::isSettable

Author: AnthonyLatsis

lib/AST/Decl.cpp

@@ -5663,6 +5663,11 @@ bool VarDecl::isSettable(const DeclContext *UseDC,
   if (!isLet())
     return supportsMutation();
 
+  // Static 'let's are always immutable.
+  if (isStatic()) {
+    return false;
+  }
+
   //
   // All the remaining logic handles the special cases where you can
   // assign a 'let'.

test/decl/protocol/protocol_enum_witness.swift

@@ -146,3 +146,10 @@ protocol MissingAccessorsVar {
 enum Bar14: MissingAccessorsVar { // OK
   case bar
 }
+
+protocol MissingAccessorsLet {
+  static let bar: Self // expected-error {{protocols cannot require properties to be immutable; declare read-only properties by using 'var' with a '{ get }' specifier}}
+}
+enum Bar15: MissingAccessorsLet { // OK
+  case bar
+}