If a function has an error type, mangle it as '(Error) -> Error'.

This lets the mangling preserve the invariant that functions always
structurally have function types. Error types don't show up in mangled
names often anyway, but it can occur when you ask for the USR of a
function with an invalid type.

Author: jrose-apple

lib/AST/ASTMangler.cpp

@@ -1570,8 +1570,11 @@ CanType ASTMangler::getDeclTypeForMangling(
     ArrayRef<Requirement> &requirements,
     SmallVectorImpl<Requirement> &requirementsBuf) {
   auto &C = decl->getASTContext();
-  if (!decl->hasInterfaceType())
-    return ErrorType::get(C)->getCanonicalType();
+  if (!decl->hasInterfaceType() || decl->getInterfaceType()->is<ErrorType>()) {
+    if (isa<AbstractFunctionDecl>(decl))
+      return CanFunctionType::get(C.TheErrorType, C.TheErrorType);
+    return C.TheErrorType;
+  }
 
   auto type = decl->getInterfaceType()->getCanonicalType();
 

lib/Demangling/Demangler.cpp

@@ -1635,7 +1635,7 @@ NodePointer Demangler::demangleSpecialType() {
       return createType(boxTy);
     }
     case 'e':
-      return createType(createNode(Node::Kind::ErrorType, StringRef()));
+      return createType(createNode(Node::Kind::ErrorType));
     default:
       return nullptr;
   }

test/SourceKit/Indexing/index.swift.response

@@ -1110,7 +1110,7 @@
     {
       key.kind: source.lang.swift.decl.function.method.instance,
       key.name: "meth()",
-      key.usr: "s:5index4methXeF",
+      key.usr: "s:5index4methXeXeF",
       key.line: 134,
       key.column: 8
     },