[TypeChecker] Add Builtin operation to trigger/test fallback diagnostic

(cherry picked from commit 64fa0ee)

Author: xedin

include/swift/AST/Builtins.def

@@ -580,6 +580,7 @@ BUILTIN_SANITIZER_OPERATION(TSanInoutAccess, "tsanInoutAccess", "")
 BUILTIN_TYPE_CHECKER_OPERATION(TypeJoin, type_join)
 BUILTIN_TYPE_CHECKER_OPERATION(TypeJoinInout, type_join_inout)
 BUILTIN_TYPE_CHECKER_OPERATION(TypeJoinMeta, type_join_meta)
+BUILTIN_TYPE_CHECKER_OPERATION(TriggerFallbackDiagnostic, trigger_fallback_diagnostic)
 
 #undef BUILTIN_TYPE_CHECKER_OPERATION
 

lib/AST/Builtins.cpp

@@ -1010,6 +1010,12 @@ static ValueDecl *getTypeJoinMetaOperation(ASTContext &Context, Identifier Id) {
   return builder.build(Id);
 }
 
+static ValueDecl *getTriggerFallbackDiagnosticOperation(ASTContext &Context,
+                                                        Identifier Id) {
+  // () -> Void
+  return getBuiltinFunction(Id, {}, Context.TheEmptyTupleType);
+}
+
 static ValueDecl *getCanBeObjCClassOperation(ASTContext &Context,
                                              Identifier Id) {
   // <T> T.Type -> Builtin.Int8
@@ -1899,7 +1905,10 @@ ValueDecl *swift::getBuiltinValueDecl(ASTContext &Context, Identifier Id) {
     return getTypeJoinInoutOperation(Context, Id);
 
   case BuiltinValueKind::TypeJoinMeta:
-    return getTypeJoinMetaOperation(Context, Id);      
+    return getTypeJoinMetaOperation(Context, Id);
+
+  case BuiltinValueKind::TriggerFallbackDiagnostic:
+    return getTriggerFallbackDiagnosticOperation(Context, Id);
   }
 
   llvm_unreachable("bad builtin value!");

lib/Sema/CSGen.cpp

@@ -1579,6 +1579,11 @@ namespace {
       if (typeOperation != TypeOperation::None)
         return CS.getASTContext().TheAnyType;
 
+      // If this is `Builtin.trigger_fallback_diagnostic()`, fail
+      // without producing any diagnostics, in order to test fallback error.
+      if (isTriggerFallbackDiagnosticBuiltin(expr, CS.getASTContext()))
+        return nullptr;
+
       // Open a member constraint for constructor delegations on the
       // subexpr type.
       if (CS.TC.getSelfForInitDelegationInConstructor(CS.DC, expr)) {
@@ -3128,6 +3133,19 @@ namespace {
       return tv;
     }
 
+    static bool isTriggerFallbackDiagnosticBuiltin(UnresolvedDotExpr *UDE,
+                                                   ASTContext &Context) {
+      auto *DRE = dyn_cast<DeclRefExpr>(UDE->getBase());
+      if (!DRE)
+        return false;
+
+      if (DRE->getDecl() != Context.TheBuiltinModule)
+        return false;
+
+      auto member = UDE->getName().getBaseIdentifier().str();
+      return member.equals("trigger_fallback_diagnostic");
+    }
+
     enum class TypeOperation { None,
                                Join,
                                JoinInout,

test/Sema/rdar38885760.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+//
+// RUN: not %target-swift-frontend -typecheck %s -parse-stdlib 2>%t/fallback_diagnostic.txt
+// RUN: %FileCheck %s --check-prefix FALLBACK-DIAGNOSTIC <%t/fallback_diagnostic.txt
+//
+// FALLBACK-DIAGNOSTIC: error: failed to produce diagnostic for expression; please file a bug report with your project
+
+import Swift
+
+Builtin.trigger_fallback_diagnostic()