[clang-repl] Improve error message on failed undos (llvm#149396)

Updated error message logic for undo function. Throws different errors
for the case of there being nothing to undo, and for the case of
requesting more undos than there are operations to undo.

Fixes llvm#143668

Author: aadanen

clang/lib/Interpreter/Interpreter.cpp

@@ -761,10 +761,18 @@ Interpreter::getSymbolAddressFromLinkerName(llvm::StringRef Name) const {
 
 llvm::Error Interpreter::Undo(unsigned N) {
 
-  if (N > getEffectivePTUSize())
+  if (getEffectivePTUSize() == 0) {
     return llvm::make_error<llvm::StringError>("Operation failed. "
-                                               "Too many undos",
+                                               "No input left to undo",
                                                std::error_code());
+  } else if (N > getEffectivePTUSize()) {
+    return llvm::make_error<llvm::StringError>(
+        llvm::formatv(
+            "Operation failed. Wanted to undo {0} inputs, only have {1}.", N,
+            getEffectivePTUSize()),
+        std::error_code());
+  }
+
   for (unsigned I = 0; I < N; I++) {
     if (IncrExecutor) {
       if (llvm::Error Err = IncrExecutor->removeModule(PTUs.back()))

clang/unittests/Interpreter/InterpreterTest.cpp

@@ -160,12 +160,12 @@ TEST_F(InterpreterTest, UndoCommand) {
 
   // Fail to undo.
   auto Err1 = Interp->Undo();
-  EXPECT_EQ("Operation failed. Too many undos",
+  EXPECT_EQ("Operation failed. No input left to undo",
             llvm::toString(std::move(Err1)));
   auto Err2 = Interp->Parse("int foo = 42;");
   EXPECT_TRUE(!!Err2);
   auto Err3 = Interp->Undo(2);
-  EXPECT_EQ("Operation failed. Too many undos",
+  EXPECT_EQ("Operation failed. Wanted to undo 2 inputs, only have 1.",
             llvm::toString(std::move(Err3)));
 
   // Succeed to undo.