[Sema][Typo Correction] Fix another infinite loop on ambiguity

See also: D67515

- For the given call expression we would end up repeatedly
   trying to transform the same expression over and over again

- Fix is to keep the old TransformCache when checking for ambiguity

Differential Revision: https://reviews.llvm.org/D69060

Author: DavidGoldman

clang/lib/Sema/SemaExprCXX.cpp

@@ -7780,8 +7780,9 @@ class TransformTypos : public TreeTransform<TransformTypos> {
 
     // If we found a valid result, double check to make sure it's not ambiguous.
     if (!IsAmbiguous && !Res.isInvalid() && !AmbiguousTypoExprs.empty()) {
-      auto SavedTransformCache = std::move(TransformCache);
-      TransformCache.clear();
+      auto SavedTransformCache =
+          llvm::SmallDenseMap<TypoExpr *, ExprResult, 2>(TransformCache);
+
       // Ensure none of the TypoExprs have multiple typo correction candidates
       // with the same edit length that pass all the checks and filters.
       while (!AmbiguousTypoExprs.empty()) {

clang/test/Sema/typo-correction-ambiguity.c

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// Check the following typo correction behavior in C:
+// - no typos are diagnosed when a call expression has ambiguous (multiple) corrections
+
+int v_63;
+
+void v_2_0(int v_452, int v_454) {}
+
+int v_3_0() {
+   for (int v_345 = 0 ; v_63;)
+       v_2_0(v_195,  // expected-error {{use of undeclared identifier 'v_195'}}
+             v_231);  // expected-error {{use of undeclared identifier 'v_231'}}
+}