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

[dgoldman]

Summary

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

clang/lib/Sema/SemaExprCXX.cpp

while (true) {
       // The transform was invalid, see if we have any TypoExprs with untried
       // correction candidates.
       if (!CheckAndAdvanceTypoExprCorrectionStreams())
         break;
     }
 
     // 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()) {
         auto TE  = AmbiguousTypoExprs.back();
 
         // TryTransform itself can create new Typos, adding them to the TypoExpr map
         // and invalidating our TypoExprState, so always fetch it instead of storing.
         SemaRef.getTypoExprState(TE).Consumer->saveCurrentPosition();

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

+// 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'}}
+}