[Sema][Typo Correction] Fix potential infite loop on ambiguity checks

[dgoldman]

Summary

This fixes a bug introduced in D62648, where Clang could infinite loop
if it became stuck on a single TypoCorrection when it was supposed to
be testing ambiguous corrections. Although not a common case, it could
happen if there are multiple possible corrections with the same edit
distance.

The fix is simply to wipe the TypoExpr from the TransformCache so that
the call to TransformTypoExpr doesn't use the CachedEntry.

[dgoldman]

• Fix method name in test

[rsmith]

test/Sema/typo-correction-ambiguity.cpp

8

Is the typo in "Ambiguous" here intentional? :)

[dgoldman]

• Fix typo in test

lib/Sema/SemaExprCXX.cpp

if (!IsAmbiguous && !Res.isInvalid() && !AmbiguousTypoExprs.empty()) {
 
         // 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();
 
         TypoCorrection TC = SemaRef.getTypoExprState(TE).Consumer->peekNextCorrection();
         TypoCorrection Next;
         do {
+          // Fetch the next correction by erasing the typo from the cache and calling
+          // `TryTransform` which will iterate through corrections in
+          // `TransformTypoExpr`.
+          TransformCache.erase(TE);
           ExprResult AmbigRes = CheckForRecursiveTypos(TryTransform(E), IsAmbiguous);
 
           if (!AmbigRes.isInvalid() || IsAmbiguous) {
             SemaRef.getTypoExprState(TE).Consumer->resetCorrectionStream();
             SavedTransformCache.erase(TE);
             Res = ExprError();
             IsAmbiguous = true;
             break;

test/Sema/typo-correction-ambiguity.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// Check the following typo correction behavior in namespaces:
+// - no typos are diagnosed when an expression has ambiguous (multiple) corrections
+// - proper iteration through multiple potentially ambiguous corrections
+
+namespace AmibguousCorrection
+{
+  void method_Bar();
+  void method_Foo();
+  void method_Zoo();
+};
+
+void testAmbiguousNoSuggestions()
+{
+  AmibguousCorrection::method_Ace(); // expected-error {{no member named 'method_Ace' in namespace 'AmibguousCorrection'}}
+}
+
+namespace MultipleCorrectionsButNotAmbiguous
+{
+  int PrefixType_Name(int value);  // expected-note {{'PrefixType_Name' declared here}}
+  int PrefixType_MIN();
+  int PrefixType_MAX();
+};
+
+int testDirectAmbiguousCorrection() {
+  int val = MultipleCorrectionsButNotAmbiguous::PrefixType_Enum(0);  // expected-error {{no member named 'PrefixType_Enum' in namespace 'MultipleCorrectionsButNotAmbiguous'; did you mean 'PrefixType_Name'?}}
+  return val;
+}