[clang] Don't typo-fix an expression in a SFINAE context.

If this is a SFINAE context, then continuing to look up names
(in particular, to treat a non-function as a function, and then
do ADL) might too-eagerly complete a type that it's not safe to
complete right now. We should just say "okay, that's a substitution
failure" and not do any more work than absolutely required.

Fixes #52970.

Differential Revision: https://reviews.llvm.org/D117603

clang/lib/Sema/Sema.cpp

@@ -2554,6 +2554,11 @@ static bool IsCPUDispatchCPUSpecificMultiVersion(const Expr *E) {
 bool Sema::tryToRecoverWithCall(ExprResult &E, const PartialDiagnostic &PD,
                                 bool ForceComplain,
                                 bool (*IsPlausibleResult)(QualType)) {
+  if (isSFINAEContext()) {
+    // If this is a SFINAE context, don't try anything that might trigger ADL
+    // prematurely.
+    return false;
+  }
   SourceLocation Loc = E.get()->getExprLoc();
   SourceRange Range = E.get()->getSourceRange();
 

clang/lib/Sema/SemaExprMember.cpp

@@ -1645,6 +1645,9 @@ static ExprResult LookupMemberExpr(Sema &S, LookupResult &R,
           << BaseType << int(IsArrow) << BaseExpr.get()->getSourceRange()
           << FixItHint::CreateReplacement(OpLoc, "->");
 
+      if (S.isSFINAEContext())
+        return ExprError();
+
       // Recurse as an -> access.
       IsArrow = true;
       return LookupMemberExpr(S, R, BaseExpr, IsArrow, OpLoc, SS,

clang/test/SemaCXX/PR52970.cpp

@@ -0,0 +1,57 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++14 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++17 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+// expected-no-diagnostics
+
+struct Incomplete;
+template <class T> struct Holder { T t; };
+
+namespace DotFollowingFunctionName {
+struct Good {
+  struct Nested {
+    int b;
+  } a;
+};
+
+struct Bad {
+  Holder<Incomplete> a();
+};
+
+template <class T>
+constexpr auto f(T t) -> decltype((t.a.b, true)) { return true; }
+constexpr bool f(...) { return false; }
+
+static_assert(DotFollowingFunctionName::f(Good{}), "");
+static_assert(!DotFollowingFunctionName::f(Bad{}), "");
+
+#if __cplusplus >= 202002L
+template <class T>
+concept C = requires(T t) { t.a.b; };
+
+static_assert(C<Good>);
+static_assert(!C<Bad>);
+#endif
+} // namespace DotFollowingFunctionName
+
+namespace DotFollowingPointer {
+struct Good {
+  int begin();
+};
+using Bad = Holder<Incomplete> *;
+
+template <class T>
+constexpr auto f(T t) -> decltype((t.begin(), true)) { return true; }
+constexpr bool f(...) { return false; }
+
+static_assert(DotFollowingPointer::f(Good{}), "");
+static_assert(!DotFollowingPointer::f(Bad{}), "");
+
+#if __cplusplus >= 202002L
+template <class T>
+concept C = requires(T t) { t.begin(); };
+
+static_assert(C<Good>);
+static_assert(!C<Bad>);
+#endif
+} // namespace DotFollowingPointer