[Sema] Naively create CallExpr when there's one lookup candidate.

[sammccall]

Summary

This is not suitable to land, just something to talk about.

clang-tools-extra/unittests/clangd/XRefsTests.cpp

const char *Tests[] = {
         class X {
           X(X&& x) = default;
         };
         X [[makeX]]() {}
         void foo() {
           auto x = m^akeX();
         }
       )cpp",
+
+      R"cpp(// Broken code: one overload, but not valid.
+        int $decl[[foo]](int);
+        void bar() {
+          ^foo(42, 43);
+        }
+      )cpp",
   };
   for (const char *Test : Tests) {
     Annotations T(Test);
     llvm::Optional<Range> WantDecl;
     llvm::Optional<Range> WantDef;
     if (!T.ranges().empty())
       WantDecl = WantDef = T.range();
     if (!T.ranges("decl").empty())

clang/lib/Sema/SemaOverload.cpp

for (const Expr *Arg : Args) {
         auto *FD = dyn_cast<FunctionDecl>(DRE->getDecl());
         if (FD &&
             !SemaRef.checkAddressOfFunctionIsAvailable(FD, /*Complain=*/true,
                                                        Arg->getExprLoc()))
           return ExprError();
       }
     }
 
+    CallExpr *FakeCE = nullptr;
+    if (AllowTypoCorrection && CandidateSet->size() == 1) {
+      auto& Candidate = *CandidateSet->begin();
+      Fn = SemaRef.FixOverloadedFunctionReference(Fn, Candidate.FoundDecl,
+                                                  Candidate.Function);
+      FakeCE =
+          CallExpr::Create(SemaRef.getASTContext(), Fn, Args,
+                           Candidate.Function->getReturnType(), VK_RValue,
+                           RParenLoc, 0, Candidate.IsADLCandidate);
+    }
+
     SemaRef.Diag(Fn->getBeginLoc(), diag::err_ovl_no_viable_function_in_call)
         << ULE->getName() << Fn->getSourceRange();
     CandidateSet->NoteCandidates(SemaRef, OCD_AllCandidates, Args);
+    if (FakeCE)
+      return FakeCE;
     break;
   }
 
   case OR_Ambiguous:
     SemaRef.Diag(Fn->getBeginLoc(), diag::err_ovl_ambiguous_call)
         << ULE->getName() << Fn->getSourceRange();
     CandidateSet->NoteCandidates(SemaRef, OCD_ViableCandidates, Args);
     break;