[Clang][Sema] Fix crash when using name of UnresolvedUsingValueDecl with template arguments (llvm#83842)

The following snippet causes a crash:
```
template<typename T>
struct A : T {
  using T::f;
  void f();

  void g() {
    f<int>(); // crash here
  }
};
```
This happens because we cast the result of `getAsTemplateNameDecl` as a
`TemplateDecl` in `Sema::ClassifyName`, which we cannot do for an
`UnresolvedUsingValueDecl`. This patch fixes the crash by considering a name
to be that of a template if _any_ function declaration is found per [temp.names] p3.3.

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -305,6 +305,8 @@ Bug Fixes to C++ Support
   our attention by an attempt to fix in (#GH77703). Fixes (#GH83385).
 - Fix evaluation of some immediate calls in default arguments.
   Fixes (#GH80630)
+- Fix a crash when an explicit template argument list is used with a name for which lookup
+  finds a non-template function and a dependent using declarator.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -1110,7 +1110,9 @@ Sema::NameClassification Sema::ClassifyName(Scope *S, CXXScopeSpec &SS,
     //   unqualified-id followed by a < and name lookup finds either one
     //   or more functions or finds nothing.
     if (!IsFilteredTemplateName)
-      FilterAcceptableTemplateNames(Result);
+      FilterAcceptableTemplateNames(Result,
+                                    /*AllowFunctionTemplates=*/true,
+                                    /*AllowDependent=*/true);
 
     bool IsFunctionTemplate;
     bool IsVarTemplate;
@@ -1120,6 +1122,7 @@ Sema::NameClassification Sema::ClassifyName(Scope *S, CXXScopeSpec &SS,
       Template = Context.getOverloadedTemplateName(Result.begin(),
                                                    Result.end());
     } else if (!Result.empty()) {
+      assert(!Result.isUnresolvableResult());
       auto *TD = cast<TemplateDecl>(getAsTemplateNameDecl(
           *Result.begin(), /*AllowFunctionTemplates=*/true,
           /*AllowDependent=*/false));

clang/lib/Sema/SemaTemplate.cpp

@@ -491,18 +491,20 @@ bool Sema::LookupTemplateName(LookupResult &Found,
     // To keep our behavior consistent, we apply the "finds nothing" part in
     // all language modes, and diagnose the empty lookup in ActOnCallExpr if we
     // successfully form a call to an undeclared template-id.
-    bool AllFunctions =
-        getLangOpts().CPlusPlus20 && llvm::all_of(Found, [](NamedDecl *ND) {
+    bool AnyFunctions =
+        getLangOpts().CPlusPlus20 && llvm::any_of(Found, [](NamedDecl *ND) {
           return isa<FunctionDecl>(ND->getUnderlyingDecl());
         });
-    if (AllFunctions || (Found.empty() && !IsDependent)) {
+    if (AnyFunctions || (Found.empty() && !IsDependent)) {
       // If lookup found any functions, or if this is a name that can only be
       // used for a function, then strongly assume this is a function
       // template-id.
       *ATK = (Found.empty() && Found.getLookupName().isIdentifier())
                  ? AssumedTemplateKind::FoundNothing
                  : AssumedTemplateKind::FoundFunctions;
-      Found.clear();
+      FilterAcceptableTemplateNames(Found,
+                                    /*AllowFunctionTemplates*/ true,
+                                    /*AllowDependent*/ true);
       return false;
     }
   }

clang/test/SemaTemplate/unqual-unresolved-using-value.cpp

@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+template<typename T>
+struct A : T {
+  using T::f;
+  using T::g;
+  using T::h;
+
+  void f();
+  void g();
+
+  void i() {
+    f<int>();
+    g<int>(); // expected-error{{no member named 'g' in 'A<B>'}}
+    h<int>(); // expected-error{{expected '(' for function-style cast or type construction}}
+              // expected-error@-1{{expected expression}}
+  }
+};
+
+struct B {
+  template<typename T>
+  void f();
+
+  void g();
+
+  template<typename T>
+  void h();
+};
+
+template struct A<B>; // expected-note{{in instantiation of member function 'A<B>::i' requested here}}