[Clang] prevent recovery call expression from proceeding with explicit attributes and undeclared templates (#107786)

Fixes #107047
Fixes #49093

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -489,6 +489,8 @@ Bug Fixes to C++ Support
 - Clang now instantiates the correct lambda call operator when a lambda's class type is
   merged across modules. (#GH110401)
 - Fix a crash when parsing a pseudo destructor involving an invalid type. (#GH111460)
+- Fixed an assertion failure when invoking recovery call expressions with explicit attributes
+  and undeclared templates. (#GH107047, #GH49093)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExpr.cpp

@@ -2470,7 +2470,15 @@ bool Sema::DiagnoseEmptyLookup(Scope *S, CXXScopeSpec &SS, LookupResult &R,
       LookupCtx ? LookupCtx : (SS.isEmpty() ? CurContext : nullptr);
   while (DC) {
     if (isa<CXXRecordDecl>(DC)) {
-      LookupQualifiedName(R, DC);
+      if (ExplicitTemplateArgs) {
+        if (LookupTemplateName(
+                R, S, SS, Context.getRecordType(cast<CXXRecordDecl>(DC)),
+                /*EnteringContext*/ false, TemplateNameIsRequired,
+                /*RequiredTemplateKind*/ nullptr, /*AllowTypoCorrection*/ true))
+          return true;
+      } else {
+        LookupQualifiedName(R, DC);
+      }
 
       if (!R.empty()) {
         // Don't give errors about ambiguities in this lookup.

clang/test/SemaTemplate/recovery-crash-cxx20.cpp

@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+
+namespace GH49093 {
+  class B {
+  public:
+    static int a() { return 0; } // expected-note {{declared as a non-template here}}
+    decltype(a< 0 >(0)) test;    // expected-error {{'a' does not refer to a template}}
+  };
+
+  struct C {
+      static int a() { return 0; } // expected-note {{declared as a non-template here}}
+      decltype(a < 0 > (0)) test;  // expected-error {{'a' does not refer to a template}}
+  };
+
+  void test_is_bool(bool t) {}
+  void test_is_bool(int t) {}
+
+  int main() {
+    B b;
+    test_is_bool(b.test);
+
+    C c;
+    test_is_bool(c.test);
+  }
+}
+
+namespace GH107047 {
+  struct A {
+    static constexpr auto test() { return 1; } // expected-note {{declared as a non-template here}}
+    static constexpr int s = test< 1 >();      // expected-error {{'test' does not refer to a template}}
+  };
+}