[clang] Fix a clang crash on invalid code in C++20 mode.

This crash is a combination of recovery-expr + new SemaInit.cpp code
introduced by by https://reviews.llvm.org/D129531.

Differential Revision: https://reviews.llvm.org/D140587

clang/lib/Sema/SemaInit.cpp

@@ -6176,7 +6176,11 @@ void InitializationSequence::InitializeFrom(Sema &S,
       // constructors. For example, conversion function.
       if (const auto *RD =
               dyn_cast<CXXRecordDecl>(DestType->getAs<RecordType>()->getDecl());
-          S.getLangOpts().CPlusPlus20 && RD && RD->isAggregate() && Failed() &&
+          // In general, we should call isCompleteType for RD to check its
+          // completeness, we don't call it here as it was already called in the
+          // above TryConstructorInitialization.
+          S.getLangOpts().CPlusPlus20 && RD && RD->hasDefinition() &&
+          RD->isAggregate() && Failed() &&
           getFailureKind() == FK_ConstructorOverloadFailed) {
         // C++20 [dcl.init] 17.6.2.2:
         //   - Otherwise, if no constructor is viable, the destination type is

clang/test/SemaCXX/recovery-expr-type.cpp

@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -frecovery-ast -frecovery-ast-type -o - %s -std=gnu++17 -fsyntax-only -verify
+// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -o - %s -std=gnu++17 -fsyntax-only -verify
+// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -o - %s -std=gnu++20 -fsyntax-only -verify
+
 
 namespace test0 {
 struct Indestructible {
@@ -171,3 +173,13 @@ void f() {
   S.m(1); // no crash
 }
 }
+
+namespace test16 {
+// verify we do not crash on incomplete class type.
+template<typename T, typename U> struct A; // expected-note 5{{template is declared here}}
+A<int, int> foo() { // expected-error {{implicit instantiation of undefined template}}
+  if (1 == 1)
+    return A<int, int>{1}; // expected-error 2{{implicit instantiation of undefined template}}
+  return A<int, int>(1); // expected-error 2{{implicit instantiation of undefined template}}
+}
+}