[Concepts] Fix a deserialization crash.

`TemplateTypeParmDecl::hasTypeConstraint` is not a safe guard for
checking `TemplateTypeParmDecl::getTypeConstraint()` result is null.

in somecases (e.g. implicit deduction guide templates synthesized from the
constructor, immediately-declared constraint is not formed because of an error),
hasTypeConstraint returns false, and getTypeConstraint returns a nullptr.

Fix https://bugs.llvm.org/show_bug.cgi?id=46790

Differential Revision: https://reviews.llvm.org/D84455

(cherry picked from commit 73c12bd)

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2912,9 +2912,11 @@ static bool isSameTemplateParameter(const NamedDecl *X,
       return false;
     if (TX->hasTypeConstraint() != TY->hasTypeConstraint())
       return false;
-    if (TX->hasTypeConstraint()) {
-      const TypeConstraint *TXTC = TX->getTypeConstraint();
-      const TypeConstraint *TYTC = TY->getTypeConstraint();
+    const TypeConstraint *TXTC = TX->getTypeConstraint();
+    const TypeConstraint *TYTC = TY->getTypeConstraint();
+    if (!TXTC != !TYTC)
+      return false;
+    if (TXTC && TYTC) {
       if (TXTC->getNamedConcept() != TYTC->getNamedConcept())
         return false;
       if (TXTC->hasExplicitTemplateArgs() != TYTC->hasExplicitTemplateArgs())

clang/test/PCH/cxx2a-constraints-crash.cpp

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -std=c++2a -emit-pch %s -o %t
+// RUN: %clang_cc1 -std=c++2a -include-pch %t -verify %s
+
+// expected-no-diagnostics
+
+#ifndef HEADER
+#define HEADER
+
+template <typename T, typename U>
+concept not_same_as = true;
+
+template <int Kind>
+struct subrange {
+  template <not_same_as<int> R>
+  subrange(R) requires(Kind == 0);
+
+  template <not_same_as<int> R>
+  subrange(R) requires(Kind != 0);
+};
+
+template <typename R>
+subrange(R) -> subrange<42>;
+
+int main() {
+  int c;
+  subrange s(c);
+}
+
+#endif