[Concepts] Fix a deserialization crash.

[hokein]

Summary

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), hasTypeConstraint returns true, and getTypeConstraint
returns a nullptr.

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

[hokein]

btw, getTypeConstraint and hasTypeConstraint APIs are quite easy to be misused (there is another similar bug in getAssociatedConstraints), I think we need to refine them (rename or so) to make them less confusing.

[nridge]

In D84455#2170461, @hokein wrote:

btw, getTypeConstraint and hasTypeConstraint APIs are quite easy to be misused (there is another similar bug in getAssociatedConstraints), I think we need to refine them (rename or so) to make them less confusing.

Agreed, this seems a bit error-prone.

[aaronpuchert]

Do I understand correctly that this means we should have a template parameter, but haven't read it yet? In that case it would seem strange that we consider them different.

But I agree with you that hasTypeConstraint returning HasTypeConstraint and getTypeConstraint checking TypeConstraintInitialized makes the interface easy to misuse. Haven't seen anywhere in the AST a concept of having, but not having initialized a property—either it's there or not.

clang/lib/Serialization/ASTReaderDecl.cpp

if (X->getKind() != Y->getKind())
     return false;
 
   if (const auto *TX = dyn_cast<TemplateTypeParmDecl>(X)) {
     const auto *TY = cast<TemplateTypeParmDecl>(Y);
     if (TX->isParameterPack() != TY->isParameterPack())
       return false;
     if (TX->hasTypeConstraint() != TY->hasTypeConstraint())
       return false;
-    if (TX->hasTypeConstraint()) {
-      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())
         return false;
       if (TXTC->hasExplicitTemplateArgs()) {
         const auto *TXTCArgs = TXTC->getTemplateArgsAsWritten();
         const auto *TYTCArgs = TYTC->getTemplateArgsAsWritten();
         if (TXTCArgs->NumTemplateArgs != TYTCArgs->NumTemplateArgs)

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

+// 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