[clang] Fixing incorrect implicit deduction guides (PR41549)

[Tyker]

Summary

bug report

Before this patch, implicit deduction guides were generated from the first declaration found by lookup.
With this patch implicit deduction guides are generated from the definition of the class template.
Also added test that was previously failing.

[rsmith]

clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp

503 ↗

(On Diff #203789)

Should this be using CTAD?

[Tyker]

fixed the issue.

[rsmith]

Thanks!

cfe/trunk/lib/Sema/SemaTemplate.cpp

NamedDecl *buildDeductionGuide(TemplateParameterList *TemplateParams,
     DC->addDecl(GuideTemplate);
     return GuideTemplate;
   }
 };
 }
 
 void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
                                           SourceLocation Loc) {
+  if (CXXRecordDecl *DefRecord =
+          cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
+    TemplateDecl *DescribedTemplate = DefRecord->getDescribedClassTemplate();
+    Template = DescribedTemplate ? DescribedTemplate : Template;
+  }
+
   DeclContext *DC = Template->getDeclContext();
   if (DC->isDependentContext())
     return;
 
   ConvertConstructorToDeductionGuideTransform Transform(
       *this, cast<ClassTemplateDecl>(Template));
   if (!isCompleteType(Loc, Transform.DeducedType))
     return;

cfe/trunk/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp

struct TestSuppression {
   TestSuppression(T) {}
 };
 TestSuppression(allow_ctad_t)->TestSuppression<void>;
 TestSuppression ta("abc");
 static_assert(__is_same(decltype(ta), TestSuppression<const char *>), "");
 }
 #pragma clang diagnostic pop
 
+namespace PR41549 {
+
+template <class H, class P> struct umm;
+
+template <class H = int, class P = int>
+struct umm {
+  umm(H h = 0, P p = 0);
+};
+
+template <class H, class P> struct umm;
+
+umm m(1);
+
+}
+
 #else
 
 // expected-no-diagnostics
 namespace undefined_warnings {
   // Make sure we don't get an "undefined but used internal symbol" warning for the deduction guide here.
   namespace {
     template <typename T>
     struct TemplDObj {