PR45124: Don't leave behind pending cleanups when declaring implicit

deduction guides.

Previously if an implicit deduction guide had a default argument with a
cleanup, we'd leave the 'pending cleanup' flag set after declaring the
implicit guide. But it turns out that there's no reason to even
substitute into the default argument when declaring an implicit
deduction guide: we only need to record that the default argument
exists, not what it is, since we never actually form a call to a
deduction guide.

(cherry picked from commit 6d894af)

Author: zygoloid

clang/lib/Sema/SemaTemplate.cpp

@@ -2173,9 +2173,15 @@ struct ConvertConstructorToDeductionGuideTransform {
     // constructor.
     ExprResult NewDefArg;
     if (OldParam->hasDefaultArg()) {
-      NewDefArg = SemaRef.SubstExpr(OldParam->getDefaultArg(), Args);
-      if (NewDefArg.isInvalid())
-        return nullptr;
+      // We don't care what the value is (we won't use it); just create a
+      // placeholder to indicate there is a default argument.
+      QualType ParamTy = NewDI->getType();
+      NewDefArg = new (SemaRef.Context)
+          OpaqueValueExpr(OldParam->getDefaultArg()->getBeginLoc(),
+                          ParamTy.getNonLValueExprType(SemaRef.Context),
+                          ParamTy->isLValueReferenceType() ? VK_LValue :
+                          ParamTy->isRValueReferenceType() ? VK_XValue :
+                          VK_RValue);
     }
 
     ParmVarDecl *NewParam = ParmVarDecl::Create(SemaRef.Context, DC,

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

@@ -507,6 +507,21 @@ umm m(1);
 
 }
 
+namespace PR45124 {
+  class a { int d; };
+  class b : a {};
+
+  struct x { ~x(); };
+  template<typename> class y { y(x = x()); };
+  template<typename z> y(z)->y<z>;
+
+  // Not a constant initializer, but trivial default initialization. We won't
+  // detect this as trivial default initialization if synthesizing the implicit
+  // deduction guide 'template<typename T> y(x = x()) -> Y<T>;' leaves behind a
+  // pending cleanup.
+  __thread b g;
+}
+
 #else
 
 // expected-no-diagnostics