[Concepts] Instantiate invented template type parameter type-constraint along with function parameters

We previously instantiated type-constraints of template type parameters along with the type parameter itself,
this caused problems when the type-constraints created by abbreviated templates refreneced other parameters
in the abbreviated templates.

When encountering a template type parameter with a type constraint, if it is implicit, delay instantiation of
the type-constraint until the function parameter which created the invented template type parameter is
instantiated.

Reland after fixing bug caused by another flow reaching SubstParmVarDecl and instantiating the TypeConstraint
a second time.

(cherry picked from commit 84959ae47f447fca9d56a9c61e8c46e993d0387a)

2 files changed, 155 insertions, 20 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index dff336f2ff2..a9357ede700 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -18,6 +18,7 @@
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/PrettyDeclStackTrace.h"
+#include "clang/AST/TypeVisitor.h"
 #include "clang/Basic/LangOptions.h"
 #include "clang/Basic/Stack.h"
 #include "clang/Sema/DeclSpec.h"
@@ -2145,6 +2146,94 @@ void Sema::SubstExceptionSpec(FunctionDecl *New, const FunctionProtoType *Proto,
   UpdateExceptionSpec(New, ESI);
 }
 
+namespace {
+
+  struct GetContainedInventedTypeParmVisitor :
+    public TypeVisitor<GetContainedInventedTypeParmVisitor,
+                       TemplateTypeParmDecl *> {
+    using TypeVisitor<GetContainedInventedTypeParmVisitor,
+                      TemplateTypeParmDecl *>::Visit;
+
+    TemplateTypeParmDecl *Visit(QualType T) {
+      if (T.isNull())
+        return nullptr;
+      return Visit(T.getTypePtr());
+    }
+    // The deduced type itself.
+    TemplateTypeParmDecl *VisitTemplateTypeParmType(
+        const TemplateTypeParmType *T) {
+      if (!T->getDecl()->isImplicit())
+        return nullptr;
+      return T->getDecl();
+    }
+
+    // Only these types can contain 'auto' types, and subsequently be replaced
+    // by references to invented parameters.
+
+    TemplateTypeParmDecl *VisitElaboratedType(const ElaboratedType *T) {
+      return Visit(T->getNamedType());
+    }
+
+    TemplateTypeParmDecl *VisitPointerType(const PointerType *T) {
+      return Visit(T->getPointeeType());
+    }
+
+    TemplateTypeParmDecl *VisitBlockPointerType(const BlockPointerType *T) {
+      return Visit(T->getPointeeType());
+    }
+
+    TemplateTypeParmDecl *VisitReferenceType(const ReferenceType *T) {
+      return Visit(T->getPointeeTypeAsWritten());
+    }
+
+    TemplateTypeParmDecl *VisitMemberPointerType(const MemberPointerType *T) {
+      return Visit(T->getPointeeType());
+    }
+
+    TemplateTypeParmDecl *VisitArrayType(const ArrayType *T) {
+      return Visit(T->getElementType());
+    }
+
+    TemplateTypeParmDecl *VisitDependentSizedExtVectorType(
+      const DependentSizedExtVectorType *T) {
+      return Visit(T->getElementType());
+    }
+
+    TemplateTypeParmDecl *VisitVectorType(const VectorType *T) {
+      return Visit(T->getElementType());
+    }
+
+    TemplateTypeParmDecl *VisitFunctionProtoType(const FunctionProtoType *T) {
+      return VisitFunctionType(T);
+    }
+
+    TemplateTypeParmDecl *VisitFunctionType(const FunctionType *T) {
+      return Visit(T->getReturnType());
+    }
+
+    TemplateTypeParmDecl *VisitParenType(const ParenType *T) {
+      return Visit(T->getInnerType());
+    }
+
+    TemplateTypeParmDecl *VisitAttributedType(const AttributedType *T) {
+      return Visit(T->getModifiedType());
+    }
+
+    TemplateTypeParmDecl *VisitMacroQualifiedType(const MacroQualifiedType *T) {
+      return Visit(T->getUnderlyingType());
+    }
+
+    TemplateTypeParmDecl *VisitAdjustedType(const AdjustedType *T) {
+      return Visit(T->getOriginalType());
+    }
+
+    TemplateTypeParmDecl *VisitPackExpansionType(const PackExpansionType *T) {
+      return Visit(T->getPattern());
+    }
+  };
+
+} // namespace
+
 ParmVarDecl *Sema::SubstParmVarDecl(ParmVarDecl *OldParm,
                             const MultiLevelTemplateArgumentList &TemplateArgs,
                                     int indexAdjustment,
@@ -2192,6 +2281,46 @@ ParmVarDecl *Sema::SubstParmVarDecl(ParmVarDecl *OldParm,
     return nullptr;
   }
 
+  // In abbreviated templates, TemplateTypeParmDecls with possible
+  // TypeConstraints are created when the parameter list is originally parsed.
+  // The TypeConstraints can therefore reference other functions parameters in
+  // the abbreviated function template, which is why we must instantiate them
+  // here, when the instantiated versions of those referenced parameters are in
+  // scope.
+  if (TemplateTypeParmDecl *TTP =
+          GetContainedInventedTypeParmVisitor().Visit(OldDI->getType())) {
+    if (const TypeConstraint *TC = TTP->getTypeConstraint()) {
+      auto *Inst = cast<TemplateTypeParmDecl>(
+          FindInstantiatedDecl(TTP->getLocation(), TTP, TemplateArgs));
+      // We will first get here when instantiating the abbreviated function
+      // template's described function, but we might also get here later.
+      // Make sure we do not instantiate the TypeConstraint more than once.
+      if (Inst && !Inst->hasTypeConstraint()) {
+        // TODO: Concepts: do not instantiate the constraint (delayed constraint
+        // substitution)
+        const ASTTemplateArgumentListInfo *TemplArgInfo
+          = TC->getTemplateArgsAsWritten();
+        TemplateArgumentListInfo InstArgs;
+
+        if (TemplArgInfo) {
+          InstArgs.setLAngleLoc(TemplArgInfo->LAngleLoc);
+          InstArgs.setRAngleLoc(TemplArgInfo->RAngleLoc);
+          if (Subst(TemplArgInfo->getTemplateArgs(),
+                    TemplArgInfo->NumTemplateArgs, InstArgs, TemplateArgs))
+            return nullptr;
+        }
+        if (AttachTypeConstraint(
+                TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
+                TC->getNamedConcept(), &InstArgs, Inst,
+                TTP->isParameterPack()
+                    ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
+                        ->getEllipsisLoc()
+                    : SourceLocation()))
+          return nullptr;
+      }
+    }
+  }
+
   ParmVarDecl *NewParm = CheckParameter(Context.getTranslationUnitDecl(),
                                         OldParm->getInnerLocStart(),
                                         OldParm->getLocation(),
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 2637b4cb6dc..0e1d5fa77c6 100755
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2525,28 +2525,34 @@ Decl *TemplateDeclInstantiator::VisitTemplateTypeParmDecl(
   Inst->setAccess(AS_public);
   Inst->setImplicit(D->isImplicit());
   if (auto *TC = D->getTypeConstraint()) {
-    // TODO: Concepts: do not instantiate the constraint (delayed constraint
-    // substitution)
-    const ASTTemplateArgumentListInfo *TemplArgInfo
-      = TC->getTemplateArgsAsWritten();
-    TemplateArgumentListInfo InstArgs;
-
-    if (TemplArgInfo) {
-      InstArgs.setLAngleLoc(TemplArgInfo->LAngleLoc);
-      InstArgs.setRAngleLoc(TemplArgInfo->RAngleLoc);
-      if (SemaRef.Subst(TemplArgInfo->getTemplateArgs(),
-                        TemplArgInfo->NumTemplateArgs,
-                        InstArgs, TemplateArgs))
+    if (!D->isImplicit()) {
+      // Invented template parameter type constraints will be instantiated with
+      // the corresponding auto-typed parameter as it might reference other
+      // parameters.
+
+      // TODO: Concepts: do not instantiate the constraint (delayed constraint
+      // substitution)
+      const ASTTemplateArgumentListInfo *TemplArgInfo
+        = TC->getTemplateArgsAsWritten();
+      TemplateArgumentListInfo InstArgs;
+
+      if (TemplArgInfo) {
+        InstArgs.setLAngleLoc(TemplArgInfo->LAngleLoc);
+        InstArgs.setRAngleLoc(TemplArgInfo->RAngleLoc);
+        if (SemaRef.Subst(TemplArgInfo->getTemplateArgs(),
+                          TemplArgInfo->NumTemplateArgs,
+                          InstArgs, TemplateArgs))
+          return nullptr;
+      }
+      if (SemaRef.AttachTypeConstraint(
+              TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
+              TC->getNamedConcept(), &InstArgs, Inst,
+              D->isParameterPack()
+                  ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
+                      ->getEllipsisLoc()
+                  : SourceLocation()))
         return nullptr;
     }
-    if (SemaRef.AttachTypeConstraint(
-            TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
-            TC->getNamedConcept(), &InstArgs, Inst,
-            D->isParameterPack()
-                ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
-                    ->getEllipsisLoc()
-                : SourceLocation()))
-      return nullptr;
   }
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
     TypeSourceInfo *InstantiatedDefaultArg =