[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)

1 files changed, 26 insertions, 20 deletions

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 =