[Clang] Implement CWG2369 "Ordering between constraints and substitution" (#102857)

This patch partially implements CWG2369 for non-lambda-constrained
functions.

Lambdas are left intact at this point because we need extra work to
correctly instantiate captures before the function instantiation.

As a premise of CWG2369, this patch also implements CWG2770 to ensure
the function parameters are instantiated on demand.

Closes https://github.com/llvm/llvm-project/issues/54440

1 files changed, 45 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 539de00..10f4920 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -846,7 +846,7 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
                                     bool ForOverloadResolution) {
   // Don't check constraints if the function is dependent. Also don't check if
   // this is a function template specialization, as the call to
-  // CheckinstantiatedFunctionTemplateConstraints after this will check it
+  // CheckFunctionTemplateConstraints after this will check it
   // better.
   if (FD->isDependentContext() ||
       FD->getTemplatedKind() ==
@@ -1111,12 +1111,55 @@ bool Sema::EnsureTemplateArgumentListConstraints(
   return false;
 }
 
-bool Sema::CheckInstantiatedFunctionTemplateConstraints(
+static bool CheckFunctionConstraintsWithoutInstantiation(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    FunctionTemplateDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
+    ConstraintSatisfaction &Satisfaction) {
+  SmallVector<const Expr *, 3> TemplateAC;
+  Template->getAssociatedConstraints(TemplateAC);
+  if (TemplateAC.empty()) {
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  LocalInstantiationScope Scope(SemaRef);
+
+  FunctionDecl *FD = Template->getTemplatedDecl();
+  // Collect the list of template arguments relative to the 'primary'
+  // template. We need the entire list, since the constraint is completely
+  // uninstantiated at this point.
+
+  // FIXME: Add TemplateArgs through the 'Innermost' parameter once
+  // the refactoring of getTemplateInstantiationArgs() relands.
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(Template, std::nullopt, /*Final=*/false);
+  SemaRef.getTemplateInstantiationArgs(
+      MLTAL, /*D=*/FD, FD,
+      /*Final=*/false, /*Innermost=*/std::nullopt, /*RelativeToPrimary=*/true,
+      /*Pattern=*/nullptr, /*ForConstraintInstantiation=*/true);
+  MLTAL.replaceInnermostTemplateArguments(Template, TemplateArgs);
+
+  Sema::ContextRAII SavedContext(SemaRef, FD);
+  std::optional<Sema::CXXThisScopeRAII> ThisScope;
+  if (auto *Method = dyn_cast<CXXMethodDecl>(FD))
+    ThisScope.emplace(SemaRef, /*Record=*/Method->getParent(),
+                      /*ThisQuals=*/Method->getMethodQualifiers());
+  return SemaRef.CheckConstraintSatisfaction(
+      Template, TemplateAC, MLTAL, PointOfInstantiation, Satisfaction);
+}
+
+bool Sema::CheckFunctionTemplateConstraints(
     SourceLocation PointOfInstantiation, FunctionDecl *Decl,
     ArrayRef<TemplateArgument> TemplateArgs,
     ConstraintSatisfaction &Satisfaction) {
   // In most cases we're not going to have constraints, so check for that first.
   FunctionTemplateDecl *Template = Decl->getPrimaryTemplate();
+
+  if (!Template)
+    return ::CheckFunctionConstraintsWithoutInstantiation(
+        *this, PointOfInstantiation, Decl->getDescribedFunctionTemplate(),
+        TemplateArgs, Satisfaction);
+
   // Note - code synthesis context for the constraints check is created
   // inside CheckConstraintsSatisfaction.
   SmallVector<const Expr *, 3> TemplateAC;