[Concepts] Constraint Enforcement & Diagnostics

Part of the C++20 concepts implementation effort.
- Associated constraints (requires clauses, currently) are now enforced when instantiating/specializing templates and when considering partial specializations and function overloads.
- Elaborated diagnostics give helpful insight as to why the constraints were not satisfied.
Phabricator: D41569

Re-commit, after fixing some memory bugs.

1 files changed, 21 insertions, 18 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 701be0411f7..2496c919311 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -363,7 +363,7 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
-    ConstraintsCheck, TemplateDecl *Template,
+    ConstraintsCheck, NamedDecl *Template,
     ArrayRef<TemplateArgument> TemplateArgs, SourceRange InstantiationRange)
     : InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::ConstraintsCheck,
@@ -372,7 +372,7 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
-    ConstraintSubstitution, TemplateDecl *Template,
+    ConstraintSubstitution, NamedDecl *Template,
     sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
     : InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::ConstraintSubstitution,
@@ -691,24 +691,27 @@ void Sema::PrintInstantiationStack() {
     case CodeSynthesisContext::Memoization:
       break;
     
-    case CodeSynthesisContext::ConstraintsCheck:
-      if (auto *CD = dyn_cast<ConceptDecl>(Active->Entity)) {
-        SmallVector<char, 128> TemplateArgsStr;
-        llvm::raw_svector_ostream OS(TemplateArgsStr);
-        CD->printName(OS);
-        printTemplateArgumentList(OS, Active->template_arguments(),
-                                  getPrintingPolicy());
-        Diags.Report(Active->PointOfInstantiation,
-                     diag::note_concept_specialization_here)
-          << OS.str()
-          << Active->InstantiationRange;
-        break;
+    case CodeSynthesisContext::ConstraintsCheck: {
+      unsigned DiagID = 0;
+      if (isa<ConceptDecl>(Active->Entity))
+        DiagID = diag::note_concept_specialization_here;
+      else if (isa<TemplateDecl>(Active->Entity))
+        DiagID = diag::note_checking_constraints_for_template_id_here;
+      else if (isa<VarTemplatePartialSpecializationDecl>(Active->Entity))
+        DiagID = diag::note_checking_constraints_for_var_spec_id_here;
+      else {
+        assert(isa<ClassTemplatePartialSpecializationDecl>(Active->Entity));
+        DiagID = diag::note_checking_constraints_for_class_spec_id_here;
       }
-      // TODO: Concepts - implement this for constrained templates and partial
-      // specializations.
-      llvm_unreachable("only concept constraints are supported right now");
+      SmallVector<char, 128> TemplateArgsStr;
+      llvm::raw_svector_ostream OS(TemplateArgsStr);
+      cast<NamedDecl>(Active->Entity)->printName(OS);
+      printTemplateArgumentList(OS, Active->template_arguments(),
+                                getPrintingPolicy());
+      Diags.Report(Active->PointOfInstantiation, DiagID) << OS.str()
+        << Active->InstantiationRange;
       break;
-      
+    }
     case CodeSynthesisContext::ConstraintSubstitution:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_constraint_substitution_here)