[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, 10 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index cc3510b0d47..1561960c99d 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -14486,8 +14486,16 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
       std::string InnerCondDescription;
       std::tie(InnerCond, InnerCondDescription) =
         findFailedBooleanCondition(Converted.get());
-      if (InnerCond && !isa<CXXBoolLiteralExpr>(InnerCond)
-                    && !isa<IntegerLiteral>(InnerCond)) {
+      if (InnerCond && isa<ConceptSpecializationExpr>(InnerCond)) {
+        // Drill down into concept specialization expressions to see why they
+        // weren't satisfied.
+        Diag(StaticAssertLoc, diag::err_static_assert_failed)
+          << !AssertMessage << Msg.str() << AssertExpr->getSourceRange();
+        ConstraintSatisfaction Satisfaction;
+        if (!CheckConstraintSatisfaction(InnerCond, Satisfaction))
+          DiagnoseUnsatisfiedConstraint(Satisfaction);
+      } else if (InnerCond && !isa<CXXBoolLiteralExpr>(InnerCond)
+                           && !isa<IntegerLiteral>(InnerCond)) {
         Diag(StaticAssertLoc, diag::err_static_assert_requirement_failed)
           << InnerCondDescription << !AssertMessage
           << Msg.str() << InnerCond->getSourceRange();