[CS] Emit fallback diagnostic if needed for IgnoreInvalidASTNode

If no other error has been emitted, make sure we emit a fallback
diagnostic rather than crashing in the ASTVerifier or SILGen.

Author: hamishknight

include/swift/Sema/ConstraintSystem.h

@@ -5736,7 +5736,7 @@ class ConstraintSystem {
 
   /// If we aren't certain that we've emitted a diagnostic, emit a fallback
   /// diagnostic.
-  void maybeProduceFallbackDiagnostic(SyntacticElementTarget target) const;
+  void maybeProduceFallbackDiagnostic(SourceLoc loc) const;
 
   /// Check whether given AST node represents an argument of an application
   /// of some sort (call, operator invocation, subscript etc.)

lib/Sema/CSApply.cpp

@@ -10021,7 +10021,7 @@ ConstraintSystem::applySolution(Solution &solution,
   {
     const auto &score = solution.getFixedScore();
     if (score.Data[SK_Fix] > numResolvableFixes || score.Data[SK_Hole] > 0) {
-      maybeProduceFallbackDiagnostic(target);
+      maybeProduceFallbackDiagnostic(target.getLoc());
       return std::nullopt;
     }
   }

lib/Sema/CSDiagnostics.cpp

@@ -236,6 +236,11 @@ bool FailureDiagnostic::conformsToKnownProtocol(
   return TypeChecker::conformsToKnownProtocol(type, protocol);
 }
 
+bool FallbackDiagnostic::diagnoseAsError() {
+  getConstraintSystem().maybeProduceFallbackDiagnostic(getLoc());
+  return true;
+}
+
 Type RequirementFailure::getOwnerType() const {
   auto anchor = getAnchor();
   // If diagnostic is anchored at assignment expression

lib/Sema/CSDiagnostics.h

@@ -210,6 +210,15 @@ class FailureDiagnostic {
                                  llvm::SmallVectorImpl<char> &scratch) const;
 };
 
+/// Emits a fallback diagnostic message if no other error has been emitted.
+class FallbackDiagnostic final : public FailureDiagnostic {
+public:
+  FallbackDiagnostic(const Solution &solution, ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 /// Base class for all of the diagnostics related to generic requirement
 /// failures, provides common information like failed requirement,
 /// declaration where such requirement comes from, etc.

lib/Sema/CSFix.cpp

@@ -2187,7 +2187,10 @@ AllowKeyPathWithoutComponents::create(ConstraintSystem &cs,
 
 bool IgnoreInvalidResultBuilderBody::diagnose(const Solution &solution,
                                               bool asNote) const {
-  return true; // Already diagnosed by `matchResultBuilder`.
+  // This should already be diagnosed by `matchResultBuilder`, emit a fallback
+  // diagnostic if not.
+  FallbackDiagnostic diag(solution, getLocator());
+  return diag.diagnose(asNote);
 }
 
 IgnoreInvalidResultBuilderBody *
@@ -2198,7 +2201,10 @@ IgnoreInvalidResultBuilderBody::create(ConstraintSystem &cs,
 
 bool IgnoreInvalidASTNode::diagnose(const Solution &solution,
                                     bool asNote) const {
-  return true; // Already diagnosed by the producer of ErrorExpr or ErrorType.
+  // This should already be diagnosed by the producer of ErrorExpr or ErrorType,
+  // emit a fallback diagnostic if not.
+  FallbackDiagnostic diag(solution, getLocator());
+  return diag.diagnose(asNote);
 }
 
 IgnoreInvalidASTNode *IgnoreInvalidASTNode::create(ConstraintSystem &cs,

lib/Sema/CSSolver.cpp

@@ -1457,7 +1457,7 @@ ConstraintSystem::solve(SyntacticElementTarget &target,
     }
 
     case SolutionResult::Error:
-      maybeProduceFallbackDiagnostic(target);
+      maybeProduceFallbackDiagnostic(target.getLoc());
       return std::nullopt;
 
     case SolutionResult::TooComplex: {

lib/Sema/ConstraintSystem.cpp

@@ -4875,8 +4875,7 @@ bool ConstraintSystem::isConformanceUnavailable(ProtocolConformanceRef conforman
 
 /// If we aren't certain that we've emitted a diagnostic, emit a fallback
 /// diagnostic.
-void ConstraintSystem::maybeProduceFallbackDiagnostic(
-    SyntacticElementTarget target) const {
+void ConstraintSystem::maybeProduceFallbackDiagnostic(SourceLoc loc) const {
   if (Options.contains(ConstraintSystemFlags::SuppressDiagnostics))
     return;
 
@@ -4888,7 +4887,7 @@ void ConstraintSystem::maybeProduceFallbackDiagnostic(
       (diagnosticTransaction && diagnosticTransaction->hasErrors()))
     return;
 
-  ctx.Diags.diagnose(target.getLoc(), diag::failed_to_produce_diagnostic);
+  ctx.Diags.diagnose(loc, diag::failed_to_produce_diagnostic);
 }
 
 SourceLoc constraints::getLoc(ASTNode anchor) {