[ASTVerifier] Move PackExpansionType::get assertions into the ASTVerifier.

These assertions are violated for invalid code, so let's only check these on
a well-formed AST.

Author: hborla

lib/AST/ASTContext.cpp

@@ -3247,17 +3247,7 @@ PackExpansionType::PackExpansionType(Type patternType, Type countType,
                                      RecursiveTypeProperties properties,
                                      const ASTContext *canCtx)
   : TypeBase(TypeKind::PackExpansion, canCtx, properties),
-    patternType(patternType), countType(countType) {
-
-  // TODO: it would be nice if the solver didn't stick PlaceholderTypes and
-  // UnresolvedTypes in here.
-  assert(countType->is<PackType>() ||
-         countType->is<TypeVariableType>() ||
-         countType->is<PackArchetypeType>() ||
-         countType->is<PlaceholderType>() ||
-         countType->is<UnresolvedType>() ||
-         countType->castTo<GenericTypeParamType>()->isParameterPack());
-}
+    patternType(patternType), countType(countType) {}
 
 CanPackExpansionType
 CanPackExpansionType::get(CanType patternType, CanType countType) {

lib/AST/ASTVerifier.cpp

@@ -603,6 +603,18 @@ class Verifier : public ASTWalker {
         abort();
       }
 
+      // Check for invalid pack expansion shape types.
+      if (auto *expansion = type->getAs<PackExpansionType>()) {
+        auto countType = expansion->getCountType();
+        if (!(countType->is<PackType>() ||
+              countType->is<PackArchetypeType>() ||
+              (countType->is<GenericTypeParamType>() &&
+               countType->castTo<GenericTypeParamType>()->isParameterPack()))) {
+          Out << "non-pack shape type" << countType->getString() << "\n";
+          abort();
+        }
+      }
+
       if (!type->hasArchetype())
         return;