[AST] Abort the process when type variable id overflows

Author: xedin

include/swift/AST/Types.h

@@ -5224,8 +5224,11 @@ TypeVariableType : public TypeBase {
   TypeVariableType(const ASTContext &C, unsigned ID)
     : TypeBase(TypeKind::TypeVariable, &C,
                RecursiveTypeProperties::HasTypeVariable) {
-    // Note: the ID may overflow, but its only used for printing.
+    // Note: the ID may overflow (current limit is 2^20 - 1).
     Bits.TypeVariableType.ID = ID;
+    if (Bits.TypeVariableType.ID != ID) {
+      llvm::report_fatal_error("Type variable id overflow");
+    }
   }
 
   class Implementation;
@@ -5261,8 +5264,10 @@ TypeVariableType : public TypeBase {
     return reinterpret_cast<Implementation *>(this + 1);
   }
 
-  /// Type variable IDs are not globally unique and are only meant as a visual
-  /// aid when dumping AST.
+  /// Type variable IDs are not globally unique and are
+  /// used in equivalence class merging (so representative
+  /// is always a type variable with smaller id), as well
+  /// as a visual aid when dumping AST.
   unsigned getID() const { return Bits.TypeVariableType.ID; }
 
   // Implement isa/cast/dyncast/etc.