[wip] disable let stored property destructuring

Author: jckarter

include/swift/AST/DiagnosticsSema.def

@@ -2136,6 +2136,9 @@ ERROR(pattern_binds_no_variables,none,
 ERROR(variable_bound_by_no_pattern,none,
       "variable %0 is not bound by any pattern",
       (const VarDecl *))
+ERROR(destructuring_let_struct_stored_property_unsupported,none,
+      "binding multiple 'let' stored properties from a single initializer expression in a struct is unsupported",
+      ())
 
 WARNING(optional_ambiguous_case_ref,none,
         "assuming you mean '%0.%2'; did you mean '%1.%2' instead?",

lib/Sema/TypeCheckConstraints.cpp

@@ -869,6 +869,7 @@ bool TypeChecker::typeCheckPatternBinding(PatternBindingDecl *PBD,
   if (hadError)
     PBD->setInvalid();
   PBD->setInitializerChecked(patternNumber);
+  
   return hadError;
 }
 

lib/Sema/TypeCheckStorage.cpp

@@ -567,6 +567,20 @@ const PatternBindingEntry *PatternBindingEntryRequest::evaluate(
                                  : GlobalVariable);
     }
   }
+  
+  // If the pattern binding appears as a compound stored `let` property with an
+  // initializer inside of a struct type, diagnose it as unsupported.
+  // This hasn't ever been implemented properly.
+  if (!binding->isStatic()
+      && binding->isInitialized(entryNumber)
+      && isa<StructDecl>(binding->getDeclContext())
+      && !pattern->getSingleVar()
+      && !vars.empty()
+      && vars[0]->isLet()) {
+    Context.Diags.diagnose(binding->getPattern(entryNumber)->getLoc(),
+                           diag::destructuring_let_struct_stored_property_unsupported);
+  }
+  
   return &pbe;
 }