[Name lookup] Diagnose attempts to reference not-yet-declared local variables.

Until the point where ASTScope-based unqualified name lookup is the
default, unqualified name lookup can still find names declared *after*
the source location. The 'hasType' check no longer makes sense, so actually
check the source location of the entity we found.

Author: DougGregor

lib/Sema/TypeCheckConstraints.cpp

@@ -474,8 +474,12 @@ resolveDeclRefExpr(UnresolvedDeclRefExpr *UDRE, DeclContext *DC) {
     ValueDecl *D = Result.Decl;
     if (!D->hasType()) validateDecl(D);
 
-    if (!D->hasType()) {
-      assert(D->getDeclContext()->isLocalContext());
+    // FIXME: The source-location checks won't make sense once
+    // EnableASTScopeLookup is the default.
+    if (Loc.isValid() && D->getLoc().isValid() &&
+        D->getDeclContext()->isLocalContext() &&
+        D->getDeclContext() == DC &&
+        Context.SourceMgr.isBeforeInBuffer(Loc, D->getLoc())) {
       if (!D->isInvalid()) {
         diagnose(Loc, diag::use_local_before_declaration, Name);
         diagnose(D, diag::decl_declared_here, Name);

test/expr/capture/order.swift

@@ -78,11 +78,14 @@ func outOfOrderEnum() {
 }
 
 func captureInClosure() {
-  var x = { (i: Int) in 
-    currentTotal += i // expected-error{{use of local variable 'currentTotal' before its declaration}}
+  let x = { (i: Int) in
+    currentTotal += i // expected-error{{cannot capture 'currentTotal' before it is declared}}
   }
 
   var currentTotal = 0 // expected-note{{'currentTotal' declared here}}
+
+  _ = x
+  currentTotal += 1
 }
 
 class X {