emit diagnostics for same type coercion. Fixes SR-11295

Author: LucianoPAlmeida

include/swift/AST/DiagnosticsSema.def

@@ -918,7 +918,9 @@ WARNING(isa_is_foreign_check,none,
 WARNING(conditional_downcast_coercion,none,
       "conditional cast from %0 to %1 always succeeds",
       (Type, Type))
-
+WARNING(unecessary_same_type_coercion,none,
+      "casting expression to %0 doesn't change the type",
+      (Type))
 WARNING(forced_downcast_noop,none,
         "forced cast of %0 to same type has no effect", (Type))
 

lib/Sema/CSApply.cpp

@@ -3533,6 +3533,15 @@ namespace {
         Expr *sub = expr->getSubExpr();
 
         cs.setExprTypes(sub);
+        
+        if (!SuppressDiagnostics) {
+          // If we are trying to perform coercion to the same type emit a warning.
+          if (cs.getType(sub)->isEqual(toType)) {
+            tc.diagnose(expr->getLoc(), diag::unecessary_same_type_coercion, toType)
+              .fixItRemove(SourceRange(expr->getLoc(),
+                                       expr->getCastTypeLoc().getSourceRange().End));
+          }
+        }
 
         if (tc.convertToType(sub, toType, cs.DC))
           return nullptr;

test/expr/cast/as_coerce.swift

@@ -135,3 +135,20 @@ _ = sr6022 as! AnyObject // expected-warning {{forced cast from '() -> Any' to '
 _ = sr6022 as? AnyObject // expected-warning {{conditional cast from '() -> Any' to 'AnyObject' always succeeds}}
 _ = sr6022_1 as! Any // expected-warning {{forced cast from '() -> ()' to 'Any' always succeeds; did you mean to use 'as'?}}
 _ = sr6022_1 as? Any // expected-warning {{conditional cast from '() -> ()' to 'Any' always succeeds}}
+
+// SR-11295
+let a = "Hello"
+_ = a as String // expected-warning {{casting expression to 'String' doesn't change the type}} {{7-17=}}
+
+let b = 1
+_ = b as Int // expected-warning {{casting expression to 'Int' doesn't change the type}} {{7-14=}}
+
+typealias Type = String
+
+let c: Type = "Hello Typealias" 
+_ = c as String // expected-warning {{casting expression to 'String' doesn't change the type}} {{7-17=}}
+
+let d = "Hello Typealias"
+_ = d as Type // expected-warning {{casting expression to 'Type' (aka 'String') doesn't change the type}} {{7-15=}}
+
+_ = "Hello" as String // expected-warning {{casting expression to 'String' doesn't change the type}} {{13-23=}}