Fix #4674: Do not emit waring if types are the same

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -91,7 +91,8 @@ object TypeTestsCasts {
 
           if (expr.tpe <:< testType)
             if (expr.tpe.isNotNull) {
-              ctx.warning(TypeTestAlwaysSucceeds(foundCls, testCls), tree.pos)
+              if (foundCls != testCls)
+                ctx.warning(TypeTestAlwaysSucceeds(foundCls, testCls), tree.pos)
               constant(expr, Literal(Constant(true)))
             }
             else expr.testNotNull

tests/pos-special/fatal-warnings/i4674.scala

@@ -0,0 +1,9 @@
+class Foo {
+  def foo: Unit = {
+    val x: String = "something"
+    x.foreach {
+      case 's' => println("s")
+      case c: Char => println(c) // no warn for `: Char` allways succeeding
+    }
+  }
+}