[Sema] Fix throwing super.init() diagnostics

The compiler now diagnoses appropriately the case where a super.init() is marked as throwing while a subclass' initializer isn't marked as throwing nor explicitly calls the super.init().

include/swift/AST/DiagnosticsSema.def

@@ -4807,6 +4807,12 @@ ERROR(return_init_non_nil,none,
       "'nil' is the only return value permitted in an initializer",
       ())
 
+ERROR(implicit_throws_super_init,none,
+      "missing call to superclass's initializer; "
+      "'super.init' is a throwing initializer and requires either an explicit "
+      "call or that this initializer is also marked as 'throws'",
+      ())
+
 ERROR(implicit_async_super_init,none,
       "missing call to superclass's initializer; "
       "'super.init' is 'async' and requires an explicit call",

lib/Sema/TypeCheckStmt.cpp

@@ -2236,7 +2236,17 @@ static bool checkSuperInit(ConstructorDecl *fromCtor,
       fromCtor->diagnose(diag::availability_unavailable_implicit_init,
                          ctor, superclassDecl->getName());
     }
-
+    
+    // Only allowed to synthesize a throwing super.init() call if the init being
+    // checked is also throwing.
+    if (ctor->hasThrows()) {
+      // Diagnose on nonthrowing or rethrowing initializer.
+      if (!fromCtor->hasThrows() || fromCtor->hasPolymorphicEffect(EffectKind::Throws)) {
+        fromCtor->diagnose(diag::implicit_throws_super_init);
+        return true; // considered an error
+      }
+    }
+    
     // Not allowed to implicitly generate a super.init() call if the init
     // is async; that would hide the 'await' from the programmer.
     if (ctor->hasAsync()) {

test/Sema/throwing_super_init.swift

@@ -0,0 +1,24 @@
+// RUN: %target-typecheck-verify-swift
+
+class ThrowingInitSuperclass {
+  init() throws { }
+}
+
+// Implicitly calling the super.init IS possible here because the initializer
+// is a throwing initializer.
+class ThrowingInitClass: ThrowingInitSuperclass {
+  init(simpleArgument: Int) throws { }
+}
+
+// Implicitly calling the super.init IS NOT possible here because the
+// initializer is not a throwing initializer.
+class NonThrowingInitClass: ThrowingInitSuperclass {
+  init(simpleArgument: Int) { } // expected-error {{missing call to superclass's initializer; 'super.init' is a throwing initializer and requires either an explicit call or that this initializer is also marked as 'throws'}}
+}
+
+// Implicitly calling the super.init IS NOT possible here because the
+// initializer is a rethrowing initializer, which means it can only
+// rethrow errors from its parameters.
+class RethrowingInitClass: ThrowingInitSuperclass {
+  init(throwingArgument: () throws -> Void) rethrows { } // expected-error {{missing call to superclass's initializer; 'super.init' is a throwing initializer and requires either an explicit call or that this initializer is also marked as 'throws'}}
+}