Diagnose differentiable functions returning Void w/o inout arguments.

include/swift/AST/DiagnosticsSema.def

@@ -5334,6 +5334,12 @@ ERROR(differentiable_function_type_invalid_result,none,
       "%select{| and satisfy '%0 == %0.TangentVector'}1, but the enclosing "
       "function type is '@differentiable%select{|(_linear)}1'",
       (StringRef, bool))
+ERROR(differentiable_function_type_void_result,
+      none,
+      "'@differentiable' function returning Void must have at least one "
+      "differentiable inout parameter, i.e. a non-'@noDerivative' parameter "
+      "whose type conforms to 'Differentiable'",
+      ())
 ERROR(differentiable_function_type_no_differentiability_parameters, 
       none,
       "'@differentiable' function type requires at least one differentiability "

lib/Sema/TypeChecker.cpp

@@ -720,6 +720,28 @@ bool TypeChecker::diagnoseInvalidFunctionType(FunctionType *fnTy, SourceLoc loc,
           diag.highlight((*repr)->getResultTypeRepr()->getSourceRange());
       }
     }
+
+    // If the result type is void, we need to have at least one differentiable
+    // inout argument
+    if (result->isVoid() &&
+        llvm::find_if(params,
+                      [&](AnyFunctionType::Param param) {
+                        if (param.isNoDerivative())
+                          return false;
+                        return param.isInOut() &&
+                          TypeChecker::isDifferentiable(param.getPlainType(),
+                                                        /*tangentVectorEqualsSelf*/ isLinear,
+                                                        dc, stage);
+                      }) == params.end()) {
+      auto diagLoc = repr ? (*repr)->getResultTypeRepr()->getLoc() : loc;
+      auto resultStr = fnTy->getResult()->getString();
+      auto diag = ctx.Diags.diagnose(
+        diagLoc, diag::differentiable_function_type_void_result);
+      hadAnyError = true;
+
+      if (repr)
+        diag.highlight((*repr)->getResultTypeRepr()->getSourceRange());
+    }
   }
 
   return hadAnyError;

test/AutoDiff/Sema/differentiable_func_type.swift

@@ -41,6 +41,10 @@ let _: @differentiable(_linear) (Float) -> NonDiffType
 
 let _: @differentiable(_linear) (Float) -> Float
 
+// expected-error @+1 {{'@differentiable' function returning Void must have at least one differentiable inout parameter, i.e. a non-'@noDerivative' parameter whose type conforms to 'Differentiable'}}
+let _: @differentiable(reverse) (Float) -> Void
+let _: @differentiable(reverse) (inout Float) -> Void // okay
+
 // expected-error @+1 {{result type '@differentiable(reverse) (U) -> Float' does not conform to 'Differentiable', but the enclosing function type is '@differentiable'}}
 func test1<T: Differentiable, U: Differentiable>(_: @differentiable(reverse) (T) -> @differentiable(reverse) (U) -> Float) {}
 // expected-error @+1 {{result type '(U) -> Float' does not conform to 'Differentiable', but the enclosing function type is '@differentiable'}}