Emit proper diagnostic when returning void from a function with a return value, fixes AssemblyScript#475

Author: dcodeIO

src/compiler.ts

@@ -1948,12 +1948,12 @@ export class Compiler extends DiagnosticEmitter {
  var module = this.module;
  var expr: ExpressionRef = 0;
  var flow = this.currentFlow;
+ var returnType = flow.returnType;
 
  // Remember that this flow returns
  flow.set(FlowFlags.RETURNS);
 
  if (statement.value) {
- let returnType = flow.returnType;
  if (returnType == Type.void) {
  this.compileExpressionRetainType(statement.value, returnType, WrapMode.NONE);
  this.error(
@@ -1974,6 +1974,13 @@ export class Compiler extends DiagnosticEmitter {
 
  // Remember whether returning a properly wrapped value
  if (!flow.canOverflow(expr, returnType)) flow.set(FlowFlags.RETURNS_WRAPPED);
+
+ } else if (returnType != Type.void) {
+ this.error(
+ DiagnosticCode.Type_0_is_not_assignable_to_type_1,
+ statement.range, "void", returnType.toString()
+ );
+ expr = module.createUnreachable();
  }
 
  // If the last statement anyway, make it the block's return value