flag "returning a statement" as a syntax error

compiler/src/prog8/compiler/astprocessing/AstChecker.kt

@@ -140,6 +140,18 @@ internal class AstChecker(private val program: Program,
                 }
             }
         }
+
+        val statements = (returnStmt.parent as IStatementContainer).statements
+        val myIndex = statements.indexOf(returnStmt)
+        if(myIndex>=0 && myIndex<statements.size-1) {
+            val stmtAfterReturn = statements[myIndex+1]
+            if(stmtAfterReturn.position.line==returnStmt.position.line) {
+                // this error is not generated by the parser, unfortunately.
+                // it parses "return somestatement"  as:  "return"  "somestatement"  (and this then only results in a warning about unreachable code).
+                errors.err("a statement is not a return value", stmtAfterReturn.position)
+            }
+        }
+
         super.visit(returnStmt)
     }
 

compiler/test/ast/TestAstChecks.kt

@@ -174,4 +174,22 @@ main {
         compileText(Cx16Target(), false, text, writeAssembly = true)  shouldNotBe null
         compileText(VMTarget(), false, text, writeAssembly = true)  shouldNotBe null
     }
+
+    test("return with a statement instead of a value is a syntax error") {
+        val src="""
+main {
+
+    sub invalid() {
+        return cx16.r0++
+    }
+
+    sub start() {
+        invalid()
+    }
+}"""
+        val errors=ErrorReporterForTests()
+        compileText(C64Target(), false, src, writeAssembly = false, errors=errors)  shouldBe null
+        errors.errors.size shouldBe 1
+        errors.errors[0] shouldContain "statement"
+    }
 })