compiler: add checks to ensure you cannot compare against Void type (…

…causes JVM verify error)

src/compiler/fan/steps/CheckErrors.fan

@@ -999,6 +999,19 @@ class CheckErrors : CompilerStep
       err("Incomparable types '$lhs.ctype' and '$rhs.ctype'", lhs.loc)
       return false
     }
+
+    if (lhs.ctype.isVoid)
+    {
+      err("Cannot compare to Void type", lhs.loc)
+      return false
+    }
+
+    if (rhs.ctype.isVoid)
+    {
+      err("Cannot compare to Void type", rhs.loc)
+      return false
+    }
+
     return true
   }
 

src/testCompiler/fan/CheckErrorsTest.fan

@@ -1884,5 +1884,32 @@ class CheckErrorsTest : CompilerTest
        ])
   }
 
+
+//////////////////////////////////////////////////////////////////////////
+// Void Compares
+//////////////////////////////////////////////////////////////////////////
+
+  Void testVoidCompares()
+  {
+   verifyErrors(
+     """class Foo
+        {
+           Void bar()
+           {
+             if (obj == foo) return  // line 5
+             if (foo != obj) return  // line 6
+             if (obj === foo) return  // line 7
+           }
+
+           Obj? obj() { null }
+
+           Void foo() {}
+         }""",
+       [
+         5, 17, "Cannot compare to Void type",
+         6, 10, "Cannot compare to Void type",
+         7, 18, "Cannot compare to Void type",
+       ])
+  }
 }