fix crash when using undefined variable in for loop

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

@@ -186,7 +186,7 @@ internal class AstChecker(private val program: Program,
             }
         }
 
-        val iterableDt = forLoop.iterable.inferType(program).getOr(DataType.forDt(BaseDataType.BYTE))
+        val iterableDt = forLoop.iterable.inferType(program).getOrUndef()
 
         if(iterableDt.isNumeric) TODO("iterable type should not be simple numeric!? "+forLoop.position) // TODO
 

compiler/test/ast/TestVariousCompilerAst.kt

@@ -1015,6 +1015,24 @@ main {
         val handler = sub.children[8] as PtSub
         handler.name shouldBe "p8s_prog8_invoke_defers"
     }
+
+    test("unknown variable in for loop gives proper errors") {
+        val src="""
+main {
+    sub start() {
+        ubyte i
+        for i in 0 to count - 1 {
+            break
+        }
+    }
+}"""
+
+        val errors = ErrorReporterForTests()
+        compileText(C64Target(), optimize=false, src, writeAssembly=false, errors = errors) shouldBe null
+        errors.errors.size shouldBe 3
+        errors.errors[0] shouldContain "loop variable can only loop over"
+        errors.errors[1] shouldContain "undefined symbol"
+    }
 }
 
 })

compilerAst/src/prog8/ast/expressions/AstExpressions.kt

@@ -241,6 +241,10 @@ class BinaryExpression(
     companion object {
         fun commonDatatype(leftDt: DataType, rightDt: DataType,
                            left: Expression?, right: Expression?): Pair<DataType, Expression?> {
+
+            if(leftDt.isUndefined || rightDt.isUndefined)
+                return DataType.forDt(BaseDataType.UNDEFINED) to null
+
             // byte + byte -> byte
             // byte + word -> word
             // word + byte -> word