fix compiler crash when using a gosub/subroutinecall in a branch statement

codeGeneration/src/prog8/compiler/target/cpu6502/codegen/AsmGen.kt

@@ -1340,9 +1340,7 @@ $repeatLabel    lda  $counterVar
             throw AssemblyError("only else part contains code, shoud have been switched already")
 
         val jump = stmt.truepart.statements.first() as? Jump
-        if(jump!=null) {
-            if(jump.isGosub)
-                throw FatalAstException("didn't expect GoSub here")
+        if(jump!=null && !jump.isGosub) {
             // branch with only a jump (goto)
             val instruction = branchInstruction(stmt.condition, false)
             out("  $instruction  ${getJumpTarget(jump)}")

codeOptimizers/src/prog8/optimizer/StatementOptimizer.kt

@@ -279,7 +279,19 @@ class StatementOptimizer(private val program: Program,
     }
 
     override fun after(jump: Jump, parent: Node): Iterable<IAstModification> {
-        if(!jump.isGosub) {
+        if(jump.isGosub) {
+            // if the next statement is return with no returnvalue, change into a regular jump if there are no parameters as well.
+            val subroutineParams = jump.identifier?.targetSubroutine(program)?.parameters
+            if(subroutineParams!=null && subroutineParams.isEmpty()) {
+                val returnstmt = jump.nextSibling() as? Return
+                if(returnstmt!=null && returnstmt.value==null) {
+                    return listOf(
+                        IAstModification.Remove(returnstmt, parent as IStatementContainer),
+                        IAstModification.ReplaceNode(jump, Jump(jump.address, jump.identifier, jump.generatedLabel, jump.position), parent)
+                    )
+                }
+            }
+        } else {
             // if the jump is to the next statement, remove the jump
             val scope = jump.parent as IStatementContainer
             val label = jump.identifier?.targetStatement(program)

examples/test.p8

@@ -3,9 +3,18 @@
 
 main {
     sub start() {
-
-        uword foobar
-
-        foobar()
+        ubyte @shared yy
+        yy = foobar()
+    }
+    sub foobar() -> ubyte {
+        if_mi {
+            foobar2()
+            foobar2()
+            return true
+        }
+        return 22
+    }
+    sub foobar2() {
+        main.start.yy++
     }
 }