fix compiler crash when trying to concatenate string var and string literal.

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

@@ -862,6 +862,14 @@ internal class AstChecker(private val program: Program,
             errors.err("right operand is not numeric or str", expr.right.position)
         if(leftDt!=rightDt)
             errors.err("left and right operands aren't the same type", expr.left.position)
+
+        if(expr.operator !in comparisonOperators) {
+            if (leftDt == DataType.STR && rightDt == DataType.STR || leftDt in ArrayDatatypes && rightDt in ArrayDatatypes) {
+                // str+str  and  str*number have already been const evaluated before we get here.
+                errors.err("no computational expressions with strings or arrays are possible", expr.position)
+            }
+        }
+
     }
 
     override fun visit(typecast: TypecastExpression) {

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

@@ -9,7 +9,7 @@ import prog8.ast.walk.IAstVisitor
 import java.util.*
 import kotlin.math.round
 
-
+// TODO capitalize for consistency
 val associativeOperators = setOf("+", "*", "&", "|", "^", "or", "and", "xor", "==", "!=")
 val comparisonOperators = setOf("==", "!=", "<", ">", "<=", ">=")
 val augmentAssignmentOperators = setOf("+", "-", "/", "*", "**", "&", "|", "^", "<<", ">>", "%", "and", "or", "xor")

docs/source/programming.rst

@@ -313,10 +313,12 @@ platform (if defined). For the C-64, that is SCREEN CODES (also known as POKE co
 This @-prefix can also be used for character byte values.
 
 
-You can concatenate two string literals using '+' (not very useful though) or repeat
+You can concatenate two string literals using '+', which can be useful to
-a string literal a given number of times using '*'. You can also assign a new string
+split long strings over separate lines. But remember that the length
-value to another string. No bounds check is done so be sure the destination string is
+of the total string still cannot exceed 255 characaters.
-large enough to contain the new value (it is overwritten in memory)::
+A string literal can also be repeated a given number of times using '*'.
+And a new string value can be assigned to another string, but no bounds check is done
+so be sure the destination string is large enough to contain the new value (it is overwritten in memory)::
 
     str string1 = "first part" + "second part"
     str string2 = "hello!" * 10
@@ -330,9 +332,6 @@ as newlines, quote characters themselves, and so on. The ones used most often ar
 ``\\``, ``\"``, ``\n``, ``\r``.  For a detailed description of all of them and what they mean,
 read the syntax reference on strings.
 
-You can use the automatic string concatenation using ``+`` to split long strings over separate
-lines, but remember that the length of the total string still cannot exceed 255 characaters.
-
 .. hint::
     Strings/arrays and uwords (=memory address) can often be interchanged.
     An array of strings is actually an array of uwords where every element is the memory

docs/source/todo.rst

@@ -3,8 +3,6 @@ TODO
 
 For next compiler release (7.6)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-fix compiler crash when trying to concatenate string var and string literal.
-
 fix return value of diskio.load() (see commment) and possibly other routines?
 
 optimization in call convention:

examples/test.p8

@@ -3,10 +3,10 @@
 
 main {
     sub start() {
-        ubyte @shared yy
+        str derp = "derp" * 4
-
+        derp = derp / "zzz"
-        if yy&64 {
+        derp = derp - "zzz"
-            yy++
+        derp = derp + "zzz"
-        }
+        txt.print(&derp+2)
     }
 }