internal/shader: bug fix: crash with an invalid operand

Closes #2989

internal/shader/expr.go

@@ -88,6 +88,10 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 			return nil, nil, nil, false
 		}
 		stmts = append(stmts, ss...)
+		if len(ts) == 0 {
+			cs.addError(e.Pos(), fmt.Sprintf("unexpected binary operator: %s", e.Y))
+			return nil, nil, nil, false
+		}
 		rhst := ts[0]
 
 		op := e.Op

internal/shader/syntax_test.go

@@ -4277,7 +4277,7 @@ func Bar() (int, int) {
 	}
 }
 
-// Issue #2926
+// Issue #2926, #2989
 func TestSyntaxNonTypeExpression(t *testing.T) {
 	if _, err := compileToIR([]byte(`package main
 
@@ -4306,6 +4306,17 @@ func Bar() float {
 func Foo() {
 }
 
+func Bar() float {
+	return 1.0 + Foo
+}
+`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+	if _, err := compileToIR([]byte(`package main
+
+func Foo() {
+}
+
 func Bar() float {
 	return Foo.x
 }