kernel: fix a crash and weirdness related to 'for' loops

The reader ("parser") did not sufficiently restrict the kind of expression
permitted as index variable in `for` loops. So one could enter e.g. this:

    for x[1] in [1] do od;

The result was a nonsensical error message. Worse, one could specify a
"constant" variable, which caused a segfault; e.g.

    for IsHPCGAP in [1] do od;

src/read.c

@@ -2050,7 +2050,9 @@ static void ReadFor(ScannerState * s, TypSymbolSet follow)
  Match(s, S_FOR, "for", follow);
 
  /* <Var> */
- ReadCallVarAss(s, follow, 'r');
+ volatile LHSRef ref = ReadVar(s, follow);
+ if (ref.type != R_INVALID)
+ EvalRef(ref, 1);
 
  /* 'in' <Expr> */
  Match(s, S_IN, "in", S_DO|S_OD|follow);

tst/testinstall/coder.tst

@@ -300,5 +300,41 @@ gap> function() Unbind(l![fail]); end();
 Error, PosObj Assignment: <position> must be a positive small integer (not the\
  value 'fail')
 
+#
+# weird corner cases in for loop index variables
+#
+gap> function() for + in [1,2,3] do od; end();
+Syntax error: Identifier expected in stream:1
+function() for + in [1,2,3] do od; end();
+ ^
+gap> function() local x; for x[1] in [1,2,3] do od; end();
+Syntax error: in expected in stream:1
+function() local x; for x[1] in [1,2,3] do od; end();
+ ^
+gap> function() local x; for x{[1]} in [1,2,3] do od; end();
+Syntax error: in expected in stream:1
+function() local x; for x{[1]} in [1,2,3] do od; end();
+ ^
+gap> function() for IsHPCGAP in [1,2,3] do od; end();
+Error, Variable: 'IsHPCGAP' is constant
+gap> function() for IsHPCGAP[1] in [1,2,3] do od; end();
+Syntax error: in expected in stream:1
+function() for IsHPCGAP[1] in [1,2,3] do od; end();
+ ^
+gap> function() for IsHPCGAP{[1]} in [1,2,3] do od; end();
+Syntax error: in expected in stream:1
+function() for IsHPCGAP{[1]} in [1,2,3] do od; end();
+ ^
+gap> function() for PrintObj in [1,2,3] do od; end();
+Error, Variable: 'PrintObj' is read only
+gap> function() for PrintObj[1] in [1,2,3] do od; end();
+Syntax error: in expected in stream:1
+function() for PrintObj[1] in [1,2,3] do od; end();
+ ^
+gap> function() for PrintObj{[1]} in [1,2,3] do od; end();
+Syntax error: in expected in stream:1
+function() for PrintObj{[1]} in [1,2,3] do od; end();
+ ^
+
 #
 gap> STOP_TEST("coder.tst", 1);

tst/testinstall/interpreter.tst

@@ -252,6 +252,42 @@ gap> Unbind(l![fail]);
 Error, PosObj Assignment: <position> must be a positive small integer (not the\
  value 'fail')
 
+#
+# weird corner cases in for loop index variables
+#
+gap> for + in [1,2,3] do od;
+Syntax error: Identifier expected in stream:1
+for + in [1,2,3] do od;
+ ^
+gap> for x[1] in [1,2,3] do od;
+Syntax error: in expected in stream:1
+for x[1] in [1,2,3] do od;
+ ^
+gap> for x{[1]} in [1,2,3] do od;
+Syntax error: in expected in stream:1
+for x{[1]} in [1,2,3] do od;
+ ^
+gap> for IsHPCGAP in [1,2,3] do od;
+Error, Variable: 'IsHPCGAP' is constant
+gap> for IsHPCGAP[1] in [1,2,3] do od;
+Syntax error: in expected in stream:1
+for IsHPCGAP[1] in [1,2,3] do od;
+ ^
+gap> for IsHPCGAP{[1]} in [1,2,3] do od;
+Syntax error: in expected in stream:1
+for IsHPCGAP{[1]} in [1,2,3] do od;
+ ^
+gap> for PrintObj in [1,2,3] do od;
+Error, Variable: 'PrintObj' is read only
+gap> for PrintObj[1] in [1,2,3] do od;
+Syntax error: in expected in stream:1
+for PrintObj[1] in [1,2,3] do od;
+ ^
+gap> for PrintObj{[1]} in [1,2,3] do od;
+Syntax error: in expected in stream:1
+for PrintObj{[1]} in [1,2,3] do od;
+ ^
+
 #
 #
 gap> STOP_TEST("interpreter.tst", 1);