Allow re-assigning to convenience variables

A customer reported somewhat odd gdb behavior, where re-assigning an
array or string to a convenience variable would yield "Too many array
elements".  A test case is:

    (gdb) p $x = "x"
    (gdb) p $x = "xyz"

This patch fixes the problem by making a special case in the evaluator
for assignment to convenience variables, which seems like the correct
behavior.

Note that a previous patch implemented this for Ada, see commit
f411722cb ("Allow re-assigning to convenience variables").

gdb/ChangeLog
2019-11-14  Tom Tromey  <tromey@adacore.com>

	* eval.c (evaluate_subexp_standard) <BINOP_ASSIGN>: Do not pass an
	expected type for the RHS if the LHS is a convenience variable.

gdb/testsuite/ChangeLog
2019-11-14  Tom Tromey  <tromey@adacore.com>

	* gdb.base/gdbvars.exp (test_convenience_variables): Add
	regression tests.

Change-Id: I5e66a2d243931a5c43c7af4bc9f6717464c2477e

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-11-14  Tom Tromey  <tromey@adacore.com>
+
+	* eval.c (evaluate_subexp_standard) <BINOP_ASSIGN>: Do not pass an
+	expected type for the RHS if the LHS is a convenience variable.
+
 2019-11-14  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* unittests/vec-utils-selftests.c (unordered_remove_tests::obj):

gdb/eval.c

@@ -2151,7 +2151,14 @@ evaluate_subexp_standard (struct type *expect_type,
 
     case BINOP_ASSIGN:
       arg1 = evaluate_subexp (NULL_TYPE, exp, pos, noside);
-      arg2 = evaluate_subexp (value_type (arg1), exp, pos, noside);
+      /* Special-case assignments where the left-hand-side is a
+	 convenience variable -- in these, don't bother setting an
+	 expected type.  This avoids a weird case where re-assigning a
+	 string or array to an internal variable could error with "Too
+	 many array elements".  */
+      arg2 = evaluate_subexp (VALUE_LVAL (arg1) == lval_internalvar
+			      ? NULL_TYPE : value_type (arg1),
+			      exp, pos, noside);
 
       if (noside == EVAL_SKIP || noside == EVAL_AVOID_SIDE_EFFECTS)
 	return arg1;

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2019-11-14  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.base/gdbvars.exp (test_convenience_variables): Add
+	regression tests.
+
 2019-11-12  Tom Tromey  <tom@tromey.com>
 
 	* lib/tuiterm.exp (_accept): Add wait_for parameter.  Check output

gdb/testsuite/gdb.base/gdbvars.exp

@@ -51,6 +51,21 @@ proc test_convenience_variables {} {
 
     gdb_test "print \$bar"		" = void" \
 	"Print contents of uninitialized convenience variable"
+
+
+    gdb_test "print \$str = \"some string\"" \
+	" = \"some string\"" \
+	"Set convenience variable to string value"
+    gdb_test "print \$str = \"some other string\"" \
+	" = \"some other string\"" \
+	"Change convenience variable to different string value"
+
+    gdb_test "print \$arr = {1, 2, 3}" \
+	" = \\{1, 2, 3\\}" \
+	"Set convenience variable to array value"
+    gdb_test "print \$arr = {0, 1, 2, 3}" \
+	" = \\{0, 1, 2, 3\\}" \
+	"Set convenience variable to different array value"
 }
 
 proc test_convenience_functions {} {