better support for `_` identifier

 * disallow variable declaration of `_`
 * prevent `_` from shadowing itself
 * prevent read access of `_`

closes #1204
closes #1320

1 files changed, 58 insertions, 0 deletions

diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index 948d212e58..56b2c51d74 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -23,6 +23,64 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
     );
 
     cases.add(
+        "`_` is not a declarable symbol",
+        \\export fn f1() usize {
+        \\    var _: usize = 2;
+        \\    return _;
+        \\}
+    ,
+        ".tmp_source.zig:2:5: error: `_` is not a declarable symbol",
+        ".tmp_source.zig:3:12: error: use of undeclared identifier '_'",
+    );
+
+    cases.add(
+        "`_` should not be usable inside for",
+        \\export fn returns() void {
+        \\    for ([]void{}) |_, i| {
+        \\        for ([]void{}) |_, j| {
+        \\            return _;
+        \\        }
+        \\    }
+        \\}
+    ,
+        ".tmp_source.zig:4:20: error: use of undeclared identifier '_'",
+    );
+
+    cases.add(
+        "`_` should not be usable inside while",
+        \\export fn returns() void {
+        \\    while (optionalReturn()) |_| {
+        \\        while (optionalReturn()) |_| {
+        \\            return _;
+        \\        }
+        \\    }
+        \\}
+        \\fn optionalReturn() ?u32 {
+        \\    return 1;
+        \\}
+    ,
+        ".tmp_source.zig:4:20: error: use of undeclared identifier '_'",
+    );
+
+    cases.add(
+        "`_` should not be usable inside while else",
+        \\export fn returns() void {
+        \\    while (optionalReturnError()) |_| {
+        \\        while (optionalReturnError()) |_| {
+        \\            return;
+        \\        } else |_| {
+        \\            if (_ == error.optionalReturnError) return;
+        \\        }
+        \\    }
+        \\}
+        \\fn optionalReturnError() !?u32 {
+        \\    return error.optionalReturnError;
+        \\}
+    ,
+        ".tmp_source.zig:6:17: error: use of undeclared identifier '_'",
+    );
+
+    cases.add(
         "while loop body expression ignored",
         \\fn returns() usize {
         \\    return 2;