std: More type checks for Thread startFn return type

Closes #4756

Author: LemonBoy

lib/std/thread.zig

@@ -6,6 +6,8 @@ const windows = std.os.windows;
 const c = std.c;
 const assert = std.debug.assert;
 
+const bad_startfn_ret = "expected return type of startFn to be 'u8', 'noreturn', 'void', or '!void'";
+
 pub const Thread = struct {
     data: Data,
 
@@ -158,15 +160,34 @@ pub const Thread = struct {
                 };
                 fn threadMain(raw_arg: windows.LPVOID) callconv(.C) windows.DWORD {
                     const arg = if (@sizeOf(Context) == 0) {} else @ptrCast(*Context, @alignCast(@alignOf(Context), raw_arg)).*;
+
                     switch (@typeInfo(@TypeOf(startFn).ReturnType)) {
-                        .Int => {
-                            return startFn(arg);
+                        .NoReturn => {
+                            startFn(arg);
                         },
                         .Void => {
                             startFn(arg);
                             return 0;
                         },
-                        else => @compileError("expected return type of startFn to be 'u8', 'noreturn', 'void', or '!void'"),
+                        .Int => |info| {
+                            if (info.bits != 8) {
+                                @compileError(bad_startfn_ret);
+                            }
+                            return startFn(arg);
+                        },
+                        .ErrorUnion => |info| {
+                            if (info.payload != void) {
+                                @compileError(bad_startfn_ret);
+                            }
+                            startFn(arg) catch |err| {
+                                std.debug.warn("error: {}\n", .{@errorName(err)});
+                                if (@errorReturnTrace()) |trace| {
+                                    std.debug.dumpStackTrace(trace.*);
+                                }
+                            };
+                            return 0;
+                        },
+                        else => @compileError(bad_startfn_ret),
                     }
                 }
             };
@@ -202,14 +223,32 @@ pub const Thread = struct {
                 const arg = if (@sizeOf(Context) == 0) {} else @intToPtr(*const Context, ctx_addr).*;
 
                 switch (@typeInfo(@TypeOf(startFn).ReturnType)) {
-                    .Int => {
-                        return startFn(arg);
+                    .NoReturn => {
+                        startFn(arg);
                     },
                     .Void => {
                         startFn(arg);
                         return 0;
                     },
-                    else => @compileError("expected return type of startFn to be 'u8', 'noreturn', 'void', or '!void'"),
+                    .Int => |info| {
+                        if (info.bits != 8) {
+                            @compileError(bad_startfn_ret);
+                        }
+                        return startFn(arg);
+                    },
+                    .ErrorUnion => |info| {
+                        if (info.payload != void) {
+                            @compileError(bad_startfn_ret);
+                        }
+                        startFn(arg) catch |err| {
+                            std.debug.warn("error: {}\n", .{@errorName(err)});
+                            if (@errorReturnTrace()) |trace| {
+                                std.debug.dumpStackTrace(trace.*);
+                            }
+                        };
+                        return 0;
+                    },
+                    else => @compileError(bad_startfn_ret),
                 }
             }
             fn posixThreadMain(ctx: ?*c_void) callconv(.C) ?*c_void {