Why slice length greater than capacity gives runtime error?

Made a slice where capacity is less than the length

package main

import "fmt"

func main() {

	type b []int
	var k = make([]b, 10, 5)
	fmt.Println(k[8])
}

This when tried to run gives following error.

> panic: runtime error: makeslice: cap out of range
>
> runtime.panic+0x9e /go/src/pkg/runtime/proc.c:1060
>
> runtime.panic(0x453b00, 0x30020390) runtime.panicstring+0x94 /go/src/pkg/runtime/runtime.c:116
>
> runtime.panicstring(0x4afd6c, 0x40d80c) runtime.makeslice+0x70 /go/src/pkg/runtime/slice.c:24
>
> runtime.makeslice(0x44302c, 0xa, 0x0, 0x5, 0x0, ...) main.main+0x45 C:/GOEXCE~1/basics/DATATY~1/slice.go:8
>
> main.main() runtime.mainstart+0xf 386/asm.s:93
> runtime.mainstart() runtime.goexit /go/src/pkg/runtime/proc.c:178
>
> runtime.goexit()
>
> ----- goroutine created by -----
>
> _rt0_386+0xbf 386/asm.s:80

My question is can capacity be less than length?<br><br>

If 'Yes' then why this error came? <br>
And if 'No'then why this is a runtime error and why not a compile time?

No, capacity cannot be less than length.

A slice is a reference to a part of an array. A slice's capacity represents the size of that backing array. If its length is greater than its capacity, then what memory is it using?

The following invariant always holds for a slice s (unless you've done something unsafe):

0 <= len(s) <= cap(s)

Your code produces a runtime error rather than a compile-time error because the error cannot always be detected statically. In your case it could be, but consider this code:

package main

import (
    "fmt"
    "rand"
)

func main() {
    k := make([]int, rand.Int(), rand.Int())
    fmt.Println(k)
}

The values passed to make cannot be known until runtime.

Read the Go Programming Language Specification.

> Length and capacity
>
> The capacity of a slice is the number of elements for which there is
> space allocated in the underlying array. At any time the following
> relationship holds:
>
> 0 <= len(s) <= cap(s)

check the runtime/slice.go

func makeslice(et *_type, len, cap int) unsafe.Pointer {
	mem, overflow := math.MulUintptr(et.size, uintptr(cap))
	if overflow || mem > maxAlloc || len < 0 || len > cap {
		// NOTE: Produce a 'len out of range' error instead of a
		// 'cap out of range' error when someone does make([]T, bignumber).
		// 'cap out of range' is true too, but since the cap is only being
		// supplied implicitly, saying len is clearer.
		// See golang.org/issue/4085.
		mem, overflow := math.MulUintptr(et.size, uintptr(len))
		if overflow || mem > maxAlloc || len < 0 {
			panicmakeslicelen()
		}
		panicmakeslicecap()
	}

	return mallocgc(mem, et, true)
}

it will check the len and the cap, and give a panic if len<0 or len>cap.