if_block0.p is not a function

[Hoss3770]

Describe the bug
This Bug appears in Version 3.29.6
I assume that this is the breaking change: #5591

I have this code:

{#if fetching}
      <li transition:slide|local>
        <svg class=spinner viewBox="0 0 50 50">
          <circle cx=25 cy=25 r=20 fill=none stroke-width=5/>
        </svg>
      </li>
{:else if query && !data.length && !error}
      <div class=no-results>لا نتائج</div>
{/if}
{#if error}
      <div class=no-results>حدثت مشكلة في البحث، حاول لاحقًا</div>
{/if}

And removing <li transition:slide|local> fixes it.

Logs
When running the code and the if statement is triggered I get this error in chrome:

Here is the error after disabling source maps:

if (~current_block_type_index) {
     if_block0 = if_blocks[current_block_type_index];
     if (!if_block0) {
          if_block0 = if_blocks[current_block_type_index] = if_block_creators[current_block_type_index](ctx);
    	  if_block0.c();
      } else {
          if_block0.p(ctx, dirty);
     }
    transition_in(if_block0, 1);
    if_block0.m(ul, t5);
} else {
    if_block0 = null;
}

on the line if_block0.p(ctx, dirty); the error is marked

Information about your Svelte project:

• I was able to reproduce the error on Chrome, Safari and Firefox

Severity
I just used another version for now

[Conduitry]

Please provide a runnable reproduction, preferably in the REPL, along with the steps to reproduce it.

[awmcclain]

#5808 has a reproduction in a REPL for this same error.

[christiankaindl]

The problem seems to be the {:else if ...} block. Here's a repl which also experiences this bug:
https://svelte.dev/repl/044e92510a0f48b399f03bbdec25ef63?version=3.31.0

When quickly focusing<->unfocusing it breaks with the same error as is the reporters case.

Screen.Recording.2020-12-27.at.15.39.54.mov

---

Here is a "fix" (more a workaround) which instead of {:else if ...} uses two separate {#if} blocks and it works very well (doesn't break):
https://svelte.dev/repl/5ebd4f257f0a43be907153fce270743c?version=3.31.0

Screen.Recording.2020-12-27.at.15.38.50.mov

[Conduitry]

This is fixed in 3.31.2 - https://svelte.dev/repl/044e92510a0f48b399f03bbdec25ef63?version=3.31.2

[braebo]

I just encountered this in version 3.49.0 with this monstrosity:

{#if gui.fftBufferArray}
  <pre>
    {#each audio ?? [] as a, i}
      {#if i > x}
        <p style="color: hsl(0, 50%, 50%)">{a}</p>
      {:else if i > y}
        <p style="color: hsl(100, 50%, 50%)">{a}</p>
      {:else if i > z}
        <p style="color: hsl(200, 50%, 50%)">{a}</p>
      {:else}
        <p style="color: hsl(0, 0%, 50%)">{a}</p>
      {/if}
    {/each}
    </pre>
{/if}

Going to use a {@const} block instead, but thought I would make note of this bug considering I haven't encountered it before today.

[Garito]

This happen on 3.49.0 too

[x4fingers]

It still happens at 3.55.1.