Repository: KaTeX/KaTeX

Issue:
`\mathop  \int` shows a parse  error while parsed by other latex implementations
**Describe the bug:**
`\mathop  \int` shows a parse  error 

**To Reproduce:**
in https://katex.org/ `\mathop  \int` prints `KaTeX parse error: Got function '\int' with no arguments as argument to '\mathop' at position 10: \mathop \̲i̲n̲t̲ ̲`

**Expected behavior:**
 should render the same as `\mathop  {\int }`
 
**Environment (please complete the following information):**
 - KaTeX Version: '0.16.25'  


Hints from issue comments:
@edemaine: Fixed by #4117 @nightwing: @edemaine should the default be changed to allowedInArgument? The issue is present with almost all other operators e.g.
```latex
${\mathop \bigcup} {\mathop \sum} {\mathop \bigwedge}$
```
and even
```latex
${\mathop \det} {\mathop \sin} $
```@edemaine: @nightwing I agree that it's worth testing various operators, but I found that `\mathop \sin` and `\mathop \max` do not work in LaTeX, and I stopped there.

However, it does look like `\mathop \sum` and `\mathop \bigcup` work in LaTeX. Would you like to do a review of what operators are supported by LaTeX in this way and send a PR to change `allowedInArgument` accordingly?

Changing the default value of `allowedInArgument` could be considered, but not without reviewing every instance of where it isn't specified...

(Out of curiosity, why are you wrapping these symbols in `\mathop`? I've never seen them used as binary operators.)@nightwing: indeed `{\mathop \sin}` doesn't work, i've made a mistake comparing with mathjax behavior. I'll do a more careful review and make a pull request.

The reason i found this issue, was that i have used Katex to render latex generated by MathType and it generates that strange construct in some cases. 

Solution patch:
diff --git a/src/functions/op.js b/src/functions/op.js
index 2c3ac13d..aef6669a 100644
--- a/src/functions/op.js
+++ b/src/functions/op.js
@@ -313,6 +313,7 @@ defineFunction({
     ],
     props: {
         numArgs: 0,
+        allowedInArgument: true,
     },
     handler({parser, funcName}) {
         let fName = funcName;



Test patch:
diff --git a/test/katex-spec.js b/test/katex-spec.js
index e850991c..19957e60 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -1579,6 +1579,8 @@ describe("An op symbol builder", function() {
         expect`\oint\nolimits_i^n`.toBuild();
         expect`\oiint\nolimits_i^n`.toBuild();
         expect`\oiiint\nolimits_i^n`.toBuild();
+        expect`\mathop{\int}`.toBuild();
+        expect`\mathop \int`.toBuild();
     });
 });