Mixins cause: TypeError: Cannot read property 'forEach' of undefined

[zslayton]

I have a Typescript codebase that employs mixins.

• Example mixin: DomValue
• Example subclass: dom.Integer

Running typedoc fails with the following stack trace:

Using TypeScript 3.7.5 from node_modules/typescript/lib
node_modules/typedoc/dist/lib/converter/nodes/class.js:65
                            && typeToInheritFrom.symbol.declarations.forEach((declaration) => {
                                                                     ^

TypeError: Cannot read property 'forEach' of undefined
    at typesToInheritFrom.forEach (node_modules/typedoc/dist/lib/converter/nodes/class.js:65:70)
    at Array.forEach (<anonymous>)
    at context.withScope (node_modules/typedoc/dist/lib/converter/nodes/class.js:62:40)
    at Context.withScope (node_modules/typedoc/dist/lib/converter/context.js:101:9)
    at ClassConverter.convert (node_modules/typedoc/dist/lib/converter/nodes/class.js:33:17)
    at Converter.convertNode (node_modules/typedoc/dist/lib/converter/converter.js:116:53)
    at Context.inherit (node_modules/typedoc/dist/lib/converter/context.js:140:24)
    at typeToInheritFrom.symbol.typeToInheritFrom.symbol.declarations.forEach (node_modules/typedoc/dist/lib/converter/nodes/class.js:66:37)
    at Array.forEach (<anonymous>)
    at typesToInheritFrom.forEach (node_modules/typedoc/dist/lib/converter/nodes/class.js:65:70)

The issue is caused by this code, reproduced here with slightly different formatting:

typesToInheritFrom.forEach((typeToInheritFrom) => {
    typeToInheritFrom.symbol
    && typeToInheritFrom.symbol.declarations.forEach((declaration) => {
       context.inherit(declaration, baseType.typeArguments);
    });
});

If I change it to:

typesToInheritFrom.forEach((typeToInheritFrom) => {
    typeToInheritFrom.symbol
    && typeToInheritFrom.symbol.declarations // <--- Make sure this is defined
    && typeToInheritFrom.symbol.declarations.forEach((declaration) => {
       context.inherit(declaration, baseType.typeArguments);
    });
});

then documentation is generated normally. I have not studied the code long enough to understand its intent, so I'm unsure if this change is the best approach.

This failure did not occur in my project until I started using mixins. Additionally, the code above was added in a PR to introduce mixin support.

• I have checked issues with bug label and found no duplicates

Expected Behavior

Typedoc should finish generating documentation successfully.

Actual Behavior

Typedoc throws an Error, preventing documentation from being generated and causing CI builds to fail.

Steps to reproduce the bug

• Example branch with failing code.
• Gruntfile.js typedoc config:

typedoc: {
  build: {
    src: 'src/**/*'
  }
}

Environment

Laptop

• Typedoc version: 0.16.9
• Node.js version: 11.13.0
• OS: mac OS mojave

Here's a link to the failing Travis CI Build, which runs on Node v8, v10, and v12 on Linux.

[Gerrit0]

This is really weird, my first instinct was that typeToInheritFrom.symbol was an internal definition that we used and thus couldn't rely on TS's declaration file for accurate types, but this doesn't seem to be the case.

I really don't want to add checks for undefined when the TS declaration file claims it will never be undefined, but maybe the TS types are just wrong here? Looking more deeply into it.

[Gerrit0]

I'm beginning to think this is a TS bug... I've tried to reduce it to a smaller example but haven't been able to. For now I'll just merge the PR with a note that this is really weird and needs further investigation.