GeezerVison

Two subtle edge cases with checking each attribute by removing it individually (withoutAttr = removeAttributeFromSelector(selector, attr.name)):

1. Multiple classes in class="...": Because removeAttributeFromSelector strips the entire class attribute and all selector.classNames at once (lines 271–275), if a parent has <ng-content select=".used" /> and a child writes <ng-content class="used unused" />, removing class changes the matched slot so the whole TextAttribute is marked as used—allowing the redundant unused class token through without a diagnostic.
2. Comma-separated / redundant selectors on the same slot: If a parent slot uses a selector list like <ng-content select="[slot], .foo" /> and a child writes <ng-content slot class="foo" />, removing slot alone still matches slot 0 via .foo, and removing class alone still matches slot 0 via [slot]. Because neither attribute changes slot when removed on its own, both attributes will be flagged as unsupported even though at least one is required to match the slot.

splitNsName(attr.name)[1] strips the namespace prefix and adds the unprefixed name to CssSelector. However, at runtime Angular's projection matcher (findAttrIndexInNode in packages/core/src/render3/node_selector_matcher.ts) explicitly skips AttributeMarker.NamespaceURI attributes altogether. If <ng-content> has a namespaced attribute (e.g. xlink:title), createContentCssSelector will match a parent's [title] slot even though it won't match at runtime. Consider skipping attributes where splitNsName(attr.name)[0] !== null.

In ngtsc's DtsMetadataReader (packages/compiler-cli/src/ngtsc/metadata/src/dts.ts), a .d.ts component with no <ng-content> slots (NgContentSelectors = never) has ngContentSelectors: [], whereas ngContentSelectors: null indicates a legacy .d.ts declaration (typeArguments.length <= 6) where projection slots were not recorded in the type definition.

Returning early here when meta.ngContentSelectors === null treats unknown slots the same as zero slots, which could produce false-positive diagnostics if <ng-content someAttr> is projected into a component from an older .d.ts.

Nit: boundEvents, parsedProperties, and references only report the first item ([0]), whereas attributes below (and validateNoAttributeBindings) loop over every item. Iterating over all elements in boundEvents, parsedProperties, and references would report all unsupported bindings on <ng-content> in one pass rather than requiring the user to fix them one at a time.