For attribute bindings that target a directive's input, the template
type checker is able to verify that the type of the input expression is
compatible with the directive's declaration for said input. This
checking adheres to the `strictNullChecks` flag as configured in the
TypeScript compilation, such that errors are reported for expressions
that include `undefined` or `null` in their type if the input's
declaration does not include those types.
There was a bug with this level of type-checking for directives that
also declare coercion members, where binding an expression that includes
the `undefined` type to a directive's input that does not include the
`undefined` type would not be reported as error.
This commit fixes the bug by changing the type-constructor in type-check
code to use an intersection type of regular inputs and coerced inputs,
instead of a union type. The union type would inadvertently allow
`undefined` types to be assigned into the regular inputs, as that would
still satisfy the characteristics of a union type.
As a result of this change, you may start to see build failures if
`strictTemplates` is enabled and `strictInputTypes` is not disabled.
These errors are legitimate and some action is required to achieve a
successful build:
1. Update the templates for which an error is reported and introduce the
non-null assertion operator at the end of the expression. This
removes the `undefined` type from the expression's type, making it
appear as a valid assignment.
2. Disable `strictNullInputTypes` in the compiler options. This will
implicitly add the non-null assertion operators similar to option 1,
but all templates in the compilation are affected.
3. Update the directive's input declaration to include the `undefined`
type, if the directive is not implemented in an external library.
PR Close#38273
7525f3afc1