inline needed to resolve weird compilation issue

[informarte]

Compiler version

3.1.2-RC1

Problem description

I am having a weird compilation issue that can be resolved by inlining a seemingly unrelated method.

Minimized code

class Foo(val id: Int) {
    inline def ==(that: Foo): Boolean = true
}
case class FooWrapper(foo: Foo)

yields

[error] 4 |    case class FooWrapper(foo: Foo)
[error]   |               ^
[error]   |               method == is declared as `inline`, but was not inlined
[error]   |
[error]   |               Try increasing `-Xmax-inlines` above 32

To make the problem go away, inline the method Bar.f.

Real-world problem

git clone https://github.com/informarte/yuck.git
cd yuck
git checkout spikes/scala-3-inlining
make unit-tests

works fine. However, rolling back the last commit (which inlines the unrelated method), breaks the compilation:

git checkout HEAD~1
make unit-tests

yields

/mill yuck.dev.test.run yuck.test.UnitTestSuite
[43/80] yuck.dev.compile 
[info] compiling 2 Scala sources to /tmp/yuck/out/yuck/dev/compile.dest/classes ...
[info] done compiling
[info] compiling 2 Scala sources to /tmp/yuck/out/yuck/dev/compile.dest/classes ...
[info] done compiling
[67/80] yuck.dev.test.compile 
[info] compiling 2 Scala sources to /tmp/yuck/out/yuck/dev/test/compile.dest/classes ...
[error] -- Error: /tmp/yuck/src/test/yuck/flatzinc/test/util/DotExporter.scala:18:23 ---
[error] 18 |    private case class VariableVertex(x: AnyVariable) extends Vertex
[error]    |                       ^
[error]    |                  method == is declared as `inline`, but was not inlined
[error]    |
[error]    |                  Try increasing `-Xmax-inlines` above 32
[error] -- Error: /tmp/yuck/src/test/yuck/flatzinc/test/util/DotExporter.scala:19:23 ---
[error] 19 |    private case class ConstraintVertex(constraint: Constraint) extends Vertex
[error]    |                       ^
[error]    |                  method == is declared as `inline`, but was not inlined
[error]    |
[error]    |                  Try increasing `-Xmax-inlines` above 32
[error] two errors found
1 targets failed
yuck.dev.test.compile Compilation failed

In fact, both AnyVariable and Constraint come with custom, inline == operators - I added those for efficiency and type safety - and they never caused any trouble with Scala 2.13.

As suggested, I increased max-inlines (up to 40) but it did not help.

Expectation

Assuming that my code is correct, I expect that it should compile without the need to inline an unrelated method.

[informarte]

@nicolasstucki, I extended my report with minimized code.

[nicolasstucki]

PostTyper (SyntheticMembers) creates the equals methods with a call to the inline == but does not set the ctx.compilationUnit.needsInlining.