Update breaking change docs

Documentation/coding-guidelines/breaking-changes.md

@@ -1,13 +1,19 @@
-Breaking Changes
-================
+# Breaking Changes
 
 We take compatibility in the .NET Framework and .NET Core extremely seriously.
 
-Although .NET Core can be deployed app local, we are engineering it such that portable libraries can target it and still run on the full desktop framework as well. This means that the behavior of the full .NET Framework constrains the implementation of any overlapping API in .NET Core.
+Although .NET Core can be deployed app local, we are engineering it such that
+portable libraries can target it and still run on the full desktop framework as
+well. This means that the behavior of the full .NET Framework constrains the
+implementation of any overlapping API in .NET Core.
 
-Below is a summary of some documentation we have internally about what kinds of things constitute breaking changes, how we categorize them, and how we decide what we're willing to take.
+Below is a summary of some documentation we have internally about what kinds of
+things constitute breaking changes, how we categorize them, and how we decide
+what we're willing to take.
 
-Note that these rules only apply to API that have shipped in a previous RTM release. New API still under development can be modified but we are still cautious not to disrupt the ecosystem unnecessarily when prerelease API change.
+Note that these rules only apply to API that have shipped in a previous RTM
+release. New API still under development can be modified but we are still
+cautious not to disrupt the ecosystem unnecessarily when prerelease API change.
 
 To help triage breaking changes, we classify them in to four buckets:
 
@@ -16,47 +22,93 @@ To help triage breaking changes, we classify them in to four buckets:
 3. Unlikely Grey Area
 4. Clearly Non-Public
 
-### Bucket 1: Public Contract
-*Clear violation of public contract.*
+## Bucket 1: Public Contract
+*Clear [violation of public contract][breaking-change].*
 
 Examples:
-* throwing a new/different exception type in an existing common scenario
+* Renaming or removing of a public type, member, or parameter
+* Changing the value of a public constant or enum member
+* Sealing a type that wasn't sealed
+* Making a virtual member abstract
+* Adding an interface to the set of base types of an interfaces
+* Removing a type or interface from the the of base types
+* Changing the return type of a member
+* ...or any other [incompatible change][breaking-change] to the shape of an API
+
+[breaking-change]: breaking-change-rules.md#source-and-binary-compatibility-changes
+
+## Bucket 2: Reasonable Grey Area
+*[Change of behavior][behavioral-changes] that customers would have reasonably
+depended on.*
+
+Examples:
+
+* Throwing a new/different exception type in an existing common scenario
 * An exception is no longer thrown
 * A different behavior is observed after the change for an input
-* renaming a public type, member, or parameter
 * decreasing the range of accepted values within a given parameter
 * A new instance field is added to a type (impacts serialization)
-* changing the value of a public constant or enum member
+* Change in timing/order of events (even when not specified in docs)
+* Change in parsing of input and throwing new errors (even if parsing behavior
+  is not specified in the docs)
 
-### Bucket 2: Reasonable Grey Area
-*Change of behavior that customers would have reasonably depended on.*
+These require judgment: how predictable, obvious, consistent was the behavior?
 
-Examples:
-* change in timing/order of events (even when not specified in docs)
-* change in parsing of input and throwing new errors (even if parsing behavior is not specified in the docs)
+[behavioral-changes]: breaking-change-rules.md#behavioral-changes
 
-These require judgment: how predictable, obvious, consistent was the behavior?
+## Bucket 3: Unlikely Grey Area
+*Change of behavior that customers could have depended on, but probably
+wouldn't.*
+
+Examples:
 
-### Bucket 3: Unlikely Grey Area
-*Change of behavior that customers could have depended on, but probably wouldn't.*
+* Correcting behavior in a subtle corner case
 
-**Examples:**
-* correcting behavior in a subtle corner case
+As with type 2 changes, these require judgment: what is reasonable and what’s
+not?
 
-As with type 2 changes, these require judgment: what is reasonable and what’s not?
+## Bucket 4: Clearly Non-Public
+*Changes to surface area or behavior that is clearly internal or non-breaking
+in theory, but breaks an app.*
 
-### Bucket 4: Clearly Non-Public
-*Changes to surface area or behavior that is clearly internal or non-breaking in theory, but breaks an app.*
+Examples:
 
-**Examples:**
 * Changes to internal API that break private reflection
 
-It is impossible to evolve a code base without making such changes, so we don't require up-front approval for these, but we will sometimes have to go back and revisit such change if there's too much pain inflicted on the ecosystem through a popular app or library.
+It is impossible to evolve a code base without making such changes, so we don't
+require up-front approval for these, but we will sometimes have to go back and
+revisit such change if there's too much pain inflicted on the ecosystem through
+a popular app or library.
+
+This bucket is painful for the machine-wide .NET Framework, but we do have much
+more latitude here in .NET Core.
+
+## What This Means for Contributors
+
+* All bucket 1, 2, and 3 breaking changes require talking to the repo owners
+  first:
+    - We generally **don't accept** change proposals that are in bucket #1.
+    - We **might accept** change proposals that are in #2 and #3 after a
+      risk-benefit analysis. See below for more details.
+    - We **usually accept** changes that are in bucket #4
+* If you're not sure in which bucket applies to a given change, contact us as
+  well.
+
+### Risk-Benefit Analysis
+
+For buckets #2 and #3 we apply a risk-benefit analysis. It doesn't matter if the
+old behavior is "wrong", we still need to think through the implications. This
+can result in one of the following outcomes: 
+
+* **Accepted with compat switch**. Depending on the estimated customer impact,
+  we may decide to add a compat switch that allows consumers to bring back the
+  old behavior if neessary.
+
+* **Accepted**. In some minor cases, we may decide to accept the change if the
+  the benefit is large and the risk is super low or if the risk is moderate and
+  quirking isn't viable.
 
-This bucket is painful for the machine-wide .NET Framework, but we do have much more latitude here in .NET Core.
+* **Rejected**. If the risk is too high and/or the improvement too minor, we may
+  decide not to accept the change proposal at all. We can help identify
+  alternatives such as introducing a new API and obsoleting the old one.
 
-### What This Means for Contributors
-* All bucket 1, 2, and 3 breaking changes require talking to the repo owners first.
-* If you're not sure in which bucket applies to a given change, contact us as well.
-* It doesn't matter if the old behavior is "wrong", we still need to think through the implications.
-* If a change is deemed too breaking, we can help identify alternatives such as introducing a new API and obsoleting the old one.