During work on JDK-8055075 for creation of golden files for negative tests, it was suggested to update error diagnostics to have an additional sub-diagnostic with more information. The suggestion came based on the golden file seen for test: langtools/test/tools/javac/InconsistentInheritedSignature.java. Golden file for this test will need updates once this issue is fixed.

Current diagnostic key:
langtools/test/tools/javac/InconsistentInheritedSignature.java:40: error: types I2 and I1 are incompatible

Quoting Maurizio's initial analysis from email:

There are 3 diagnostics of that kind:
# 0: type, 1: type, 2: string
compiler.err.types.incompatible.diff.ret=\
    types {0} and {1} are incompatible; both define {2}, but with unrelated return types

# 0: kind, 1: type, 2: name, 3: list of type, 4: symbol, 5: symbol
compiler.err.types.incompatible.unrelated.defaults=\
    {0} {1} inherits unrelated defaults for {2}({3}) from types {4} and {5}

# 0: kind, 1: type, 2: name, 3: list of type, 4: symbol, 5: symbol
compiler.err.types.incompatible.abstract.default=\
    {0} {1} inherits abstract and default for {2}({3}) from types {4} and {5}


They are inconsistent - as they should all say: 'types ... are incompatible ; reason'. All 3 diagnostics need to made more uniform - perhaps by structuring them using a common header:

# 0: type, 1: type, 2: fragment
compiler.err.types.incompatible=\
    types {0} and {1} are incompatible;
    {2}

And then multiple subdiagnostics - once for each 'reason' (passed as argument #2):

# 0: string
compiler.misc.diff.ret=
    both define {2}, but with unrelated return types

# 0: kind, 1: type, 2: name, 3: list of type, 4: symbol, 5: symbol
compiler.misc.incompatible.unrelated.defaults=
    {0} {1} inherits unrelated defaults for {2}({3}) from types {4} and {5}

# 0: kind, 1: type, 2: name, 3: list of type, 4: symbol, 5: symbol
compiler.misc.incompatible.abstract.default=\
    {0} {1} inherits abstract and default for {2}({3}) from types {4} and {5}