RFR: 7039014: Confusing error message for method conflict

[Archie L. Cobbs]

This fixes a confusing error message.

Currently, this input:

interface A<T> {
    byte m(String x);
    char m(T x);
}

interface B extends A<String> {
}

produces this error:

A.java:6: error: types A<String> and A<String> are incompatible;
interface B extends A<String> {
^
  both define m(String), but with unrelated return types
1 error

This patch detects when the two types are actually the same type and gives this error message instead:

A.java:6: error: type A<String> defines m(String) more than once with unrelated return types
interface B extends A<String> {
^
1 error

-------------

Commit messages:
 - 7039014: Confusing error message for method conflict

Changes: https://git.openjdk.org/jdk/pull/10752/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10752&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7039014
  Stats: 44 lines in 4 files changed: 41 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10752.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10752/head:pull/10752

PR: https://git.openjdk.org/jdk/pull/10752