ADDITIONAL SYSTEM INFORMATION :
The github issue and Travis report shows different JDK 11 environment where the error occurs, here's one of them corresponding to a recent JDK11 :

openjdk version "11-ea" 2018-09-25
OpenJDK Runtime Environment 18.9 (build 11-ea+13)
OpenJDK 64-Bit Server VM 18.9 (build 11-ea+13, mixed mode)

A DESCRIPTION OF THE PROBLEM :
AssertJ Core open source project (https://github.com/joel-costigliola/assertj-core) fails to compile under JDK 11 when it compiles successfully with JDK 8, JDK 9 and JDK 10.

The issue has been in reported in https://github.com/joel-costigliola/assertj-core/issues/1232.

The compilation error occurs in different environments and JDK 11 EA versions as per the previous issue description and the following Travis CI build report: https://travis-ci.org/joel-costigliola/assertj-core/builds/375047906?utm_source=github_status&utm_medium=notification


REGRESSION : Last worked in version 10.0.1

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
clone assertj core: git clone https://github.com/joel-costigliola/assertj-core.git
compile it including the tests: mvn clean test

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Successful compilation
ACTUAL -
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/api/AutoCloseableBDDSoftAssertionsTest.java:[62,13] reference to then is ambiguous
  both method <T>then(T) in org.assertj.core.api.Java6AbstractBDDSoftAssertions and method then(java.lang.CharSequence) in org.assertj.core.api.Java6AbstractBDDSoftAssertions match
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/perf/SoftAssertionsPerfTest.java:[214,13] reference to assertThat is ambiguous
  both method <T>assertThat(T) in org.assertj.core.api.Java6AbstractStandardSoftAssertions and method assertThat(java.lang.CharSequence) in org.assertj.core.api.Java6AbstractStandardSoftAssertions match
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/api/SoftAssertionsTest.java:[219,13] reference to assertThat is ambiguous
  both method <T>assertThat(T) in org.assertj.core.api.Java6AbstractStandardSoftAssertions and method assertThat(java.lang.CharSequence) in org.assertj.core.api.Java6AbstractStandardSoftAssertions match
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/api/Assertions_assertThat_with_CharSequence_Test.java:[35,61] reference to assertThat is ambiguous
  both method assertThat(java.lang.CharSequence) in org.assertj.core.api.Assertions and method <T>assertThat(T) in org.assertj.core.api.Assertions match
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/api/Assertions_assertThat_with_CharSequence_Test.java:[41,61] reference to assertThat is ambiguous
  both method assertThat(java.lang.CharSequence) in org.assertj.core.api.Assertions and method <T>assertThat(T) in org.assertj.core.api.Assertions match
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/api/BDDSoftAssertionsTest.java:[216,13] reference to then is ambiguous
  both method <T>then(T) in org.assertj.core.api.Java6AbstractBDDSoftAssertions and method then(java.lang.CharSequence) in org.assertj.core.api.Java6AbstractBDDSoftAssertions match
[ERROR] /home/joel/prog/assertj/assertj-core-3.x/src/test/java/org/assertj/core/api/AutoCloseableSoftAssertionsTest.java:[71,13] reference to assertThat is ambiguous
  both method <T>assertThat(T) in org.assertj.core.api.Java6AbstractStandardSoftAssertions and method assertThat(java.lang.CharSequence) in org.assertj.core.api.Java6AbstractStandardSoftAssertions match
[INFO] 7 errors

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git clone https://github.com/joel-costigliola/assertj-core.git
compile it including the tests: mvn clean test

I'm going to try write a smaller test case for this issue, in the meantime, this is the steps to reproduce this issue.
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FREQUENCY : always