ADDITIONAL SYSTEM INFORMATION :
OS: CentOS Linux release 7.2.1511 (Core)
# uname -a
Linux SZE-L0163338 3.10.0-862.9.1.el7.x86_64 #1 SMP Mon Jul 16 16:29:36 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Java: have tried 12.0.1, and 14 early access:
# ./java -version
java version "12.0.1" 2019-04-16
Java(TM) SE Runtime Environment (build 12.0.1+12)
Java HotSpot(TM) 64-Bit Server VM (build 12.0.1+12, mixed mode, sharing)

# java -version
openjdk version "14-ea" 2020-03-17
OpenJDK Runtime Environment (build 14-ea+2-29)
OpenJDK 64-Bit Server VM (build 14-ea+2-29, mixed mode, sharing)


A DESCRIPTION OF THE PROBLEM :
The following compiler error is expected on line 9. In fact, compilation is succeed after line 14-16 has been commented, and then it failed in execution because of ClassCastException as expected.

Expected compiler error examples:
CallGGNSGM.java:14: error: incompatible types: Map<String,String> cannot be converted to Map<String,Object>
        GGSGM.call(map);
                   ^
CallGGNSGM.java:15: error: incompatible types: Map<String,String> cannot be converted to Map<String,Object>
        (new NSGM()).call(map);
                          ^
CallGGNSGM.java:16: error: incompatible types: Map<String,String> cannot be converted to Map<String,Object>
        SGM.call(map);
                 ^

Main class (CallGGNSGM) implementation:

import java.util.HashMap;
import java.util.Map;

public class CallGGNSGM {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap();
        map.put("k", "v");
        System.out.println("CallGGSGM try gg_instantiated_gm");
        (new GGNSGM()).call(map); //Compiler error(as lines 14-16) expected here
        System.out.println(map);
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
        }
// GGSGM.call(map);
// (new NSGM()).call(map);
// SGM.call(map);
    }
}

GGNSGM:
import java.util.Map;

public class GGNSGM<T> {
    public void call(Map<String, Object> map) {
        System.out.println("call gg_instantiated_gm_map<String, Object> " + map);
        map.put("k2", new Integer(1));
    }
}

GGSGN:
import java.util.Map;

public class GGSGM<T> {
    public static void call(Map<String, Object> map) {
        System.out.println("call gg_static_gm_map<String, Object> " + map);
    }
}

NSGM:
import java.util.Map;

public class NSGM {
    public void call(Map<String, Object> map) {
        System.out.println("call instantiated_gm_map<String, Object> " + map);
    }
}

SGM:
import java.util.Map;

public class SGM {
    public static void call(Map<String, Object> map) {
        System.out.println("call static_gm_map<String, Object> " + map);
    }
}


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Try compile the attached source files: CallGGNSGM.java, GGNSGM.java, GGSGM.java, NSGM.java, SGM.java, and then launch CallGGNSGM.class

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Compiler error expected while compiling.
ACTUAL -
Compilation succeeded, and then failed in execution because of ClassCastException.

---------- BEGIN SOURCE ----------
Main class (CallGGNSGM) implementation:

import java.util.HashMap;
import java.util.Map;

public class CallGGNSGM {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap();
        map.put("k", "v");
        System.out.println("CallGGSGM try gg_instantiated_gm");
        (new GGNSGM()).call(map); //Compiler error(as lines 14-16) expected here
        System.out.println(map);
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
        }
// GGSGM.call(map);
// (new NSGM()).call(map);
// SGM.call(map);
    }
}

GGNSGM:
import java.util.Map;

public class GGNSGM<T> {
    public void call(Map<String, Object> map) {
        System.out.println("call gg_instantiated_gm_map<String, Object> " + map);
        map.put("k2", new Integer(1));
    }
}

GGSGN:
import java.util.Map;

public class GGSGM<T> {
    public static void call(Map<String, Object> map) {
        System.out.println("call gg_static_gm_map<String, Object> " + map);
    }
}

NSGM:
import java.util.Map;

public class NSGM {
    public void call(Map<String, Object> map) {
        System.out.println("call instantiated_gm_map<String, Object> " + map);
    }
}

SGM:
import java.util.Map;

public class SGM {
    public static void call(Map<String, Object> map) {
        System.out.println("call static_gm_map<String, Object> " + map);
    }
}
---------- END SOURCE ----------

FREQUENCY : always