ADDITIONAL SYSTEM INFORMATION :
Microsoft Windows [Version 10.0.17134.523]

openjdk version "13-ea" 2019-09-17
OpenJDK Runtime Environment (build 13-ea+6)
OpenJDK 64-Bit Server VM (build 13-ea+6, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
The Manifest Class-Path entry is specified as a relative URL.
https://docs.oracle.com/en/java/javase/11/docs/specs/jar/jar.html#class-path-attribute

But the JVM ClassLoader accepts all kinds of URLs:
- my%20dir/mylib.jar
- /C:/my%20dir/mylib.jar
- file:/C:/my%20dir/mylib.jar
- file:///C:/my%20dir/mylib.jar

The Java Compiler is treating the entries as Paths. And the behavior has changed since Java 9.

Behavior of javac 8:
- my%20dir/mylib.jar = ignored
- /C:/my%20dir/mylib.jar = ignored
- file:/C:/my%20dir/mylib.jar = ignored

Behavior of javac 9-13:
- my%20dir/mylib.jar = ignored
- /C:/my%20dir/mylib.jar = Illegal char <:> at index 2
- file:/C:/my%20dir/mylib.jar = Illegal char <:> at index 4

I expect that:
1. The Java Compiler treats the entries as URLs, according to spec.
2. The behavior of the JVM ClassLoader and the Java Compiler is equal
3. The behavior is backwards compatible
So the Java Compiler must accept all the URLs that the JVM ClassLoader also accepts nowadays.

Should be fixed in Java 11 and master.

REGRESSION : Last worked in version 8u192

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
This windows batch file demonstrates the issue.
Change the Class-Path to any other URI to see the difference. (Double % is to escape)

=====================repro.bat=====================
echo public class Hello {> Hello.java
echo 	public static String hello() {>> Hello.java
echo 		return "hello";>> Hello.java
echo 	}>> Hello.java
echo }>> Hello.java

echo public class Main {> Main.java
echo 	public static void main(String[] args) {>> Main.java
echo 		System.out.println(Hello.hello());>> Main.java
echo 	}>> Main.java
echo }>> Main.java

javac Hello.java
mkdir "Program Libs"
jar -cf "Program Libs/hello.jar" Hello.class
del Hello.class

echo Class-Path: Program%%20Libs/hello.jar > manifest.mf
jar -cfm hellocp.jar manifest.mf
del manifest.mf

javac -cp "hellocp.jar" Main.java
rem This failed. Now I add the hello.jar directly to the classpath to be able to compile
pause
javac -cp "hellocp.jar;Program Libs/hello.jar" Main.java

java -cp hellocp.jar;. Main

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
C:\javacbug>java -cp hellocp.jar;. Main
hello
ACTUAL -
C:\javacbug>javac -cp "hellocp.jar" Main.java
Main.java:3: error: cannot find symbol
                System.out.println(Hello.hello());
                                   ^
  symbol:   variable Hello
  location: class Main
1 error

C:\javacbug>javac -cp "hellocp.jar" Main.java
error: illegal argument for --class-path: Illegal char <:> at index 2: /C:/javacbug/Program%20Libs/hello.jar

C:\javacbug>javac -cp "hellocp.jar" Main.java
error: illegal argument for --class-path: Illegal char <:> at index 4: file:/C:/javacbug/Program%20Libs/hello.jar

FREQUENCY : always