So I thought it would be cool to create a mod for the very first version of Forge, which is for Beta 1.7.3. (you can find it here) So I set up MCP, got everything, set up the Eclipse workspace. But when I playtest the game and try to create a new world, I get this error.
The method populateBiome(fd, kd, int, int) in the type MinecraftForge is not applicable for the arguments (World, BiomeGenBase, int, int)
Now I'm not sure what the issue is here. For all I know, it's using the incorrect arguments.
Here's an example of it being called in ChunkProviderGenerate.java
I have no experience with Forge but the wording of the error suggests that Forge itself is not deobfuscated; it expects "fd" and "kd" instead of the arguments given (which I assume are deobfuscated since it lists "World" and "BiomeGenBase"). I assume that MCP does not decompile/recompile the Forge libraries since such an error otherwise seems impossible at runtime (javac should complain that the parameters of the wrong type; it should also verify references to libraries outside of the Minecraft code).
Also, I found a Reddit post which may help; they say they got it to work by decompling a Minecraft jar with Forge installed, although I don't know if renaming all the imports/packages is right (this could result in mods that only work within MCP since they have the incorrect imports for a normal installation. Also, if this does work you should run "updatemd5" after fixing errors to make sure only your own mod's classes are reobfuscated):