[Issue 20563] New: module conflicts with package confusing error message

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20563

          Issue ID: 20563
           Summary: module conflicts with package confusing error message
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: diagnostic
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: schveiguy at yahoo.com

If I declare two packages with the same name accidentally, then the compiler
tells me there's an error, but doesn't tell me where the conflict is. In some
cases, this might be obvious, but in others it's not

For example:

----- foo/package.d

module foo;

----- bar/package.d

module foo;

-----

dmd foo/package.d bar/package.d
bar/package.d(1): Error: module foo from file bar/package.d conflicts with
package name foo

This error is OK, since I realize that bar/package.d shouldn't be foo, and I
can correct it. However, if I change the order of compilation:

dmd bar/package.d foo/package.d
foo/package.d(1): Error: module foo from file foo/package.d conflicts with
package name foo

This error is super-confusing, because of COURSE I want foo/package.d to be
package module foo, how does it conflict with the package foo?

The error should specify where the previous definition of module-package foo
is.

I looked at the code for this, the relevant lines are here:

https://github.com/dlang/dmd/blob/ee19c0b5c0465dfe1581111bb20042988a32dfdb/src/dmd/dmodule.d#L1081-L1094

Looking at that code, it is telling me there's an error because the package was
already resolved to be a package or a module. If I look through the code, I
can't find any place where it's set to a package_, (there is a function
resolvePKGUnknown, but it's never called, I'm assuming it may have been called
at some point in the past) so I assume that this can only happen with two
package modules that are competing for the right one.

Note that if I just declare a foo.d with module foo, then I get a similar
error:

foo.d(1): Error: module foo from file foo.d conflicts with package name foo

Again, if it said something like:

bar/package.d(1): previously declared here

then I can instantly see the problem.

We might also want to change the message to clarify it's not conflicting with a
package, but a declaration of a package module (which must be true since it's
never declared a package_ type).

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