Go: fix DefinedType.getBaseType

[owen-mc]

Consider this situation:

type T1 = T2

type T2 struct {}

Previously, DefinedType.getBaseType just gave the underlying type, so calling it on T1 would give struct {}. This PR fixes it so that DefinedType.getBaseType gives the right hand side of the type declaration, and hence calling it on T1 gives T2.

Note that this requires us to have extracted the declaration of the type, so it is only available for types declared in the project being analyzed. It is not defined for types defined in external packages. Since this information is not stored in the information that we already extract for external packages, I cannot see any way to improve this situation except for expanding what we look at from external packages.

I have also fixed a test which was the only use of DefinedType.getBaseType outside of the definition of DefinedType, and added a proper test for DefinedType.getBaseType.

[Copilot]

Pull Request Overview

This PR changes DefinedType.getBaseType to return the alias’s right‐hand side (the declared type) rather than its underlying type, updates related methods to maintain correct semantics, and refreshes library tests accordingly.

• Reimplement getBaseType to read from the declaration AST so aliases return their RHS.
• Update getMethod and getUnderlyingType to use the new separation between base and underlying types.
• Revise existing method‐resolution tests and add new tests for getBaseType.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
go/ql/test/library-tests/semmle/go/Types/MethodTypes.ql	Use Type.hasMethod and add a declaration existence check
go/ql/test/library-tests/semmle/go/Types/MethodTypes.expected	Adjust expected methods list after alias semantics change
go/ql/test/library-tests/semmle/go/Types/DefinedType_getBaseType.ql	New test querying DefinedType.getBaseType()
go/ql/test/library-tests/semmle/go/Types/DefinedType_getBaseType.expected	Expected output for the new getBaseType test
go/ql/lib/semmle/go/Types.qll	Change getBaseType, getUnderlyingType, and related logic

Comments suppressed due to low confidence (1)

go/ql/test/library-tests/semmle/go/Types/DefinedType_getBaseType.ql:4

• Consider adding a test case to verify that getBaseType returns no result for types declared in external packages, ensuring the new behavior is validated against external definitions.

where tp = dt.getBaseType()

[Copilot]

Fix the plural form here: change "type declared in external packages" to "types declared in external packages" for grammatical consistency.

Suggested change

	* analyzed. It will not be defined for type declared in external packages.
	* analyzed. It will not be defined for types declared in external packages.