cmd/go: OOB panic in cmd/go/internal/list.runList

[gopherbot]

#!watchflakes
default <- pkg ~ `gopls` && output ~ `index out of range [0] with length 0` && output ~ `cmd/go/internal/list.runList`

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestMatchingPortsStdlib
    port_test.go:43: err: exit status 2: stderr: panic: runtime error: index out of range [0] with length 0
        
        goroutine 1 [running]:
        cmd/go/internal/list.runList({0x7ff704ffbd80, 0x7ff7050c0600}, 0xb93ce242990?, {0xb93ce13c1c0, 0x2, 0x4})
        	cmd/go/internal/list/list.go:887 +0x30cd
        main.invoke(0x7ff7050698c8, {0xb93ce13c110, 0xd, 0xf})
        	cmd/go/main.go:340 +0x8bf
        main.main()
        	cmd/go/main.go:219 +0xee5
        
--- FAIL: TestMatchingPortsStdlib (44.89s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "golang.org/x/tools/gopls/internal/cache" && test == "TestMatchingPortsStdlib"

2026-05-29 19:22 x_tools-gotip-windows-amd64-longtest tools@d46bcbe4 go@d7b3da6e x/tools/gopls/internal/cache.TestMatchingPortsStdlib (log)

=== RUN   TestMatchingPortsStdlib
    port_test.go:43: err: exit status 2: stderr: panic: runtime error: index out of range [0] with length 0
        
        goroutine 1 [running]:
        cmd/go/internal/list.runList({0x7ff704ffbd80, 0x7ff7050c0600}, 0xb93ce242990?, {0xb93ce13c1c0, 0x2, 0x4})
        	cmd/go/internal/list/list.go:887 +0x30cd
        main.invoke(0x7ff7050698c8, {0xb93ce13c110, 0xd, 0xf})
        	cmd/go/main.go:340 +0x8bf
        main.main()
        	cmd/go/main.go:219 +0xee5
        
--- FAIL: TestMatchingPortsStdlib (44.89s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "golang.org/x/tools/gopls/internal/cache" && test == "TestMatchingPortsStdlib"

2026-05-30 01:00 x_tools-gotip-darwin-amd64-longtest tools@3d7b5fba go@b7b03d9e x/tools/gopls/internal/cache.TestMatchingPortsStdlib (log)

=== RUN   TestMatchingPortsStdlib
    port_test.go:43: err: exit status 2: stderr: panic: runtime error: index out of range [0] with length 0
        
        goroutine 1 [running]:
        cmd/go/internal/list.runList({0xec45570, 0xecdd7e0}, 0x25ea4655a6a8?, {0x25ea465541a0, 0x2, 0x2})
        	cmd/go/internal/list/list.go:887 +0x30cd
        main.invoke(0xecabd48, {0x25ea465540f0, 0xd, 0xd})
        	cmd/go/main.go:340 +0x8bf
        main.main()
        	cmd/go/main.go:219 +0xf05
        
--- FAIL: TestMatchingPortsStdlib (35.68s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "golang.org/x/tools/gopls/internal/cache" && test == "TestMatchingPortsStdlib"

2026-05-30 01:00 x_tools-gotip-darwin-amd64-longtest tools@3d7b5fba go@d7b3da6e x/tools/gopls/internal/cache.TestMatchingPortsStdlib (log)

=== RUN   TestMatchingPortsStdlib
    port_test.go:43: err: exit status 2: stderr: panic: runtime error: index out of range [0] with length 0
        
        goroutine 1 [running]:
        cmd/go/internal/list.runList({0x10bd8570, 0x10c707e0}, 0x10e337d0c6a8?, {0x10e337d061a0, 0x2, 0x2})
        	cmd/go/internal/list/list.go:887 +0x30cd
        main.invoke(0x10c3ed48, {0x10e337d060f0, 0xd, 0xd})
        	cmd/go/main.go:340 +0x8bf
        main.main()
        	cmd/go/main.go:219 +0xf05
        
--- FAIL: TestMatchingPortsStdlib (27.40s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "golang.org/x/tools/gopls/internal/cache" && test == "TestMatchingPortsStdlib"

2026-05-30 01:00 x_tools-gotip-darwin-amd64-longtest tools@3d7b5fba go@555dd469 x/tools/gopls/internal/cache.TestMatchingPortsStdlib (log)

=== RUN   TestMatchingPortsStdlib
    port_test.go:43: err: exit status 2: stderr: panic: runtime error: index out of range [0] with length 0
        
        goroutine 1 [running]:
        cmd/go/internal/list.runList({0xf658570, 0xf6f07e0}, 0xe4d9ddb85e8?, {0xe4d9ddca0c0, 0x2, 0x2})
        	cmd/go/internal/list/list.go:887 +0x30cd
        main.invoke(0xf6bed48, {0xe4d9ddca010, 0xd, 0xd})
        	cmd/go/main.go:340 +0x8bf
        main.main()
        	cmd/go/main.go:219 +0xf05
        
--- FAIL: TestMatchingPortsStdlib (33.16s)

— watchflakes

[adonovan]

This logic in list.go assumes a correspondence between Imports and Concat(RawImports, CompiledImports) that is violated by the logic to add implicit linker deps of main (see addImport calls in pkg.go)

	// Record non-identity import mappings in p.ImportMap.
	for _, p := range pkgs {
		nRaw := len(p.Internal.RawImports)
		for i, path := range p.Imports {
			var srcPath string
			if i < nRaw {
				srcPath = p.Internal.RawImports[i]
			} else {
				// This path is not within the raw imports, so it must be an import
				// found only within CompiledGoFiles. Those paths are found in
				// CompiledImports.
				srcPath = p.Internal.CompiledImports[i-nRaw]
			}

			if path != srcPath {
				if p.ImportMap == nil {
					p.ImportMap = make(map[string]string)
				}
				p.ImportMap[srcPath] = path
			}
		}
	}