cl: tolerate invalid receivers during syntax preload

[cpunion]

Depends on #2222.

Summary

• make syntax preload skip only function declarations whose receiver AST cannot be converted safely, instead of panicking
• normalize legal parenthesized, pointer, and generic receiver forms through a single checked conversion path
• keep processing later valid declarations and their linkname directives
• match the exact same-line recovery diagnostics for fixedbugs/issue20789.go and remove that xfail

The regression starts with the valid-but-invalid-receiver declaration func ([]int) m() {}. It also covers the review counterexamples ((*T)), (*(T)), generic value receivers, and parenthesized generic pointer receivers, so the checked path does not reject legal receiver syntax.

Validation

• go test ./cl -run "^(TestParsePkgSyntaxSkipsInvalidReceiver|TestAstAndTypesFuncNameCoverage|TestParsePkgSyntax.*)$" -count=1
• go test ./internal/packages ./cmd/internal/compile ./test/goroot -count=1
• fixedbugs/issue20789.go with an empty xfail set on Go 1.24.11, 1.25.0, 1.26.0, and 1.26.5
• all remaining Go 1.26 compile/errorcheck xfails on Go 1.26.5
• git diff --check

No broad package parse-error gate is added; unrelated declarations and packages keep their existing preload behavior.

[fennoai]

Review summary

Reviewed the full 8-commit diff (cl/import.go core change + test/goroot recovery-pairing harness and xfail removals). The core refactor is correct and well-guarded:

• recvTypeNameInfo / astFuncNameOK fail safe on every non-Ident/non-simple-index receiver shape (including nil), correctly normalize parenthesized, pointer, and generic (G[P], ((*G[P]))) forms via ast.Unparen, and ParsePkgSyntax now continues on an unconvertible receiver instead of panicking.
• The retained panic("unreachable") in recvTypeName / astFuncName is only reachable from the post-typecheck initFiles path, so it is a genuine invariant guard rather than a latent crash on malformed source.
• The test coverage (fail-open: missing/mismatched/wrong-line/wrong-file/line-directive primaries; TestParsePkgSyntaxSkipsInvalidReceiver) is thorough, and the diagnosticPathResolver.resolve tightening is a good hardening.

No bugs, regressions, security, or performance issues found. Doc comments match final-state code. One minor maintainability nit posted inline; the remaining observations below are optional.

Optional (non-blocking)

• test/goroot/runner_test.go — parserRecoverySecondaries / parserRecoverySourceSecondaries / parserRecoverySecondaryGroups form a hand-maintained catalog of exact GOROOT source shapes and exact go/parser diagnostic spellings. This is intentional and well-commented, but the literals are version-sensitive (see the invalid use of ... Go-version difference already captured) and will silently rot on a Go bump. Consider consolidating the three functions into a single data table (primary, optional exact-source, secondary-groups) plus a note on how to regenerate/verify the golden spellings when bumping Go.
• cl/import.go:385,426 — a one-line comment on each panic wrapper noting that the invalid-receiver path is handled by astFuncNameOK/recvTypeNameInfo and this variant asserts a checked (post-typecheck) invariant would prevent a future reader from treating the panic as dead code or a latent crash.
• test/goroot/runner_test.go hasLineDirective — the //line branch requires start-of-line-only preceding whitespace while the /*line branch intentionally does not; this is correct per Go's spec but looks asymmetric. A one-line comment would prevent a future "fix" from breaking it.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/import.go	78.94%	4 Missing and 4 partials ⚠️

📢 Thoughts on this report? Let us know!

[cpunion]

Rebased onto the updated #2222 head after #2218 merged, then addressed the review and Codecov gaps in 51d67b23c.

The update documents the two post-typecheck invariant wrappers and adds direct counterexamples for an indexed non-identifier receiver base, an invalid array receiver, an index-list receiver, nil declarations and names, empty and nil receiver fields, invalid receiver types, and both invariant panic paths.

Validation after rebase:

• focused cl receiver/parser tests
• go test ./internal/packages ./cmd/internal/compile ./test/goroot -count=1
• Go 1.26.5 fixedbugs/issue20789.go
• git diff --check

The new CI and coverage run is in progress.