cmd/go: report importer-specific positions in go list dependency errors

src/cmd/go/internal/list/list.go

@@ -936,11 +936,23 @@ func collectDeps(p *load.Package) {
 func collectDepsErrors(p *load.Package) {
 	depsErrors := make(map[*load.PackageError]bool)
 
-	for _, p := range p.Internal.Imports {
-		if p.Error != nil {
-			depsErrors[p.Error] = true
+	for _, dep := range p.Internal.Imports {
+		if dep.Error != nil {
+			// The cached Package for dep may be shared by multiple importers.
+			// Clone the error and, if Pos refers to the importer side,
+			// replace it with p's own import position (#78183).
+			depErr := new(dep.Error)
+			if depErr.IsImporterPos() && p.Internal.Build != nil {
+				if pos := p.Internal.Build.ImportPos[dep.ImportPath]; len(pos) > 0 {
+					p0 := pos[0]
+					p0.Filename = base.ShortPath(p0.Filename)
+					depErr.Pos = p0.String()
+				}
+			}
+			depsErrors[&depErr] = true
 		}
-		for _, q := range p.DepsErrors {
+
+		for _, q := range dep.DepsErrors {
 			depsErrors[q] = true
 		}
 	}

src/cmd/go/internal/load/pkg.go

@@ -343,6 +343,7 @@ func (p *Package) setLoadPackageDataError(err error, path string, stk *ImportSta
 	top, ok := stk.Top()
 	if ok && path != top.Pkg {
 		p.Error.setPos(importPos)
+		p.Error.setImporterPos(importPos)
 	}
 }
 
@@ -457,6 +458,7 @@ type PackageError struct {
 	Err              error       // the error itself
 	IsImportCycle    bool        // the error is an import cycle
 	alwaysPrintStack bool        // whether to always print the ImportStack
+	importerPos      bool        // Pos refers to the importer, not the imported package
 }
 
 func (p *PackageError) Error() string {
@@ -509,6 +511,18 @@ func (p *PackageError) setPos(posList []token.Position) {
 	pos := posList[0]
 	pos.Filename = base.ShortPath(pos.Filename)
 	p.Pos = pos.String()
+	p.importerPos = true
+}
+
+func (p *PackageError) setImporterPos(posList []token.Position) {
+	p.setPos(posList)
+	p.importerPos = true
+}
+
+// IsImporterPos reports whether Pos refers to the import statement
+// in the importing package, not to the imported package's source.
+func (p *PackageError) IsImporterPos() bool {
+	return p.importerPos
 }
 
 // ImportPathError is a type of error that prevents a package from being loaded
@@ -781,17 +795,20 @@ func loadImport(loaderstate *modload.State, ctx context.Context, opts PackageOpt
 			}
 			p.Incomplete = true
 			p.Error.setPos(importPos)
+			p.Error.setImporterPos(importPos)
 		}
 	}
 
 	// Checked on every import because the rules depend on the code doing the importing.
 	if perr := disallowInternal(loaderstate, ctx, srcDir, parent, parentPath, p, stk); perr != nil {
 		perr.setPos(importPos)
+		perr.setImporterPos(importPos)
 		return p, perr
 	}
 	if mode&ResolveImport != 0 {
 		if perr := disallowVendor(srcDir, path, parentPath, p, stk); perr != nil {
 			perr.setPos(importPos)
+			perr.setImporterPos(importPos)
 			return p, perr
 		}
 	}
@@ -802,6 +819,7 @@ func loadImport(loaderstate *modload.State, ctx context.Context, opts PackageOpt
 			Err:         ImportErrorf(path, "import %q is a program, not an importable package", path),
 		}
 		perr.setPos(importPos)
+		perr.setImporterPos(importPos)
 		return p, perr
 	}
 
@@ -817,6 +835,7 @@ func loadImport(loaderstate *modload.State, ctx context.Context, opts PackageOpt
 			Err:         err,
 		}
 		perr.setPos(importPos)
+		perr.setImporterPos(importPos)
 		return p, perr
 	}
 
@@ -1802,6 +1821,7 @@ func (p *Package) load(loaderstate *modload.State, ctx context.Context, opts Pac
 			top, ok := stk.Top()
 			if ok && path != top.Pkg && len(importPos) > 0 {
 				p.Error.setPos(importPos)
+				p.Error.setImporterPos(importPos)
 			}
 		}
 	}

src/cmd/go/testdata/script/golist_missingimport.txt

@@ -0,0 +1,26 @@
+# Test that go list reports correct Pos for each import of a missing package.
+# When two different packages import the same missing package, each should
+# report the position of its own import statement, not reuse the first one.
+# See issue #78183.
+
+env GOWORK=off
+
+go list -e -f '{{range .DepsErrors}}package {{$.ImportPath}}: error at {{.Pos}}: {{.Err}}{{end}}' ./...
+
+stdout 'package example.com/test/p: error at p/p.go:3:8: no required module'
+stdout 'package example.com/test/q: error at q/q.go:3:8: no required module'
+
+-- go.mod --
+module example.com/test
+
+go 1.22
+
+-- p/p.go --
+package p
+
+import _ "example.com/missing"
+
+-- q/q.go --
+package q
+
+import _ "example.com/missing"