go/internal/srcimporter: report reimport of incomplete packages

See the issue below for details.

For #19337.

Change-Id: I7637dcd4408f1bc4a9b3050a107aadb4de6f950b
Reviewed-on: https://go-review.googlesource.com/37620Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/srcimporter/srcimporter.go

@@ -92,10 +92,15 @@ func (p *Importer) ImportFrom(path, srcDir string, mode types.ImportMode) (*type
 		if pkg == &importing {
 			return nil, fmt.Errorf("import cycle through package %q", bp.ImportPath)
 		}
-		if pkg.Complete() {
+		if !pkg.Complete() {
+			// package exists but is not complete - we cannot handle this
+			// at the moment since the source importer replaces the package
+			// wholesale rather than augmenting it (see #19337 for details)
+			return nil, fmt.Errorf("reimported partially imported package %q", bp.ImportPath)
+		}
 		return pkg, nil
 	}
-	} else {
+
 	p.packages[bp.ImportPath] = &importing
 	defer func() {
 		// clean up in case of error
@@ -106,7 +111,6 @@ func (p *Importer) ImportFrom(path, srcDir string, mode types.ImportMode) (*type
 			p.packages[bp.ImportPath] = nil
 		}
 	}()
-	}
 
 	// collect package files
 	bp, err = p.ctxt.ImportDir(bp.Dir, 0)

src/go/internal/srcimporter/srcimporter_test.go

@@ -132,3 +132,19 @@ func TestImportedTypes(t *testing.T) {
 		}
 	}
 }
+
+func TestReimport(t *testing.T) {
+	if runtime.GOOS == "nacl" {
+		t.Skip("no source code available")
+	}
+
+	// Reimporting a partially imported (incomplete) package is not supported (see issue #19337).
+	// Make sure we recognize the situation and report an error.
+
+	mathPkg := types.NewPackage("math", "math") // incomplete package
+	importer := New(&build.Default, token.NewFileSet(), map[string]*types.Package{mathPkg.Path(): mathPkg})
+	_, err := importer.ImportFrom("math", ".", 0)
+	if err == nil || !strings.HasPrefix(err.Error(), "reimport") {
+		t.Errorf("got %v; want reimport error", err)
+	}
+}