go/doc: tune factory method association logic

Ignore predeclared types (such as error) in result parameter lists when determining
with which result type a method should be associated with. This change will again
associate common factory functions with the first result type even if there are more
than one result, as long as the others are predeclared types.

Fixes #27928

Change-Id: Ia2aeaed15fc4c8debdeeaf729cc7fbba1612cafb
Reviewed-on: https://go-review.googlesource.com/c/141617
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/doc/reader.go

@@ -365,6 +365,12 @@ func (r *reader) readType(decl *ast.GenDecl, spec *ast.TypeSpec) {
 	}
 }
 
+// isPredeclared reports whether n denotes a predeclared type.
+//
+func (r *reader) isPredeclared(n string) bool {
+	return predeclaredTypes[n] && r.types[n] == nil
+}
+
 // readFunc processes a func or method declaration.
 //
 func (r *reader) readFunc(fun *ast.FuncDecl) {
@@ -398,29 +404,30 @@ func (r *reader) readFunc(fun *ast.FuncDecl) {
 		return
 	}
 
-	// Associate factory functions with the first visible result type, if that
-	// is the only type returned.
+	// Associate factory functions with the first visible result type, as long as
+	// others are predeclared types.
 	if fun.Type.Results.NumFields() >= 1 {
 		var typ *namedType // type to associate the function with
 		numResultTypes := 0
 		for _, res := range fun.Type.Results.List {
-			// exactly one (named or anonymous) result associated
-			// with the first type in result signature (there may
-			// be more than one result)
 			factoryType := res.Type
 			if t, ok := factoryType.(*ast.ArrayType); ok {
 				// We consider functions that return slices or arrays of type
 				// T (or pointers to T) as factory functions of T.
 				factoryType = t.Elt
 			}
-			if n, imp := baseTypeName(factoryType); !imp && r.isVisible(n) {
+			if n, imp := baseTypeName(factoryType); !imp && r.isVisible(n) && !r.isPredeclared(n) {
 				if t := r.lookupType(n); t != nil {
 					typ = t
 					numResultTypes++
+					if numResultTypes > 1 {
+						break
+					}
 				}
 			}
 		}
-		// If there is exactly one result type, associate the function with that type.
+		// If there is exactly one result type,
+		// associate the function with that type.
 		if numResultTypes == 1 {
 			typ.funcs.set(fun, r.mode&PreserveAST != 0)
 			return
@@ -494,7 +501,7 @@ func (r *reader) readFile(src *ast.File) {
 		}
 	}
 
-	// add all declarations
+	// add all declarations but for functions which are processed in a separate pass
 	for _, decl := range src.Decls {
 		switch d := decl.(type) {
 		case *ast.GenDecl:
@@ -548,8 +555,6 @@ func (r *reader) readFile(src *ast.File) {
 					}
 				}
 			}
-		case *ast.FuncDecl:
-			r.readFunc(d)
 		}
 	}
 
@@ -586,6 +591,15 @@ func (r *reader) readPackage(pkg *ast.Package, mode Mode) {
 		}
 		r.readFile(f)
 	}
+
+	// process functions now that we have better type information
+	for _, f := range pkg.Files {
+		for _, decl := range f.Decls {
+			if d, ok := decl.(*ast.FuncDecl); ok {
+				r.readFunc(d)
+			}
+		}
+	}
 }
 
 // ----------------------------------------------------------------------------

src/go/doc/testdata/issue12839.0.golden

@@ -14,9 +14,15 @@ FUNCTIONS
 	// F1 should not be associated with T1 
 	func F1() (*T1, *T2)
 
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
 	// F4 should not be associated with a type (same as F1) 
 	func F4() (a T1, b T2)
 
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
 
 TYPES
 	// 
@@ -28,6 +34,18 @@ TYPES
 	// F3 should be associated with T1 because b.T3 is from a ...
 	func F3() (a T1, b p.T3)
 
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
 	// 
 	type T2 struct{}
 

src/go/doc/testdata/issue12839.1.golden

@@ -14,9 +14,15 @@ FUNCTIONS
 	// F1 should not be associated with T1 
 	func F1() (*T1, *T2)
 
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
 	// F4 should not be associated with a type (same as F1) 
 	func F4() (a T1, b T2)
 
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
 
 TYPES
 	// 
@@ -28,6 +34,18 @@ TYPES
 	// F3 should be associated with T1 because b.T3 is from a ...
 	func F3() (a T1, b p.T3)
 
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
 	// 
 	func (t T1) hello() string
 

src/go/doc/testdata/issue12839.2.golden

@@ -14,9 +14,15 @@ FUNCTIONS
 	// F1 should not be associated with T1 
 	func F1() (*T1, *T2)
 
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
 	// F4 should not be associated with a type (same as F1) 
 	func F4() (a T1, b T2)
 
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
 
 TYPES
 	// 
@@ -28,6 +34,18 @@ TYPES
 	// F3 should be associated with T1 because b.T3 is from a ...
 	func F3() (a T1, b p.T3)
 
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
 	// 
 	type T2 struct{}
 

src/go/doc/testdata/issue12839.go

@@ -5,6 +5,7 @@
 // Package issue12839 is a go/doc test to test association of a function
 // that returns multiple types.
 // See golang.org/issue/12839.
+// (See also golang.org/issue/27928.)
 package issue12839
 
 import "p"
@@ -36,3 +37,33 @@ func F3() (a T1, b p.T3) {
 func F4() (a T1, b T2) {
 	return T1{}, T2{}
 }
+
+// F5 should be associated with T1.
+func F5() (T1, error) {
+	return T1{}, nil
+}
+
+// F6 should be associated with T1.
+func F6() (*T1, error) {
+	return &T1{}, nil
+}
+
+// F7 should be associated with T1.
+func F7() (T1, string) {
+	return T1{}, nil
+}
+
+// F8 should be associated with T1.
+func F8() (int, T1, string) {
+	return 0, T1{}, nil
+}
+
+// F9 should not be associated with T1.
+func F9() (int, T1, T2) {
+	return 0, T1{}, T2{}
+}
+
+// F10 should not be associated with T1.
+func F10() (T1, T2, error) {
+	return T1{}, T2{}, nil
+}