go/types: fix method set computation

When computing method sets, any struct field that "shadows" a
method at a lower embedding level eliminates that method from
the method set. Treat any field at a given level as a "collision"
for any methods at lower embedding level.

Method sets are not directly used by go/types (except for self-
verification in debug mode); they are a functionality provided
by go/types. Thus, the method sets that go/types is using were
not affected by this bug.

Fixes golang#37081.

Change-Id: Ic1937e01891b3614a6f7965d4384aeb485f3fe3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/218617
Reviewed-by: Alan Donovan <[email protected]>

src/go/types/example_test.go

@@ -120,6 +120,9 @@ import "fmt"
 type Celsius float64
 func (c Celsius) String() string  { return fmt.Sprintf("%g°C", c) }
 func (c *Celsius) SetF(f float64) { *c = Celsius(f - 32 / 9 * 5) }
+
+type S struct { I; m int }
+type I interface { m() byte }
 `
 	fset := token.NewFileSet()
 	f, err := parser.ParseFile(fset, "celsius.go", input, 0)
@@ -147,13 +150,21 @@ func (c *Celsius) SetF(f float64) { *c = Celsius(f - 32 / 9 * 5) }
 		fmt.Println()
 	}
 
+	// Print the method set of S.
+	styp := pkg.Scope().Lookup("S").Type()
+	fmt.Printf("Method set of %s:\n", styp)
+	fmt.Println(types.NewMethodSet(styp))
+
 	// Output:
 	// Method set of temperature.Celsius:
 	// method (temperature.Celsius) String() string
 	//
 	// Method set of *temperature.Celsius:
 	// method (*temperature.Celsius) SetF(f float64)
 	// method (*temperature.Celsius) String() string
+	//
+	// Method set of temperature.S:
+	// MethodSet {}
 }
 
 // ExampleInfo prints various facts recorded by the type checker in a

src/go/types/methodset.go

@@ -166,17 +166,15 @@ func NewMethodSet(T Type) *MethodSet {
 			}
 		}
 
-		// Multiple fields with matching names collide at this depth and shadow all
-		// entries further down; add them as collisions to base if no entries with
-		// matching names exist already.
-		for k, f := range fset {
-			if f == nil {
-				if _, found := base[k]; !found {
-					if base == nil {
-						base = make(methodSet)
-					}
-					base[k] = nil // collision
+		// Add all fields at this depth as collisions (since they will hide any
+		// method further down) to base if no entries with matching names exist
+		// already.
+		for k := range fset {
+			if _, found := base[k]; !found {
+				if base == nil {
+					base = make(methodSet)
 				}
+				base[k] = nil // collision
 			}
 		}
 
@@ -233,7 +231,7 @@ func (s fieldSet) add(f *Var, multiples bool) fieldSet {
 
 // A methodSet is a set of methods and name collisions.
 // A collision indicates that multiple methods with the
-// same unique id appeared.
+// same unique id, or a field with that id appeared.
 type methodSet map[string]*Selection // a nil entry indicates a name collision
 
 // Add adds all functions in list to the method set s.