Bubble location from LocationForPC's ProvideOption up to errors (#332)

Previously, if a location was passed to Provide via the
LocationForPC ProvideOption, the location would not be taken
into account in the case of an error where a constructor could
not be Provided.

This change updates the error returned, making it more readable by
basing the error message on alternate function provided by
LocationForPC rather than the constructor.

For example, the following error:
```
Failed: cannot provide function "reflect".makeFuncStub (/usr/local/Cellar/go/1.17.6/libexec/src/reflect/asm_amd64.s:30):
cannot provide *fx_test.a[name="a"] from [0].Field0: already provided by
"go.uber.org/fx_test".TestAnnotate.func1 (/gocode/src/github.com/uber-go/fx/annotated_test.go:515)
```

Becomes:
```
Failed: cannot provide function "go.uber.org/fx_test".TestAnnotate.func1 (/gocode/src/github.com/uber-go/fx/annotated_test.go:515):
cannot provide *fx_test.a[name="a"] from [0].Field0: already provided by
"go.uber.org/fx_test".TestAnnotate.func1 (/gocode/src/github.com/uber-go/fx/annotated_test.go:515)
```

Co-authored-by: Sung Yoon Whang <sungyoon@uber.com>

dig_test.go

@@ -2417,6 +2417,30 @@ func testProvideFailures(t *testing.T, dryRun bool) {
 		assert.Contains(t, err.Error(), "cannot provide io.Reader")
 		assert.Contains(t, err.Error(), "already provided")
 	})
+
+	t.Run("error should refer to location given by LocationForPC ProvideOption", func(t *testing.T) {
+		c := digtest.New(t)
+		type A struct{ idx int }
+		type ret struct {
+			dig.Out
+
+			A1 A // same type A provided twice
+			A2 A
+		}
+
+		locationFn := func() {}
+
+		err := c.Provide(func() ret {
+			return ret{
+				A1: A{idx: 1},
+				A2: A{idx: 2},
+			}
+		}, dig.LocationForPC(reflect.ValueOf(locationFn).Pointer()))
+		require.Error(t, err, "provide must return error")
+		dig.AssertErrorMatches(t, err,
+			`cannot provide function "go.uber.org/dig_test".testProvideFailures.func\d+.1`,
+		)
+	})
 }
 
 func TestInvokeFailures(t *testing.T) {
@@ -3515,6 +3539,20 @@ func TestEndToEndSuccessWithAliases(t *testing.T) {
 		)
 	})
 
+	t.Run("duplicate provide with LocationForPC", func(t *testing.T) {
+		c := digtest.New(t)
+		c.RequireProvide(func(x int) float64 {
+			return testStruct{}.TestMethod(x)
+		}, dig.LocationForPC(reflect.TypeOf(testStruct{}).Method(0).Func.Pointer()))
+		err := c.Provide(func(x int) float64 {
+			return testStruct{}.TestMethod(x)
+		}, dig.LocationForPC(reflect.TypeOf(testStruct{}).Method(0).Func.Pointer()))
+
+		require.Error(t, err)
+		require.Contains(t, err.Error(), `cannot provide function "go.uber.org/dig_test".testStruct.TestMethod`)
+		require.Contains(t, err.Error(), `already provided by "go.uber.org/dig_test".testStruct.TestMethod`)
+	})
+
 	t.Run("named instances", func(t *testing.T) {
 		c := digtest.New(t)
 		type A1 struct{ s string }

provide.go

@@ -417,8 +417,15 @@ func (s *Scope) Provide(constructor interface{}, opts ...ProvideOption) error {
 	}
 
 	if err := s.provide(constructor, options); err != nil {
+		var errFunc *digreflect.Func
+		if options.Location == nil {
+			errFunc = digreflect.InspectFunc(constructor)
+		} else {
+			errFunc = options.Location
+		}
+
 		return errProvide{
-			Func:   digreflect.InspectFunc(constructor),
+			Func:   errFunc,
 			Reason: err,
 		}
 	}