lsp/completion: fix ranking of *types.PkgName candidates

In Go 1.18 types.AssignableTo() started reporting that an invalid type
is assignable to any interface. *types.PkgName (i.e. an import at the
top of the file) has an invalid type for its Type(), so we started
thinking all in scope imports were great candidates when the expected
type was an interface.

Fix by wrapping the AssignableTo (and AssertableTo) to explicitly
return false if either operand is invalid.

Updates golang/go#53595

Change-Id: Ie5a84b7f410ff5c73c6b7870e052bafaf3e21e99
Reviewed-on: https://go-review.googlesource.com/c/tools/+/415595
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>

internal/lsp/source/completion/completion.go

@@ -2314,7 +2314,7 @@ func (ci candidateInference) applyTypeNameModifiers(typ types.Type) types.Type {
 // matchesVariadic returns true if we are completing a variadic
 // parameter and candType is a compatible slice type.
 func (ci candidateInference) matchesVariadic(candType types.Type) bool {
-	return ci.variadic && ci.objType != nil && types.AssignableTo(candType, types.NewSlice(ci.objType))
+	return ci.variadic && ci.objType != nil && assignableTo(candType, types.NewSlice(ci.objType))
 }
 
 // findSwitchStmt returns an *ast.CaseClause's corresponding *ast.SwitchStmt or
@@ -2640,7 +2640,7 @@ func (ci *candidateInference) candTypeMatches(cand *candidate) bool {
 			return false
 		}
 
-		if ci.convertibleTo != nil && types.ConvertibleTo(candType, ci.convertibleTo) {
+		if ci.convertibleTo != nil && convertibleTo(candType, ci.convertibleTo) {
 			return true
 		}
 
@@ -2728,7 +2728,7 @@ func considerTypeConversion(from, to types.Type, path []types.Object) bool {
 		return false
 	}
 
-	if !types.ConvertibleTo(from, to) {
+	if !convertibleTo(from, to) {
 		return false
 	}
 
@@ -2777,7 +2777,7 @@ func (ci *candidateInference) typeMatches(expType, candType types.Type) bool {
 
 	// AssignableTo covers the case where the types are equal, but also handles
 	// cases like assigning a concrete type to an interface type.
-	return types.AssignableTo(candType, expType)
+	return assignableTo(candType, expType)
 }
 
 // kindMatches reports whether candType's kind matches our expected
@@ -2840,7 +2840,7 @@ func (ci *candidateInference) assigneesMatch(cand *candidate, sig *types.Signatu
 			assignee = ci.assignees[i]
 		}
 
-		if assignee == nil {
+		if assignee == nil || assignee == types.Typ[types.Invalid] {
 			continue
 		}
 
@@ -2894,7 +2894,7 @@ func (c *completer) matchingTypeName(cand *candidate) bool {
 		//
 		// Where our expected type is "[]int", and we expect a type name.
 		if c.inference.objType != nil {
-			return types.AssignableTo(candType, c.inference.objType)
+			return assignableTo(candType, c.inference.objType)
 		}
 
 		// Default to saying any type name is a match.

internal/lsp/source/completion/util.go

@@ -321,3 +321,23 @@ func (c *completer) editText(from, to token.Pos, newText string) ([]protocol.Tex
 		NewText: newText,
 	}})
 }
+
+// assignableTo is like types.AssignableTo, but returns false if
+// either type is invalid.
+func assignableTo(x, to types.Type) bool {
+	if x == types.Typ[types.Invalid] || to == types.Typ[types.Invalid] {
+		return false
+	}
+
+	return types.AssignableTo(x, to)
+}
+
+// convertibleTo is like types.ConvertibleTo, but returns false if
+// either type is invalid.
+func convertibleTo(x, to types.Type) bool {
+	if x == types.Typ[types.Invalid] || to == types.Typ[types.Invalid] {
+		return false
+	}
+
+	return types.ConvertibleTo(x, to)
+}

internal/lsp/testdata/deep/deep.go

@@ -28,6 +28,13 @@ func _() {
 	wantsContext(c) //@rank(")", ctxBackground),rank(")", ctxTODO)
 }
 
+func _() {
+	var cork struct{ err error }
+	cork.err         //@item(deepCorkErr, "cork.err", "error", "field")
+	context          //@item(deepContextPkg, "context", "\"context\"", "package")
+	var _ error = co //@rank(" //", deepCorkErr, deepContextPkg)
+}
+
 func _() {
 	// deepCircle is circular.
 	type deepCircle struct {

internal/lsp/testdata/summary.txt.golden

@@ -6,7 +6,7 @@ CompletionSnippetCount = 106
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 163
+RankedCompletionsCount = 164
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 37
 FoldingRangesCount = 2

internal/lsp/testdata/summary_go1.18.txt.golden

@@ -6,7 +6,7 @@ CompletionSnippetCount = 116
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 173
+RankedCompletionsCount = 174
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 37
 FoldingRangesCount = 2