internal/lsp/source: fix completion crash with untyped assignee

We were panicking when a LHS assignee's type was nil, such as:

    // "foo" has not been declared
    foo = 123

A recent refactoring changed (*completer).typeMatches() to no longer
gracefully handle a nil types.Type for its first parameter. That
behavior seems fine, so fix the problematic caller of typeMatches to
check for nil before calling.

Change-Id: Ie11e4a2d374ab1efbf6fd13fbe214e06d359fca0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/221020
Reviewed-by: Rebecca Stambler <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
(cherry picked from commit 8d77031)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222979
Reviewed-by: Heschi Kreinick <[email protected]>

Author: muirdm

internal/lsp/source/completion.go

@@ -2026,6 +2026,10 @@ func (ci *candidateInference) assigneesMatch(cand *candidate, sig *types.Signatu
 			assignee = ci.assignees[i]
 		}
 
+		if assignee == nil {
+			continue
+		}
+
 		allMatch, _ = ci.typeMatches(assignee, sig.Results().At(i).Type())
 		if !allMatch {
 			break

internal/lsp/testdata/lsp/primarymod/multireturn/multi_return.go

@@ -31,6 +31,8 @@ func _() {
 	var s string
 	_, s := f //@rank(" //", multiF2Str, multiF2)
 
+	banana, s = f //@rank(" //", multiF2, multiF3)
+
 	var variadic func(int, ...int)
 	variadic() //@rank(")", multiF1, multiF0),rank(")", multiF2, multiF0),rank(")", multiF3, multiF0)
 }

internal/lsp/testdata/lsp/summary.txt.golden

@@ -5,7 +5,7 @@ CompletionSnippetCount = 68
 UnimportedCompletionsCount = 11
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 111
+RankedCompletionsCount = 112
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 39
 FoldingRangesCount = 2