gopls/internal/golang: implementation: fix nil panic

+ test

Fixes golang/go#74305

Change-Id: I1093255b448332743726f1e35bbe48689d17b522
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682955
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>

Author: adonovan

gopls/internal/golang/implementation.go

@@ -1117,7 +1117,9 @@ func beneathFuncDef(cur inspector.Cursor) bool {
 // Tested via ../test/marker/testdata/implementation/signature.txt.
 func dynamicFuncCallType(info *types.Info, call *ast.CallExpr) types.Type {
 	if typesinternal.ClassifyCall(info, call) == typesinternal.CallDynamic {
-		return info.Types[call.Fun].Type.Underlying()
+		if tv, ok := info.Types[call.Fun]; ok {
+			return tv.Type.Underlying()
+		}
 	}
 	return nil
 }

gopls/internal/test/marker/testdata/implementation/issue74305.txt

@@ -0,0 +1,11 @@
+Regression test for a crash, #74305.
+
+-- go.mod --
+module example.com
+
+go 1.18
+
+-- a/a.go --
+package a
+
+var _ = undefined() //@ diag("undefined", re"undefined"), implementation("(", err="not a dynamic function call")