gopls/internal/golang/completion: fix crash in instance conversion

The new inference logic assumed that a CallExpr surrounding an instance
was a function, but it could be a conversion, builtin, or invalid type.
Add the missing check.

Fixes golang/go#70889

Change-Id: I0b05a90cca671196cf5cfd8782b6863e17485cc1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/637635
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Commit-Queue: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/completion/completion.go

@@ -2708,7 +2708,7 @@ func reverseInferTypeArgs(sig *types.Signature, typeArgs []types.Type, expectedR
 	return substs
 }
 
-// inferExpectedTypeArg gives a type param candidateInference based on the surroundings of it's call site.
+// inferExpectedTypeArg gives a type param candidateInference based on the surroundings of its call site.
 // If successful, the inf parameter is returned with only it's objType field updated.
 //
 // callNodeIdx is the index within the completion path of the type parameter's parent call expression.
@@ -2722,9 +2722,12 @@ func (c *completer) inferExpectedTypeArg(callNodeIdx int, typeParamIdx int) type
 	if !ok {
 		return nil
 	}
+	sig, ok := c.pkg.TypesInfo().Types[callNode.Fun].Type.(*types.Signature)
+	if !ok {
+		return nil
+	}
 
-	// Infer the type parameters in a function call based on it's context
-	sig := c.pkg.TypesInfo().Types[callNode.Fun].Type.(*types.Signature)
+	// Infer the type parameters in a function call based on context
 	expectedResults := inferExpectedResultTypes(c, callNodeIdx)
 	if typeParamIdx < 0 || typeParamIdx >= sig.TypeParams().Len() {
 		return nil

gopls/internal/test/marker/testdata/completion/type_params.txt

@@ -15,11 +15,13 @@ package typeparams
 
 func one[a int | string]()            {}
 func two[a int | string, b float64 | int]() {}
+type three[a any] int
 
 func _() {
 	one[]() //@rank("]", string, float64)
 	two[]() //@rank("]", int, float64)
 	two[int, f]() //@rank("]", float64, float32)
+	int(three[]) //@rank("]") // must not crash (golang/go#70889)
 }
 
 func slices[a []int | []float64]() {} //@item(tpInts, "[]int", "[]int", "type"),item(tpFloats, "[]float64", "[]float64", "type")