gopls/internal/golang: fall back when ( is not a dynamic function call

When implementation is requested at the "(" paren of a call, implFuncs
first checks whether the call is dynamic. For calls that are not dynamic
function calls (for example, method calls), that check fails.

Instead of returning a hard error, return errNotHandled so the request
falls back to the regular method-set implementation path.

This restores the fallback behavior that regressed in
25a90be.

Fixes golang/go#77784

Author: Mani Zeygham

gopls/internal/golang/implementation.go

@@ -940,7 +940,7 @@ func implFuncs(pkg *cache.Package, curSel inspector.Cursor, start, end token.Pos
 			if inToken(n.Lparen, "(", start, end) {
 				t := dynamicFuncCallType(info, n)
 				if t == nil {
-					return nil, fmt.Errorf("not a dynamic function call")
+					return nil, errNotHandled
 				}
 				// Case 2b: dynamic call of function value.
 				// Report declarations of corresponding concrete functions.

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

@@ -8,4 +8,4 @@ go 1.18
 -- a/a.go --
 package a
 
-var _ = undefined() //@ diag("undefined", re"undefined"), implementation("(", err="not a dynamic function call")
+var _ = undefined() //@ diag("undefined", re"undefined"), implementation("(", err="no identifier found")

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

@@ -69,12 +69,11 @@ func _(
 	struct{x Nullary}{}.x() //@ loc(fieldCall, "("), implementation("(", nullaryFunc)
 
 	// Calls that are not dynamic function calls:
-	_ = len("")          //@ implementation("(", err="not a dynamic function call")
-	_ = int(0)           //@ implementation("(", err="not a dynamic function call")
-	_ = error.Error(nil) //@ implementation("(", err="not a dynamic function call")
-	_ = err.Error()      //@ implementation("(", err="not a dynamic function call")
-	_ = f4(0)            //@ implementation("(", err="not a dynamic function call"), loc(f4Call, "(")
+	_ = len("")          //@ implementation("(", err="no identifier found")
+	_ = int(0)           //@ implementation("(", err="no identifier found")
+	_ = error.Error(nil) //@ implementation("(", err="no identifier found")
+	_ = err.Error()      //@ implementation("(", err="no identifier found")
+	_ = f4(0)            //@ implementation("(", err="no identifier found"), loc(f4Call, "(")
 }
 
 
-