Warn about shadowing names when importing a lexical sub whose name ma…

…tches a package one

builtin.c

@@ -55,6 +55,14 @@ Perl_prepare_export_lexical(pTHX)
 #define export_lexical(name, sv)  S_export_lexical(aTHX_ name, sv)
 static void S_export_lexical(pTHX_ SV *name, SV *sv)
 {
+    if(ckWARN(WARN_SHADOW) && SvPVX(name)[0] == '&') {
+        CV *cv = get_cvn_flags(SvPVX(name)+1, SvCUR(name)-1, GV_NOTQUAL);
+        if(cv)
+            warner(packWARN(WARN_SHADOW),
+                "Lexical import of subroutine %" SVf_QUOTEDPREFIX " masks package subroutine",
+                SVfARG(name));
+    }
+
     PADOFFSET off = pad_add_name_sv(name, padadd_STATE, 0, 0);
     SvREFCNT_dec(PL_curpad[off]);
     PL_curpad[off] = SvREFCNT_inc(sv);

lib/builtin.t

@@ -276,6 +276,7 @@ package FetchStoreCounter {
 # imported builtins can be unexported
 {
     package UnimportTest;
+    no warnings 'shadow';
 
     sub true() { return "true" };
 

pod/perldiag.pod

@@ -3573,6 +3573,12 @@ The number of items in a hash can be obtained by doing:
 
     scalar(keys %hash);
 
+=item Lexical import of subroutine "%s" masks package subroutine
+
+(W shadow) The current package already contains a subroutine whose name
+matches that of a lexically imported subroutine.  The latter will become
+visible and make the package one inaccessible by name.
+
 =item Lexing code attempted to stuff non-Latin-1 character into Latin-1 input
 
 (F) An extension is attempting to insert text into the current parse

t/lib/warnings/builtin

@@ -100,3 +100,10 @@ use builtin qw(true false);
 use builtin ':5.39';
 use builtin ':5.39';
 EXPECT
+########
+# builtin.c - import warns of collisions with package subs
+use warnings 'shadow';
+sub true { "ok" }
+use builtin 'true';
+EXPECT
+Lexical import of subroutine "&true" masks package subroutine at - line 4.