[ELF] Do not report undefined weak references in shared libraries

This fixes an issue introduced in D101996.

A weak reference in a shared library could be incorrectly reported if
there is another library that has a strong reference to the same symbol.

Differential Revision: https://reviews.llvm.org/D115041

lld/ELF/InputFiles.cpp

@@ -1536,7 +1536,7 @@ template <class ELFT> void SharedFile::parse() {
       Symbol *s = symtab->addSymbol(
           Undefined{this, name, sym.getBinding(), sym.st_other, sym.getType()});
       s->exportDynamic = true;
-      if (s->isUndefined() && !s->isWeak() &&
+      if (s->isUndefined() && sym.getBinding() != STB_WEAK &&
           config->unresolvedSymbolsInShlib != UnresolvedPolicy::Ignore)
         requiredSymbols.push_back(s);
       continue;

lld/test/ELF/allow-shlib-undefined-weak.s

@@ -0,0 +1,48 @@
+# REQUIRES: x86
+
+# RUN: rm -rf %t.dir
+# RUN: split-file %s %t.dir
+# RUN: cd %t.dir
+
+## Verify that in the following case:
+##
+##   <exec>
+##   +- ref.so (weak reference to foo)
+##   +- wrap.so (non-weak reference to foo)
+##      +- def.so (defines foo)
+##
+## we don't report that foo is undefined in ref.so when linking <exec>.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 ref.s -o ref.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 wrap.s -o wrap.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 def.s -o def.o
+# RUN: ld.lld -shared ref.o -o ref.so
+# RUN: ld.lld -shared def.o -soname def.so -o def.so
+# RUN: ld.lld -shared wrap.o def.so -o wrap.so
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 start.s -o start.o
+# RUN: ld.lld --no-allow-shlib-undefined start.o wrap.so ref.so -o /dev/null 2>&1 | count 0
+
+#--- start.s
+.globl _start
+_start:
+  callq wrap_get_foo@PLT
+
+#--- ref.s
+.weak foo
+.globl ref_get_foo
+ref_get_foo:
+  movq foo@GOTPCREL(%rip), %rax
+  retq
+
+#--- wrap.s
+.globl wrap_get_foo
+wrap_get_foo:
+  movq foo@GOTPCREL(%rip), %rax
+  retq
+
+#--- def.s
+.data
+.globl foo
+foo:
+  .long 0