Missing some symbol definitions in module build

[hokein]

Originally reported in https://issues.chromium.org/issues/390537876.

The minimal testcase is at https://github.com/hokein/module-bugs/tree/main/undefine

$ CLANG=/llvm-project/build-debug/bin/clang ./run.sh

$
... 
util.cc:(.text._Z1fv[_Z1fv]+0x5): undefined reference to `void x<int>()'
clang: error: linker command failed with exit code 1 (use -v to see invocation)                  

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Haojian Wu (hokein)

Originally reported in https://issues.chromium.org/issues/390537876.

The minimal testcase is at https://github.com/hokein/module-bugs/tree/main/undefine

$ CLANG=/llvm-project/build-debug/bin/clang ./run.sh

$
... 
util.cc:(.text._Z1fv[_Z1fv]+0x5): undefined reference to `void x<int>()'
clang: error: linker command failed with exit code 1 (use -v to see invocation)                  

[hokein]

Dumping the FD at the place

llvm-project/clang/lib/CodeGen/CodeGenAction.cpp

Line 294 in 38cb693

auto Loc = FD->getASTContext().getFullLoc(FD->getLocation());

:

FunctionDecl 0x55fabe132e20 <util.cc:3:1, line:5:1> line:3:5 main 'int ()'
`-CompoundStmt 0x55fabe1335f8 <col:12, line:5:1>
  `-CallExpr 0x55fabe1335d8 <line:4:3, col:5> 'void'
    `-ImplicitCastExpr 0x55fabe1335c0 <col:3> 'void (*)()' <FunctionToPointerDecay>
      `-DeclRefExpr 0x55fabe1330d0 <col:3> 'void ()' lvalue Function 0x55fabe132f70 'f' 'void ()'
FunctionDecl 0x55fabe132f70 <./hoge.h:5:1, col:29> col:13 imported in hoge hidden used f 'void ()' inline
`-CompoundStmt 0x55fabe133560 <col:17, col:29>
  `-CallExpr 0x55fabe133540 <col:19, col:26> 'void'
    `-ImplicitCastExpr 0x55fabe1334f8 <col:19, col:24> 'void (*)()' <FunctionToPointerDecay>
      `-DeclRefExpr 0x55fabe1330f0 <col:19, col:24> 'void ()' lvalue Function 0x55fabe1333e0 'x' 'void ()' (FunctionTemplate 0x55fabe133150 'x')
FunctionDecl 0x55fabe1333e0 <./shared_ptr2.h:4:1, col:12> col:6 imported in hoge hidden used x 'void ()' implicit_instantiation
`-TemplateArgument type 'int'
  `-BuiltinType 0x55fabe0d8e30 'int'

The x FunctionDecl is missing a function body, which causes the undefined symbol error in the linker.